Delia Radicum

Kathleen Menacer
email: kathleen.menacer@gmail.com

Maxime R Hervé

Anne Marie Cortesero

Tom Aujames

Sylvia Anton

Sex-and maturity-dependent antennal detection of host plant volatiles in the cabbage root fly

Keywords: cabbage root fly (Delia radicum), olfactory plasticity, host plant volatiles, antennal sensitivity, electroantennogram

Highlights

-Dose-dependent electoantennogram responses were obtained for all tested compounds -Mature insect antennae respond more to several host-derived compounds -Immature insect antennae respond more to a behaviourally attractive flower volatile -Male and female antennae respond with different sensitivity to certain compounds

Introduction

Insect olfactory-guided behaviour is known to differ between males and females and to show strong plasticity as a function of adult maturity (age), and mating state [START_REF] Gadenne | Plasticity in insect olfaction: To smell or not to smell?[END_REF]. This plasticity helps insects to make optimal decisions when responding to sex pheromones, host-, food-, or oviposition site odours. In several moth species, recently mated male moths do not respond to female-emitted sex pheromones, because they would not be able to mate successfully [START_REF] Gadenne | Transient post-mating inhibition of behavioural and central nervous responses to sex pheromone in an insect[END_REF]. Blood-feeding insects such as mosquitoes and hematophagous bugs also adapt their responses to host odours as a function of age and mating state, thus avoiding taking unnecessary risks during the approach and attack of a host animal ( [START_REF] Gadenne | Plasticity in insect olfaction: To smell or not to smell?[END_REF] and references therein).

More recently, differences in behavioural responses to plant volatiles as a function of sex, maturity and mating state have been investigated in various phytophagous insects. In the fruit fly Bactrocera tyroni (Froggatt) (Diptera: Tephritidae), mated females were more attracted to their host fruit volatiles than unmated females [START_REF] Devescovi | Mating-induced changes in responses of female Queensland fruit fly to male pheromones and fruit: A mechanism for mating-induced sexual inhibition[END_REF]. In the tomato fruit fly, Neoceratitis cyanescens (Bezzi) (Diptera: Tephritidae), both mated and unmated females responded to ripe tomato fruits, whereas males were rather attracted by odours from unripe fruits [START_REF] Brevault | Flower and fruit volatiles assist host-plant location in the Tomato fruit fly Neoceratitis cyanescens[END_REF]. In the cotton leafworm, Spodoptera littoralis (Boisduval) (Lepidoptera, Noctuidae), females moth change their behavioural preferences from flower odours to odours emitted by plants used for oviposition after mating (Saveer et al., 2012). As a last example, in several species, male moths are only rarely attracted by host plant volatiles, but host plant volatiles can synergize responses to the female-emitted sex pheromone [START_REF] Masante-Roca | Attraction of the grapevine moth to host and non-host plant parts in the wind tunnel: effects of plant phenology, sex, and mating status[END_REF][START_REF] Ochieng | Host plant volatiles synergize responses of sex pheromonespecific olfactory receptor neurons in male Helicoverpa zea[END_REF][START_REF] Reddy | Interactions of insect pheromones and plant semiochemicals[END_REF][START_REF] Varela | Synergism of pheromone and host-plant volatile blends in the attraction of Grapholita molesta males: Pheromone and plant volatile synergism in Grapholita molesta[END_REF][START_REF] Yang | Host Plant Volatiles Synergize Response to Sex Pheromone in Codling Moth, Cydia pomonella[END_REF].

The physiological mechanisms underlying behavioural plasticity have been investigated in detail specifically in male moths responding to sex pheromones. Here, depending on the species, either changes in the peripheral or central olfactory system have been described [START_REF] Gadenne | Plasticity in insect olfaction: To smell or not to smell?[END_REF] and references therein). In the noctuid moth Agrotis ipsilon (Hufnagel) (Lepidoptera: Noctuidae), male responses to the sex pheromone increase with maturity, but not responses to plant volatiles [START_REF] Greiner | Central Processing of Plant Volatiles in Agrotis ipsilon Males is Age-independent in Contrast to Sex Pheromone Processing[END_REF]. However central, but not peripheral neuron responses to a flower volatile increased after mating in A. ipsilon, possibly facilitating a switch from searching for a mate to feeding behaviour [START_REF] Barrozo | Mating-induced differential coding of plant odour and sex pheromone in a male moth[END_REF]. In some moth species, on the contrary, both peripheral and central nervous responses to host plant volatiles have been found to vary between sexes and as a function of maturity and mating state, in line with behavioural changes. For example in the grapevine moth Lobesia botrana (Denis and Schiffermüller) (Lepidoptera: Tortricidae), antennal lobe neurons in unmated males and in mated females responded more often to selected plant compounds than in mated males and unmated females [START_REF] Masante-Roca | Attraction of the grapevine moth to host and non-host plant parts in the wind tunnel: effects of plant phenology, sex, and mating status[END_REF]. Antennal responses of Spodoptera littoralis females to plant odours, on the other hand, were generally stronger in unmated females as compared to mated females [START_REF] Martel | Peripheral modulation of olfaction by physiological state in the Egyptian leaf worm Spodoptera littoralis (Lepidoptera: Noctuidae)[END_REF]. Interestingly, glomerular responses to a flower odour measured by calcium imaging activity representing essentially receptor neuron responses in the same species decreased after mating, whereas glomerular responses to host plant volatiles (cotton) increased after mating (Saveer et al., 2012). Finally, differential expression of antennal olfactory genes has been shown in different insects, such as Drosophila suzukii (Matsumura) (Diptera: Drosophilidae), Ceratitis capitata (Wiedemann) (Diptera: Tephritidae) and Bactrocera dorsalis (Hendel) (Diptera: Tephritidae) after mating [START_REF] Crava | Functional transcriptome analyses of Drosophila suzukii antennae reveal mating-dependent olfaction plasticity in females[END_REF][START_REF] Siciliano | Sniffing out chemosensory genes from the Mediterranean fruit fly, Ceratitis capitata[END_REF][START_REF] Jin | Olfactory Plasticity: Variation in the Expression of Chemosensory Receptors in Bactrocera dorsalis in Different Physiological States[END_REF].

Adults of the cabbage root fly, Delia radicum L. (Diptera: Anthomyiidae), feed from the nectar of a large diversity of plant species, whereas females are specialists of Brassicaceae plants for oviposition [START_REF] Finch | Ecological Considerations in the Management of Delia Pest Species in Vegetable Crops[END_REF][START_REF] Rännbäck | Flower attractiveness and nectar accessibility for Delia radicum (Diptera: Anthomyiidae) with implications for the control by Trybliographa rapae (Hymenoptera:Figitidae)[END_REF]. The cabbage root fly represents an interesting model to study sex differences and physiological plasticity in olfaction, because adults need several days to reach maturity and males and females might potentially respond differently to host-derived compounds, due to a differential role of host plants between the sexes. In addition to visual cues, females have been shown to use certain host plant-emitted odours, such as allyl isothiocyanate (AITC), to localize oviposition sites in the field [START_REF] Tuttle | Role of visual and olfactory stimuli in host finding of adult cabbage root flies, Delia radicum[END_REF]. Also cis-3-hexenyl acetate, a volatile emitted by damaged host plants has been shown to be attractive for females [START_REF] Kergunteuil | Selecting volatiles to protect brassicaceous crops against the cabbage root fly, Delia radicum: Selecting volatiles against Delia radicum[END_REF]. Other compounds such as dimethyl disulfide (DMDS) strongly decrease oviposition [START_REF] Ferry | Field evaluation of the combined deterrent and attractive effects of dimethyl disulfide on Delia radicum and its natural enemies[END_REF][START_REF] Kergunteuil | Selecting volatiles to protect brassicaceous crops against the cabbage root fly, Delia radicum: Selecting volatiles against Delia radicum[END_REF][START_REF] Lamy | Artificially applied plant volatile organic compounds modify the behavior of a pest with no adverse effect on its natural enemies in the field: Improving the push-pull strategy against a major Brassicaceae pest[END_REF]. Upon emergence and for the first few days of their life, unmated females and males search first for food sources and seem not to respond to host plant odours [START_REF] Hawkes | Physiological Condition of Adult Cabbage Root Fly (Erioischia brassicae (Bouche)) Attracted to Host-Plants[END_REF][START_REF] Nottingham | Host-plant finding for oviposition by adult cabbage root fly, Delia radicum[END_REF]. Females usually start oviposition after a maturation phase that lasts about five days after emergence [START_REF] Hawkes | Physiological Condition of Adult Cabbage Root Fly (Erioischia brassicae (Bouche)) Attracted to Host-Plants[END_REF][START_REF] Nottingham | Host-plant finding for oviposition by adult cabbage root fly, Delia radicum[END_REF], but it is unknown whether this maturation, in addition to mating, contributes to changes in plant odour responses. Much less is known about how D. radicum males find their mating partners and if they use odour cues for orientation [START_REF] Finch | Upwind flight by the cabbage root fly, Delia radicum[END_REF]. In particular, no sex pheromone has been identified so far. Males show some behavioural responses to AITC, but lower than females. This response could potentially help them to find a mating partner as this compound is only emitted by brassicaceous plants [START_REF] Finch | Upwind flight by the cabbage root fly, Delia radicum[END_REF] and mating usually takes place near the emergence site and therefore near the host plants (Coaker andFinch , 1971 in Nottingham, 1988). The main olfactory organ of D. radicum, the funiculus of the antenna, carries large numbers of four types of olfactory sensilla: trichoid, basiconic, grooved and clavate (Ross andAnderson, 1991, 1987). Interestingly, female antennae carry about 40 % more sensilla than male antennae and lower sensillum numbers in males were observed for all types, which correlates with behavioural differences between sexes [START_REF] Ross | Morphology of the antennal sensilla of the cabbage root fly, Delia radicum L. (Diptera : Anthomyiidae)[END_REF].

As a basis for future behavioural and physiological studies, we examine here for the first time antennal responses of D. radicum to a selection of host-derived compounds using electroantennographic recordings. We reveal dose-dependent antennal responses for all tested compounds, showing that the tested compounds are all detected by the cabbage root fly antenna. Our secondary aim was to identify volatile organic compounds, previously identified from intact and damaged host plants, which might be detected differentially between the sexes or as a function of maturity, in order to choose compounds to investigate variation in behavioural responses in the future. To detect such potential differences, we compared dose-response curves and mean response amplitudes between sexes, and between recently hatched immature, and mature mated insects.

Materials and Methods

Experimental insects

in the field in 2019 (Pleumeur-Gautier, Brittany, France). Flies were reared on rutabagas in a climate chamber at 20 ± 2 °C, 16L:8D, and 60 ± 10 % RH and fed with a milk powder: yeast: sugar (1:1:1) mixture as described in Neveu Bernard-Griffiths (1998). For EAG experiments, pupae were transferred to small cages, which were changed every two days to a new cage, controlling thus for age of emerged insects. Males and females were kept together to obtain mated mature males and females, which were fed ad libitum as under rearing conditions. Mature, assumedly mated insects were used at five to seven days of age. In favourable condition such as these, mating is known to take place on the 4th day after emergence (Coaker andFinch, 1971 in Nottingham, 1988). Immature, unmated insects were obtained by transferring individual pupae into Eppendorf cups with a small amount of food and a moist piece of cotton. Emerging insects were checked every day and insects were used within 24 h after emergence.

Assuming that all immature insects were unmated, and all mature insects were mated, we use only the terms "immature" for immature unmated flies and "mature" for mature mated flies in the remaining text.

EAG recording

Whole insects were used for EAG recordings, with flies slipped into a plastic disposable pipette tip with the head and antennae protruding, secured with dental wax. Glass pipettes filled with Beadle Ephrussi Ringer were used as electrodes. The reference electrode was inserted into a small opening in the cuticle of the eye and the recording electrode was placed tightly in the centre of the undamaged funiculus similarly to EAG recordings in other flies (e.g. [START_REF] Hieu | Behavioural and electroantennogram responses of the stable fly (Stomoxys calcitrans L.) to plant essential oils and their mixtures with attractants: Effects of repellent oil with attractants on stable fly[END_REF]. The electrodes were connected to an amplifier (axoclamp 2B, Molecular Devices, San Jose, CA, USA), and an IDAC-4 device to digitize signals (Syntech, Kirchzarten, Germany). EAG Pro software (Syntech, Kirchzarten, Germany) was used to record the antennal responses.

Stimulation procedure

The antennae were superfused by a constant air-stream (0.3 m.s -1 , 17 mL.s -1 ) through a glass tube.

Stimuli were applied onto a piece of filter paper inserted into a Pasteur Pipette and the pipette was introduced into the glass tube through a hole in the sidewall. A stimulus controller (CS 55, Syntech, Kirchzarten, Germany) was used to apply a 500 ms air pulse through the stimulation pipette. During the stimulation, the airflow was switched from the continuous to the stimulus flow, which allowed the mechanoreceptive component to be minimized. Inter-stimulus intervals lasted at least 1 s. For each stimulus, increasing doses in decadic steps from 0.01 to 1000 µg were applied to obtain dose-response curves and the solvent, mineral oil, was applied before and after each stimulus series. The mean response to mineral oil before and after a stimulus dose series was subtracted from each amplitude value (= net amplitude) to compensate for amplitude differences between insects and over time. Recordings from fly antennae were, however, extremely stable over several hours, so that we did not need to further normalize data as a function of time. For each stimulus, sex, and maturity state 10 individuals were tested per session (see below).

Volatile organic compounds used for antennal stimulation

All compounds tested were previously identified in odour blends emitted by host plants of D. radicum (Supplementary Table A). Fifteen compounds were retained, which were taken as representatives for a variety of chemical classes having demonstrated effects on phytophagous insects in general and specialists of Brassicaceae in particular: sulfur compounds (AITC, DMDS), green leaf volatiles (cis-3hexenyl acetate), defense hormones (methyl salicylate), aromatic compounds (ethylacetophenone), sesquiterpenes (α-farnesene and β-caryophyllene) and monoterpenes (α-pinene, α-thujene, eucalyptol, limonene, linalool, myrcene, ocimene and sabinene). Whereas differences in behavioural responses between the sexes and between immature and mature flies have not been studied so far, some of these compounds have been identified as having effects on mature D. radicum females. Indeed, AITC and cis-3-hexenyl acetate have been shown to attract mature flies toward oviposition sites [START_REF] Finch | Upwind flight by the cabbage root fly, Delia radicum[END_REF][START_REF] Kergunteuil | Selecting volatiles to protect brassicaceous crops against the cabbage root fly, Delia radicum: Selecting volatiles against Delia radicum[END_REF][START_REF] Nottingham | Host-plant finding for oviposition by adult cabbage root fly, Delia radicum[END_REF][START_REF] Tuttle | Role of visual and olfactory stimuli in host finding of adult cabbage root flies, Delia radicum[END_REF] and to stimulate oviposition in field [START_REF] Kergunteuil | Selecting volatiles to protect brassicaceous crops against the cabbage root fly, Delia radicum: Selecting volatiles against Delia radicum[END_REF][START_REF] Lamy | One more step toward a push-pull strategy combining both a trap crop and plant volatile organic compounds against the cabbage root fly Delia radicum[END_REF] and laboratory conditions [START_REF] Traynier | Stimulation of oviposition by the cabbage root fly Erioischia brassicae[END_REF][START_REF] Traynier | Chemostimulation of Oviposition by the Cabbage Root Fly Erioischia brassicae (Bouché)[END_REF]. DMDS [START_REF] Ferry | Field evaluation of the combined deterrent and attractive effects of dimethyl disulfide on Delia radicum and its natural enemies[END_REF][START_REF] Kergunteuil | Selecting volatiles to protect brassicaceous crops against the cabbage root fly, Delia radicum: Selecting volatiles against Delia radicum[END_REF][START_REF] Lamy | Artificially applied plant volatile organic compounds modify the behavior of a pest with no adverse effect on its natural enemies in the field: Improving the push-pull strategy against a major Brassicaceae pest[END_REF][START_REF] Lamy | One more step toward a push-pull strategy combining both a trap crop and plant volatile organic compounds against the cabbage root fly Delia radicum[END_REF], eucalyptol [START_REF] Lamy | Artificially applied plant volatile organic compounds modify the behavior of a pest with no adverse effect on its natural enemies in the field: Improving the push-pull strategy against a major Brassicaceae pest[END_REF] and limonene [START_REF] Kergunteuil | Des odeurs pour protéger les cultures: utilisation de composés volatils pour modifier le comportement de la mouche du chou, Delia radicum et de ses ennemis naturels[END_REF], when applied in dispensers close to the host plants, were found to reduce oviposition. Moreover, ethylacetophenone is emitted by flowers of many species including Lobularia maritima, a brassicaceous species whose flowers are particularly attractive to D. radicum females [START_REF] Kergunteuil | Selecting volatiles to protect brassicaceous crops against the cabbage root fly, Delia radicum: Selecting volatiles against Delia radicum[END_REF][START_REF] Rännbäck | Flower attractiveness and nectar accessibility for Delia radicum (Diptera: Anthomyiidae) with implications for the control by Trybliographa rapae (Hymenoptera:Figitidae)[END_REF], and methyl salicylate is a herbivoreinduced plant volatile emitted by damaged brassicaceous plants in particular [START_REF] Pierre | Differences in Volatile Profiles of Turnip Plants Subjected to Single and Dual Herbivory Above-and Belowground[END_REF]. All other tested compounds are found in the volatile organic compounds (VOC) profiles of brassicaceous plants, but have not been tested for their effect on behavioural responses of D. radicum adults yet [START_REF] Kergunteuil | Characterizing Volatiles and Attractiveness of Five Brassicaceous Plants with Potential for a 'Push-Pull' Strategy Toward the Cabbage Root Fly, Delia radicum[END_REF][START_REF] Pierre | Differences in Volatile Profiles of Turnip Plants Subjected to Single and Dual Herbivory Above-and Belowground[END_REF][START_REF] Tollsten | Headspace volatiles of whole plants and macerated plant parts of Brassica and Sinapis[END_REF]. Three of the fifteen compounds (namely ethylacetophenone, α-thujene and sabinene) were not available at the beginning of the experiments. Therefore, compounds were tested in two experimental sessions separated by a few weeks, with different individuals used in each session. Nine compounds were tested in the first session only, three compounds were tested in the second session only, and three compounds were tested in both sessions to ensure consistency in the interpretation of the results of the two sessions (Table 1).

Data processing and statistical analyses

All analyses were performed using the R software version 4.1.3 (R Core Team 2022). EAG amplitude responses were analyzed both univariately and multivariately. First, for each compound, the mean amplitude response was analyzed using Wald tests applied to Linear Mixed Models in which the sex, maturity status and dose (log-transformed), as well as all interactions between these variables, were considered as independent variables and the individual was considered as a random factor (R packages 'car' [START_REF] Fox | An {R} Companion to Applied Regression, Third Edition[END_REF], 'lme4' [START_REF] Bates | Fitting linear mixed-effects models Usinglme4[END_REF], 'RVAideMemoire' [START_REF] Hervé | RVAideMemoire: Testing and Plotting Procedures for Biostatistics[END_REF].

Responses were log-transformed to ensure model fit and, since some negative responses were obtained, normalized by computing 𝑥 + |min (𝑥)| + 0.01 to avoid zeroes that would cause infinite logs. Pairwise comparisons of Estimated Marginal Means (EMMeans) were computed with p-values adjusted using the False Discovery Rate correction [START_REF] Benjamini | Controlling the False Discovery Rate: a practical and powerful approach to multiple testing[END_REF] (R package 'emmeans' [START_REF] Lenth | Characterization of Plant Volatiles Reveals Distinct Metabolic Profiles and Pathways among 12 Brassicaceae Vegetables[END_REF]). The multivariate approach consisted in two Redundancy Analyses (RDAs, R package 'vegan' [START_REF] Oksanen | Community Ecology Package. R package version 2[END_REF], i.e. one per session. Responses were the overall mean EAG amplitude per individual and per compound (i.e. all doses confounded), which were normalized as above for consistency with the univariate approach and autoscaled. Independent variables were the sex, the maturity status and the interaction between these two factors. The effect of these factors was tested using a permutation F test with 9 999 permutations [START_REF] Legendre | Numerical ecology[END_REF]. Pairwise comparisons were performed also using permutation F-tests and p-values corrected with the False Discovery Rate method.

Results

EAG response characteristics and dose-response curves vary between tested compounds

A significant dose effect was observed for all compounds showing that antennae of D. radicum responded in a dose-dependent manner to the fifteen volatiles tested (Table 1, Fig. 1, Fig. 2) The EAG amplitudes of responses varied between individual insects and reached maximum net amplitudes of up different thresholds and slopes were found for different compounds (Fig. 1, Fig. 2). Generally, antennae responded already at the lowest tested dose to ocimene, β-caryophyllene and DMDS, but responses increased only slightly when increasing the tested doses (Fig. 3). For other compounds such as eucalyptol and α-pinene, responses only started at a dose of 100 μg and increased at a dose of 1 000 μg (Fig. 3). For still other compounds such as linalool and AITC, responses started at 1 or 10 μg and increased steeply at high doses (Fig. 3).

EAG responses to a majority of compounds are sex-and maturity-dependent

Mean amplitudes of responses, as well as slopes of the dose-response curves, varied significantly between sexes, maturity status or both for 10 of the 15 compounds tested (Table 1). These effects were very similar between the two experimental sessions and are therefore described together.

A significant, independent effect of the sex was observed for two compounds (Table 1, Fig. 4a, Fig. 1).

In males larger responses were found for β-caryophyllene than in females, whereas female antennae exhibited larger responses for ocimene than male antennae. An interaction of dose and sex was only significant for two of the 15 compounds tested (Table 1). In males larger responses were found at high stimulus doses for eucalyptol than in females (Fig. 1). In females, on the other hand, larger responses were found at high stimulus doses for myrcene than in males (Fig. 2). A significant, sex-independent effect of maturity was observed for four compounds (Table 1, Fig. 4a,d, Fig. 1, Fig. 2). Antennae of immature flies exhibited larger responses to α-farnesene and ethylacetophenone, while larger responses were found in mature flies for DMDS and β-caryophyllene. An interaction of dose and maturity was significant for five compounds (Table 1). Responses at high doses of AITC, cis-3-hexenyl acetate, limonene, myrcene and ocimene exhibited larger amplitudes in mature compared to immature flies (Fig. 1).

When focusing on the mean amplitude of response per compound, both the univariate and multivariate approaches gave a consistent picture (Table 1, Fig. 4). In the first test session, a significant global effect of the sex was observed (F = 2.57, p = 0.019, Fig. 4b). Male antennae respond more intensely to methyl salicylate, β-caryophyllene and α-pinene (Fig. 4a,c). An independent, non-interacting (F = 0.72, p = 0.703), effect of the maturity status was observed (F = 2.86, p = 0.008, Fig. 4b). Antennae of immature individuals of both sexes responded more strongly to α-farnesene, limonene, and myrcene, in contrast to mature individuals who responded more strongly to AITC and DMDS (Fig. 4a,c). In the second session, only a significant effect of the maturity status was observed (F = 4.48, p = 0.004, Fig. 4e), with a stronger antennal response of immature individuals to ethylacetophenone and α-thujene, while mature individuals responded more strongly to AITC (Fig 4d,f,). No significant effect of the sex or the interaction between the sex and the maturity status was observed in the second session (respectively F = 0.22, p = 0.165; F = 1.01, p = 0.395) (Fig. 4d,f). Session 1 AITC 128,53 1 < 0,001 0,42 1 0,519 0,05 1 0,824 0,05 1 0,815 8,22 1 0,004 2,16 1 0,142 1,85 1 0,174 DMDS 14,76 1 < 0,001 1,68 1 0,195 7,78 1 0,005 0,07 1 0,797 1,74 1 0,187 0,94 1 0,333 0,31 1 0,579 cis-3-Hexenyl acetate 254,43 1 < 0,001 0,01 1 0,918 2,21 1 0,137 0,23 1 0,630 8,63 1 0,003 0,86 1 0,354 0,11 1 0,743

Methyl salicylate 162,23 1 < 0,001 1,08 1 0,298 3,39 1 0,065 0,94 1 0,331 0,06 1 0,805 0,74 1 0,391 2,33 1 0,127 α-Farnesene 77,08 1 < 0,001 0,08 1 0,775 6,19 1 0,013 1,53 1 0,216 1,85 1 0,174 2,23 1 0,135 0,51 1 0,477 β-Caryophyllene 48,25 1 < 0,001 9,98 1 0,002 6,92 1 0,009 0,46 1 0,498 2,25 1 0,133 0,02 1 0,879 0,18 1 0,674 α-Pinene 99,68 1 < 0,001 3,64 1 0,056 0,09 1 0,767 0,74 1 0,389 2,74 1 0,098 0,03 1 0,870 1,08 1 0,299 Eucayptol 159,40 1 < 0,001 0,92 1 0,336 0,08 1 0,777 4,34 1 0,037 1,65 1 0,198 1,09 1 0,296 1,52 1 0,217 Limonene 140,33 1 < 0,001 1,96 1 0,162 3,48 1 0,062 0,12 1 0,726 5,84 1 0,016 0,37 1 0,541 0,02 1 0,882 1 Linalool 379,59 1 < 0,001 0,69 1 0,406 0,28 1 0,598 0,95 1 0,329 0,28 1 0,597 0,83 1 0,364 1,19 0,276 Myrcene 144,22 1 < 0,001 0,06 1 0,799 5,42 1 0,020 10,31 1 0,001 6,15 1 0,013 0,74 1 0,389 1,63 0,202 Ocimene 124,60 1 < 0,001 4,69 1 0,030 2,05 1 0,152 0,10 1 0,752 4,35 1 0,037 0,01 1 0,928 0,64 0,423 Session 2 AITC 214,44 1 < 0,001 0,38 1 0,537 7,46 1 0,006 0,04 1 0,848 1,10 1 0,294 2,22 1 0,136 2,16 0,142 Ethylacetophenone 386,01 1 < 0,001 0,13 1 0,714 6,44 1 0,011 2,31 1 0,128 0,21 1 0,647 0,08 1 0,782 0,01 0,934 α -Thujene 47,81 1 < 0,001 0,07 1 0,789 2,16 1 0,141 0,01 1 0,939 0,06 1 0,804 0,10 1 0,758 0,20 0,651 Myrcene 158,87 1 < 0,001 14,05 1 < 0,001 6,11 1 0,013 6,09 1 0,014 1,82 1 0,177 0,02 1 0,883 0,85 0,357 Ocimene 215,26 1 < 0,001 3,44 1 0,064 0,80 1 0,372 3,58 1 0,059 0,28 1 0,597 0,05 1 0,828 0,85 0,356 Sabinene 216,50 1 < 0,001 2,59 1 0,107 0,07 1 0,790 0,03 1 0,855 0,10 1 0,746 2,11 1 0,146 1,08 0,300

Discussion

Our results show that all tested plant-derived compounds are detected by D. radicum antennae in a dosedependent manner and reveal highly variable responses to these compounds, as a function of sex and maturity. Interestingly, the differences in antennal responses between mature and immature flies, as well as between males and females, depend strongly on the tested compound. Behavioural significance of the tested compounds is known only for a few of them and almost exclusively in mature females so far.

Therefore, we can here only speculate on the behavioural significance of the differences in antennal detection, which we observed between the sexes and immature and mature flies. Two of the three compounds, known to reduce oviposition in D. radicum females, DMDS and limonene, elicited stronger responses in mature than in immature flies of both sexes, at least at high doses. The observed reduced oviposition rate of female flies exposed to these compounds [START_REF] Lamy | Artificially applied plant volatile organic compounds modify the behavior of a pest with no adverse effect on its natural enemies in the field: Improving the push-pull strategy against a major Brassicaceae pest[END_REF]Liu et al., 2018) might thus be due to a high sensitivity to these compounds, mediating a potential repulsive effect. However, behavioural assays with immature insects need to confirm whether the sensitivity difference is indeed responsible for such an effect only in mature insects. Allyl isothiocyanate and cis-3-hexenyl acetate, compounds known to attract flies towards oviposition sites [START_REF] Traynier | Chemostimulation of Oviposition by the Cabbage Root Fly Erioischia brassicae (Bouché)[END_REF][START_REF] Traynier | Stimulation of oviposition by the cabbage root fly Erioischia brassicae[END_REF]Städler, 1978), elicited increased EAG responses, at least at high doses, in mature compared to immature adults.

Whereas increased sensitivity in mature females correlates with the stimulation of oviposition, it would now be interesting to evaluate behavioural effects of these compounds on mature males. A compound emitted by flowers used for nectar feeding in D. radicum [START_REF] Rännbäck | Flower attractiveness and nectar accessibility for Delia radicum (Diptera: Anthomyiidae) with implications for the control by Trybliographa rapae (Hymenoptera:Figitidae)[END_REF], ethylacetophenone, elicited much stronger EAG responses in immature insects than in mature insects. This could be related to the importance to find food sources in immature flies. Interestingly many of the compounds shown to be emitted in various host plants of D. radicum elicited stronger responses in mature than in immature insects (at least at higher doses) and some compounds elicited stronger responses in females than in males at higher doses. This increased sensitivity could indicate that the concerned compounds might play a role in host finding behaviour. However, the occasionally observed higher sensitivity in males needs to be further investigated.

Our results on the plasticity of antennal responses to host plant odours in this species provide a complex dataset, allowing to choose compounds of interest for future behavioural experiments as a function of sex and maturity of this insect. However, even though the compounds not eliciting any differences in antennal responses between sexes and maturity might be potentially of less importance for adult flies, one should not exclude these compounds from further investigations, because modulation of plant odour sensitivity might well occur at the central nervous level, as previously observed in different moth species [START_REF] Barrozo | Mating-induced differential coding of plant odour and sex pheromone in a male moth[END_REF][START_REF] Masante-Roca | Attraction of the grapevine moth to host and non-host plant parts in the wind tunnel: effects of plant phenology, sex, and mating status[END_REF].

Differences in antennal olfactory sensitivity to plant-derived volatiles between mature and immature insects and between the sexes have rarely been investigated systematically. In other studies, single doses of different components have been used as stimuli [START_REF] Martel | Peripheral modulation of olfaction by physiological state in the Egyptian leaf worm Spodoptera littoralis (Lepidoptera: Noctuidae)[END_REF], but this limits the conclusions that can be drawn. Indeed, as shown by our results, dose-response curves with different thresholds and different slopes can occur and thus differences can be hidden when testing only a single dose.

Nevertheless significant differences in responses to a panel of plant-derived compounds have been found in females of the noctuid moth Spodoptera littoralis as a function of mating and adult age [START_REF] Martel | Peripheral modulation of olfaction by physiological state in the Egyptian leaf worm Spodoptera littoralis (Lepidoptera: Noctuidae)[END_REF]. Also in the Mediterranean fruit fly, Ceratitis capitata, EAG responses to plant headspace of different host plants varied between the sexes, but also in both males and females as a function of mating state and the origin of the flies (laboratory-reared or wildtype) [START_REF] Sollai | Differences in the Olfactory Sensitivity of Ceratitis capitata to Headspace of Some Host Plants in Relation to Sex, Mating Condition and Population[END_REF]. EAG responses of female blowflies to attractive and repellent compounds similarly depended on the maturity: response amplitudes increased with age [START_REF] Crnjar | Influence of age on the electroantennogram response of the female blowfly (Phormia regina) (Diptera, Calliphoridae)[END_REF]. Our results on the cabbage root fly confirm that indeed differences in plant volatile responses between the sexes and maturity of insects exist already at the detection level.

Whereas modulatory mechanisms of central olfactory sensitivity have been described in different insects, among others as a function of maturity [START_REF] Gadenne | Plasticity in insect olfaction: To smell or not to smell?[END_REF][START_REF] Ignell | Juvenile-hormone-mediated plasticity of aggregation behaviour and olfactory processing in adult desert locusts[END_REF][START_REF] Jarriault | Age-dependent plasticity of sex pheromone response in the moth, Agrotis ipsilon: Combined effects of octopamine and juvenile hormone[END_REF], the mechanisms underlying peripheral modulation of olfactory sensitivity are not well understood. Up-and down-regulation of the expression of olfaction-related antennal genes, such as olfactory receptors and odorant binding proteins, for example, seem to correlate with antennal sensitivity. In C. capitata for example, odorant binding protein expression levels varied as a function of sex, sexual maturity and mating state, correlated with recently found EAG differences in response to volatile organic compounds [START_REF] Siciliano | Sniffing out chemosensory genes from the Mediterranean fruit fly, Ceratitis capitata[END_REF][START_REF] Sollai | Differences in the Olfactory Sensitivity of Ceratitis capitata to Headspace of Some Host Plants in Relation to Sex, Mating Condition and Population[END_REF]. In the oriental fruit fly Bactrocera dorsalis, mating leads to changes in the expression of various olfactory receptor genes, including ORs, GRs and IRs, but the corresponding ligands are not known [START_REF] Jin | Olfactory Plasticity: Variation in the Expression of Chemosensory Receptors in Bactrocera dorsalis in Different Physiological States[END_REF]. In the same species, changes of EAG sensitivity to a compound present in host plants as a function of starvation depended on the presence of short neuropeptide F and the expression of its receptor [START_REF] Jiang | The short neuropeptide F modulates olfactory sensitivity of Bactrocera dorsalis upon starvation[END_REF]. With the recently sequenced genome of D. radicum available [START_REF] Sontowski | A high-quality functional genome assembly of Delia radicum L. (Diptera: Anthomyiidae) annotated from egg to adult[END_REF], it will be possible in the future to identify olfactory genes involved in the differences in antennal sensitivity between the sexes and as a function of maturity. Another potential mechanism is modulation of peripheral olfactory sensitivity by modulatory feedback neurons from the brain and reaching the antennae, as it has been shown for serotoninergic neurons in mosquitoes [START_REF] Siju | Immunocytochemical localization of serotonin in the central and peripheral chemosensory system of mosquitoes[END_REF]. Immunostaining of D. radicum antennae would be necessary to potentially reveal such a mechanism.

CRediT author contribution statement

Kathleen Menacer: Conceptualization, Formal analysis, Writing. Maxime R. Hervé:

Conceptualization, Formal analysis, Writing, Supervision, Project administration, Funding acquisition.

Anne

Fig. 1

 1 Fig. 1 Dose-response curves (mean ± SEM) of D. radicum EAG amplitudes in response to the different VOCs tested in session 1. Colored lowercase letters indicate significant difference in doseresponse slopes. The inserts represent the mean amplitude (± SE) (i.e., all doses confounded) in response to the different VOCs. Note that in the inserts, different uppercase letters indicate significant differences between sexes whereas different lowercase letters indicate significant differences between maturity statuses. Light pink = immature females, dark pink = mature females, light blue = immature males, dark blue = mature males. AITC: allyl isothiocyanate; DMDS: dimethyl disulfide

Fig. 4

 4 Fig. 4 Sex and maturity-dependent EAG responses to plant volatile compounds. Heatmaps of mean EAG amplitudes (i.e., all doses confounded) in response to the tested compounds according to sex and maturity of Delia radicum individuals, for session 1 (a) and session 2 (d). Mean amplitudes are pictured independently for each compound and represented by a color gradient from light yellow (lowest level)to dark red (highest level). Note that results are not comparable between lines but only between columns on a given line, due to compound-specific statistical standardisation of mean response amplitudes. Score plots and associated correlation circles of the redundancy analyses performed on mean EAG response amplitudes for each individual, for session 1 (b)(c) and session 2 (e)(f). AITC: allyl isothiocyanate, EAP: ethylacetophenone.

Table 1 : Wald tests on the effect of sex, maturity status, dose as well as all interactions between these variables on the mean EAG amplitude of Delia radicum for the compounds tested according to sessions.

 1 Significant p-values are indicated in bold

		Dose			Sex			Maturity			Dose:Sex		Dose:Maturity	Sex:Maturity	Dose:Sex:Maturity
	Compound
	χ 2	df	P	χ 2	df	P	χ 2	df	P	χ 2	df	P	χ 2	df	P	χ 2	df	P	χ 2	df	P

  Marie Cortesero: Conceptualization, Writing, supervision, Funding acquisition. Tom Aujames:

					Found in the volatile profiles of different D. radicum Brassica	Tollsten & Bergstrom, 1988; Pierre et al.,
	β -Caryophyllene	sesquiterpene	≥ 98.5%	Sigma	hosts species	2011; Kergunteuil et al., 2015
					Predominant in the VOC profile of oil seed rape
	α-Pinene	monoterpene	98%	Aldrich	Found in the VOC profiles of different D. radicum Brassica hosts species	Tollsten & Bergstrom, 1988; Pierre et al., 2011; Kergunteuil et al., 2015
	α-Thujene	monoterpene	95 %	Toronto Research Chemicals	Found in the VOC profiles of different D. radicum Brassica hosts species,	Kergunteuil et al., 2015
					Reduces oviposition
	Eucalyptol	monoterpene	99%	Aldrich		Lamy et al., 2017; Liu et al., 2018
					Found in some Brassica VOC profiles
					Found in volatile profiles of different D. radicum Brassica
	Limonene	monoterpene	97%	Sigma Aldrich	hosts species	Pierre et al., 2011; Kergunteuil, 2013; Kergunteuil et al., 2015; Liu et al., 2018
					Reduces plant colonization
	Investigation. Sylvia Anton: Conceptualization, Methodology, Investigation, Formal analysis, Writing-Linalool monoterpene 97% Aldrich Predominant in the VOC profile of Chinese cabbage	Kergunteuil et al., 2015
	original draft, Supervision.
	Myrcene	monoterpene	Technical grade	Aldrich	Found in the volatile profiles of different D. radicum Brassica host species	Tollsten & Bergstrom, 1988; Kergunteuil et al., 2015
	Ocimene (mixture of isomers)	monoterpene	≥ 90%	Sigma-Aldrich	Found in the volatile profiles of different D. radicum Brassica hosts species	Tollsten & Bergstrom, 1988
	Sabinene	monoterpene	98%	abcr chemicals	Found in the volatile profiles of different D. radicum Brassica hosts species	Tollsten & Bergstrom, 1988
	1

Acknowledgements

We thank Christophe Lunel for help with insect rearing and shipping.

Funding

This work was supported by a doctoral fellowship from Université Rennes 1 and a Research grant from the department Plant Health and Environment of INRAE, France.

Declaration of Competing Interest:

The authors declare that they have no known competing financial interests or personal relationships that would have appeared to influence the work reported in this paper. [START_REF] Tollsten | Headspace volatiles of whole plants and macerated plant parts of Brassica and Sinapis[END_REF][START_REF] Pierre | Differences in Volatile Profiles of Turnip Plants Subjected to Single and Dual Herbivory Above-and Belowground[END_REF][START_REF] Kergunteuil | Characterizing Volatiles and Attractiveness of Five Brassicaceous Plants with Potential for a 'Push-Pull' Strategy Toward the Cabbage Root Fly, Delia radicum[END_REF]

Supplementary


Alexandra Vignolles
email: avignolles@groupeinseec.com

Paul-Emmanuel Pichon
email: paul.pichon@voila.fr

The marketing Use of nostalgia : An exploratory study and a nostalgic perception scale

INTRODUCTION

Food consumption often takes back people into their past.

It calls on several senses and can be a valuable vector of nostalgia. The links between nostalgia and food consumption seem to be self-evident; yet, very little research work has been conducted so far [START_REF] Baker | My favourite recipes: recreating emotions and memories through cooking[END_REF]. Food consumption is a recurring theme when dealing with the concept of nostalgia and nostalgic evocations are quite often associated with food consumption experiences. Every consumer will remember an experience, a product or a brand related to a nostalgic episode. Our paper has a twofold purpose: gaining a more thorough understanding of the themes and emotions connected with nostalgic food consumption and suggesting a nostalgic perception scale in order to help businesses improve their position on the market and consumer segmentation.

As part of this work, a first exploratory study was conducted with 300 persons interviewed on the subject of nostalgic consumption in general. Out of the 300 answers, over a 100 allude to food consumption. Most consumers seem to establish a link between nostalgia and food. One can talk of « the Proust little Madeleine crumb » effect, that is travelling back into one's childhood memories either with elation, regret or with a bittersweet emotional reaction.

In the first part of this article, we will focus on the main results of this exploratory qualitative analysis ; then we will present the construction and the validation of the nostalgic perception scale.

FOOD CONSUMPTION AND NOSTALGIA : CLARIFYING THE CONCEPT.

We will first introduce and elucidate the concepts described in this research work. It is indeed esential to understand that nowadays the food context is unsettled and that nostalgia can be a relevant concept to comfort the consumers looking for information or in their purchasing and consumption activities.

Food consumption : a disrupted context

In the last twenty years, food crises have occurred repeatedly at the national and international level. Food and its risks are of great concern to consumers. A recent report published by the European Commission states that food health problems identified in the EU have increased by 22% in 2005 even if it is to be noted that this trend is also related to a larger number of controls and of systematic warnings. 1According to a recent survey by the CREDOCfood behaviour and consumption2 (september 2005) -85% of the respondent households think that their food pattern has an impact on their health as against 79% in 2000 and 75% in 1997. This explains why the issues linked to food hazards have been of great interest to marketing researchers in the last few years (e.g. [START_REF] Kapferer | les marques, base de la confiance ?[END_REF][START_REF] Sirieix | La consommation Alimentaire : Problématiques, Approches et Voies de Recherche[END_REF][START_REF] Sirieix | Le discours des consommateurs sur la confiance : l'apport des analyses lexicales et structurales[END_REF][START_REF] Sirieix | Confiance des consommateurs et choix des lieux d'achat :le cas de l'achat de vin[END_REF]Kreziak and Joly, 2000 ;[START_REF] Marouseau | Sécurité Sanitaire en Agroalimentaire : Evolution conjointe du droit et de la gestion des risques[END_REF][START_REF] Cazes-Valette | Le comportement du consommateur décodé par l'anthropologie : le cas des crises de la vache folle[END_REF]Dandouau, 2001 ;[START_REF] Filser | Crise alimentaire ou crise du marketing ? Revue Française du Marketing[END_REF][START_REF] Fischler | Le besoin de réassurance en consommation alimentaire[END_REF][START_REF] Gurviez | Le rôle de la confiance dans la perception des risques alimentaires par les consommateurs[END_REF][START_REF] Loisel | Une typologie des attitudes vis-à-vis du risque alimentaire[END_REF][START_REF] Brunel | Les Stratégies d'ajustement au risque alimentaire : modèle théorique et test empirique[END_REF][START_REF] Muraro-Cochart | L'utilité des réducteurs de risque alimentaire dans un contexte de crise, in les Actes du 2nd atelier de Recherche de l'AFM[END_REF][START_REF] Pichon | Perception et réduction du risque lors de l'achat de produits alimentaires en grande distribution : facteurs d'influence et rôle de la confiance[END_REF]. More recent work considers food hazards as a vital consumption hazard and as a current trend in our western societies [START_REF] Bergadaà | Le risque alimentaire comme risque vital perçu de consommation : émergences, adaptation et gestion[END_REF].

The list of possible threats is long : BSE, the Buffalo Grill scandal in 2002, the foot and mouth disease, the link between food patterns and heart diseases. Modern food is a source of deep anxiety; as a result, consumers no longer trust what they eat. The fear of choosing the wrong product creates an anxiety and a strong feeling of guilt. «If you lose control over the nature and purity of what you eat , you may also lose control of your health and even of your identity » [START_REF] Fischler | Le besoin de réassurance en consommation alimentaire[END_REF]. This means major consequences for the consumers. The feeling of discomfort, anxiety and the loss of confidence in food products can be accounted for by four main factors [START_REF] Poulain | Les nouvelles pratiques alimentaires, entre commensalisme et vagabondage[END_REF] : the lack of product identification, food abundance, contradictory statements and the influence of the media. Some nutrients of the agrifood industry arouse suspicion and doubt in the consumers' minds whereas traditional food is considered as safe. The consumers seeking authenticity in the origin, symbolic dimension and unicity of products [START_REF] Camus | L'authenticité marchande perçue : Pour une échelle de mesure appliquée au domaine alimentaire, 20ème Congrès de l'AFM[END_REF] are faced with a dilemma : on the one hand they are attracted by cheap convenience food and on the other hand they are scared because very little is known in this respect.

The concept of nostalgia and its implications in marketing. deserve closer scrutiny. In the next paragraph, we will present its main features.

Nostalgia and food behaviour

Nostalgia comes from the Greek words nostos (return) and algos (pain). In its first first meaning, « nostalgia »was the homesickness caused by the yearning to return to one's homeland. This concept was already visible in Biblical books or in Greek mythology (Homer, The Odyssey )

The first studies on the concept of nostalgia were conducted in medicine with [START_REF] Hofer | Medical dissertation on nostalgia (traduit du latin par Anspach C. K.)[END_REF] who reviewed its effects on Swiss soldiers. In his thesis, he described the clinical physical and psychological signs of nostalgia. Gradually, the word meaning has changed. Nowadays, nostalgia no longer relates to a pathology or an ailment but rather to " a yearning for a past personal or reported experience" (Petit Larousse, 1994). Some marketing researchers have pursued work on the concept of nostalgia in the last few years Several definitions suggested by [START_REF] Holbrook | Echoes of the Dear Departed Past: Some Work in Progress on Nostalgia[END_REF][START_REF] Holak | Nostalgia: An Exploratory Study of Themes and Emotions in the Nostalgia Experience[END_REF][START_REF] Divard | La nostalgie: un thème récent dans la recherche en marketing[END_REF] are summarized in the table below (table 1).

Nostalgia is considered as a mood, a preference or more often as an emotional reaction. Nostalgia is necessary to the human being for different reasons, i.e for its positive or negative role. According to [START_REF] Sedikides | Nostalgia: Conceptual Issues and Existential Functions, Handbook of Experimental Existential Psychology[END_REF], nostalgia fulfills existential functions and acts « as a stock of emotions and experiences which people resort to in order to cope with their existential fears ». Exploring the highly complex and extraordinarily abundant concept of nostalgia can provide additional tools to better understand consumers and purchasers'behaviours.

Many agrifood businesses choose products, design, packaging, sales outlets or communication based on the concept of nostalgia.

Let us mention the Bannette baguette, the Saint Michel galettes or the Bonne Maman jams with their Vichy patterned lids which remind us of home-made jams. Other products recall an "à l'ancienne" design even if the recipe or the product is not at all old, such as for instance Vico « à l'ancienne crisps» or some frozen food recipes like the Tipiak products praised by the famous grannies from Brittany. The Paul bakery shops concept gathers all the necessary ingredients (furniture, employees'clothing, smell of freshly baked bread) necessary to recreate nostalgia of times past.

If we agree to say that nostalgia helps to reassure people, the study of its links with food consumption should be enlightening. Food consumption is largely characterized by risktaking which is more often subjective than objective. Even if food hazards are objectively less nowadays, the general public has got the feeling that these hazards are more severe (Apfelbaum, 1998).In this context, nostalgia could be a reassuring factor for the consumers.

Distinction between personal and collective nostalgia

Several research work have helped to better understand nostalgia. [START_REF] Stern | Historical and personal nostalgia in advertising text: the Fin de siecle effect[END_REF] makes the distinction between two types of nostalgia : on the one hand « personal nostalgia » idealizing one's past life and « historical nostalgia » which corresponds to the wish to return to a past (not necessarily identified) which is considered as being better than the present. This first definition shows that personal experience is not mandatory to recreate nostalgia which can be experienced «by surrogacy». An illustration of this is the example of the «yuppies » buying the New Volkswagen Beetle car although they may have never known the authentic original model. [START_REF] Baker | Death by Nostalgia: A Diagnostic of Context-Specific-Cases[END_REF] identify three different types of nostalgia:

-«real nostalgia »: symbol of a time related to a direct experience (e.g a song of one's youth),

-«simulated nostalgia»: symbol of a time evoked by close relationships (e/g Abba fans born in the eighties),

-«collective nostalgia» symbol of a culture, a nation or a generation (e.g the 1998 Football

World Cup in France).

This classification highlights the interest of the collective nostalgia concept which is not specific to an individual but which concerns a group of individuals with common recollections or shared memories. [START_REF] Belk | The Role of Possessions in Constructing and Maintaining a Sense of Past[END_REF] suggests that collective memory is attached to the notion of generation. Some tastes, products or brands are specific to some generational cohorts. Young adults' food references, for example, are quite different from those of the adults in their forties, fifties or even of seniors (e.g Tang drinks, Pepito chocolate biscuits or Car-en-Sac sweets) [START_REF] Havlena | Exploring Nostalgia Imagery Trough he Use of Consumer Collages[END_REF] identify four types of nostalgia based on the following two dimensions: private/collective and direct/indirect (cf. graph1). Personal (or autobiographic) nostalgia corresponds to personal memories and to events really experienced. Interpersonal (or intergenerational) nostalgia is specific to the individual and linked to family or next of kin recollections such as recipes transmitted from generation to generation. Food educationfor instance educating children's taste -can be listed in this category.

Cultural nostalgia includes memories common to people of a same culture and sharing the same symbols. Traditional dishes such as « pot-au-feu or blanquette » are two examples of cultural nostalgia for the French. Virtual nostalgia is related to a reported collective experience reflecting the need to borrow elements from a different culture; it is illustrated in theme chain restaurants like the Hard Rock Cafés. These restaurants stage objects whichused to belong to American film or music stars.

Other products borrowed from a foreign culture can also recreate a virtual nostalgia experience such as for instance the Oreo biscuits or Vegemite. Graph 1 summarizes the works by [START_REF] Havlena | Exploring Nostalgia Imagery Trough he Use of Consumer Collages[END_REF] and shows examples of different key types of nostalgia applied to food consumption. Cultural and virtual nostalgia which trigger the same type of response among a group of individuals or on a market segment are the most widely used classes of nostalgia by marketers.

EXPLORATORY QUALITATIVE STUDY OF NOSTALGIC FOOD CONSUMPTION ACTIVITIES

The qualitative data used in this paper are derived from a PhD work dealing with the influence of nostalgia on consumer behaviour [START_REF] Vignolles | Nostalgie et comportement du consommateur : test de deux échelles de propension à la nostalgie et voies de recherche sur le concept[END_REF][START_REF] Vignolles | Expression d'expériences nostalgiques chez les jeunes adultes par la technique projective des collages : étude empirique exploratoire, 6èmes Journées Normandes de Recherche sur la Consommation[END_REF]. The qualitative chapter of this work completed in September and October 2006 surveyed nostalgic consumptions in general and was not actually focusing on food behaviours.Yet, the semantic and lexical processing of the data with the NVivo 2.0 software revealed that most statements were dealing with nostalgic food consumption experiences. The other classes of nostalgic consumption activities were by increasing order : cultural products, clothes, perfumes, cars, home furnishings and places.

Methodology

A qualitative study of the influence of nostalgia on consumer behaviour has been performed.

Three hundred interviewees aged 16 to 84 (half males, half females) have been asked to complete a self-administered questionnaire. The respondents had to answer the three following questions :

1. How would you define the word nostalgia?

2. Describe a situation in which you have experienced nostalgia and detail all the emotions you have felt on this occasion.

3. Describe a nostalgic situation associated with a consumption activity and all the emotions you have felt.

We have tried to better understand the individual perceptions, the situations and the emotions related to nostalgia as well as the recurring nostalgic consumption activities. The 300 respondents have also given their own definition of the word nostalgia, described one nostalgic situation and one nostalgic consumption behaviour 4 . The results we have considered as relevant for our study are those dealing with nostalgic consumption. Out of the 224 descriptions of nostalgic consumptions, approx. half (46%) deal with food consumption.

In the next paragraph, we are going to describe the themes mentioned by the respondents and illustrate them with statements.

Themes specific to food nostalgia

The analysis of the results has highlighted six main themes:

-food products related to childhood -yearning for no longer available products -substitute products -products related to homesickness -products for special occasions -rediscovering products Nearly half of these food consumption activities involve food products related to childhood, for instance time spent in the family, more particularly with the mother or the grandmother.

The respondents' answers clearly show the importance of rituals in nostalgic food consumption. These rituals imply a common place, a precise time and the presence of people around (family or friends).

Next comes the yearning for products which are no longer available or whose production has changed. This feeling is mostly found with people over 60 who think that « it was better » before. This yearning for their past corresponds to the bitter dimension of nostalgia . This finding is in line with Davis' view (1979) according to which elderly people find solace and comfort in their past to better cope with their shrinking prospects.

Other people very well know that the products they consume are just copies of what they used to have; still, they conjure up a happy past. In this case, the products act as substitutes filling a gap and replacing those that are now out of reach because the person who used to prepare or cook them has gone away or is dead. This is true for family recipes rather than for brand products.

Although nostalgia is mainly caused by time »gone by » [START_REF] Divard | La nostalgie: un thème récent dans la recherche en marketing[END_REF], it can also be due to homesickness. Some respondents who have left their homeland, experience nostalgia because they cannot find in their host country the products of their country of origin. Nostalgic recollections may also be linked to products which are only found in very limited geographical areas. These nostalgic memories are quite similar to nostalgia as defined by Hofer (1688) who considered it as equivalent to homesickness.

Some products are consumed on special occasions. As an example, the respondents mention the products they have on feastdays. These scarce moments are engraved in people's minds and can be idealized in their autobiographic memory but they are always associated with intense and highly emotional occasions, triggering precise recollections such as the date, the name of the persons attending the event or the emotions then experienced.

Lastly, some respondents associate the nostalgic experience with a food product that they have not had for long and that they rediscover. The fact of rediscovering a product long forgotten makes it easier to buy it because the consumer is willing to recreate past emotions and their associated memories.

The question asked as a basis for this work was dealing with nostalgia in general. The respondents did not have to quote any specific brand but rather a consumption activity evoking nostalgia; they had to try and describe the emotions felt at the time of consumption as well as all the elements meant to help repeat the nostalgic experience. The results show that nostalgic food consumption activities involve branded products (38%) and unbranded products (62%) to a larger extent. The memories and rituals which go with food consumption impress the individuals much more than the brand itself.

The design of the product, more than its taste, seems to summon a feeling of nostalgia which once more triggers the will to buy or the purchasing act itself. In some other cases, the old packaging itself triggers the purchasing act much more readily than a revamping of the initial packaging as illustrated with the release of the original Coca Cola bottles or of the early 20th century Maxwell packaging in the States .

After defining the themes inherent to food nostalgia, we are going to review the different types of food nostalgia described in literature.

The different types of food nostalgia

Three types of food nostalgia have been evidenced. Researchers agree to consider nostalgia as an ambivalent emotion [START_REF] Belleli | Une émotion ambiguë : la nostalgie[END_REF][START_REF] Holbrook | Echoes of the Dear Departed Past: Some Work in Progress on Nostalgia[END_REF][START_REF] Batcho | Nostalgia: A Psychological Perspective[END_REF][START_REF] Divard | La nostalgie: un thème récent dans la recherche en marketing[END_REF].

Nostalgia can be a sweet, a bitter or a bittersweet reaction. Food consumption can take the consumers back into a past period and arouse past emotions or feelings.

The responses have been encoded in three different categories and defined as follows :

-Positive food nostalgia (soft): a food product is associated with positive memories and with emotional reactions such as joy, happiness, comfort or peacefulness.

-Negative food nostalgia (bitter); a food product is associated with negative memories and with emotional reactions such as regret, sadness or even anger.

-Ambivalent food nostalgia (bittersweet): a food product is associated with both positive and negative memories , emotional reactions are ambivalent and imply contradictory emotions such as joy and sadness.

The encoding has been performed with the NVivo 2.0 software by three researchers in management sciences. The comparisons of the encoding match in 97% of the cases.

Most answers belong to the positive food nostalgia (51%) class. The products mentioned are products that people have not consumed for long, sometimes since childhood and which conjure up happy moments. Most of the time, people buy these products to retrieve their memories.

Sweets, biscuits, cakes, jam are the most often quoted products whatever the age of the respondents. To a much lesser extent, some salted products are also mentioned, mainly by elderly respondents and in connection with homesickness.

Sweet food is associated with the sweet pleasures of childhood.

In decreasing order of importance, food consumption linked to ambivalent food nostalgia comes next (33%). Products call forth pleasant memories but are kind of blurred with a feeling of sadness or regret because time has elapsed. This ambivalent nostalgia is similar to what [START_REF] Batcho | Nostalgia: A Psychological Perspective[END_REF] reported in his work, that is a lack, something missing -either people, feelings or objects.

Negative food nostalgia comes last (16%): people allude to the yearning for a product or a brand no longer available or produced in a different manner. This feeling is linked to yearning for savours of yore and also to a distaste for current products. It can elicit very strong reactions. The products can no longer be considered in a positive way because they are associated with people who are no longer present or alive.

PROPOSITION OF A NOSTALGIC PERCEPTION SCALE

The nostalgic perception scale has been designed according to Churchill paradigm (1979). It aims at measuring the nostalgic perception of products and brands. We have observed that consumers make the difference between individual nostalgic perception and collective nostalgic perception. In some cases, the nostalgic experience is inherent to the individuals which experience it for products and brands as they would for an object (eg a gift or a family photograph). In other cases, the consumers consider a product as nostalgic because of its design and of the references to its past or to the brand history without having necessarily a direct relationship with the product itself.

The distinction between personal and collective nostalgia should help to better segment populations and to discuss the elements to incorporate in a communication strategy. Which symbols or references should be included function of the target? i.e either the consumers who have known the original product or those who only know its updated or revamped version.

Results of the exploratory analysis

Based on the previously collected corpus, we have tried to understand whether the four facets of nostalgiaas [START_REF] Havlena | Exploring Nostalgia Imagery Trough he Use of Consumer Collages[END_REF] understand them-could be identified in the respondents' answers. Several themes had been pre-selected in the interview outline in order to study the nostalgic conumption experiences. [START_REF] Havlena | Exploring Nostalgia Imagery Trough he Use of Consumer Collages[END_REF] dimensions are not all present in the respondents'answers. Virtual nostalgia, for instance, is not reported in the statements.

A second reading of the respondents'answers has led to a simplified two-dimensional distinction : on the one hand, personal nostalgia (i.e personal and interpersonal nostalgia) including all the individual autobiographic memories and also those reported by the family or friends but still part of the personal history and on the other hand collective nostalgia which does not directly involve the individuals but that they can also consider as nostalgia. This distinction is consistent with [START_REF] Divard | La nostalgie: un thème récent dans la recherche en marketing[END_REF] definition since « nostalgia brings the individuals back into a period or an event derived from an idealized past, part or not of their own personal experience. »

We are going to see that in the case of collective nostalgia, businesses use elements referring to the past or to the brand history to « package » the nostalgic experience.

The products can evoke either personal or collective nostalgia. In the case or personal nostalgia, products can remind of life experiences such as childhood or youth experiences. In this case, products enable the individuals to take a leap back into their past with more or less intensity.

In many cases, the design of the product, much more than its taste, plays a major part in the evocation of nostalgia. Design, C The use of the original design (Banania chocolate breakfast mix) or the stability of the design over time (Nestlé mini cartons) evoke both collective nostalgia (the original design is engraved in collective memory) and personal nostalgia.

So, we will take into account the references to the brand history and the capacity of the brand to recall the past in the design of the nostalgic perception scale.

The points of sale described are said to be nostalgic for two main reasons: some of them belong to a chain using the concept of nostalgia to design its stores. The most often quoted examples in the food sector are Paul bakery and O §Co (olive oils and gourmet specialties) shops. It is interesting to note, though, that some people may be skeptical vis-à-vis the use of nostalgic elements pertaining to French collective memory such as the use of second-hand antiques or employees' clothes (white apron and baker's hat).

The second reason for considering these outlets as nostalgic is the fact that consumers who used to go there can no longer do so because they have closed down. The image of authenticity is then closely connected wih the nostalgic experience.

Communication schemes using nostalgia are rather numerous in the food industry. Their aim is to reassure the consumers by bringing back childhood memories or by using the notion of gentle way of life. Let us quote Bonne Maman and its slogan « a sweet taste of yore » , Tipiak foodstuff and the grannies from Britanny or the Herta cakes whose flavour reminds the character in the ad of the cake baked by mum (even if the pastry is now ready for use)

In other cases, respondents distance themselves from ads. They understand that brands bank on authenticity and intergenerational links [START_REF] Camus | L'authenticité marchande perçue : Pour une échelle de mesure appliquée au domaine alimentaire, 20ème Congrès de l'AFM[END_REF][START_REF] Camus | La marque authentique, Les contours du concept à partir de la revue de la littérature, des pratiques commerciales et des discours des consommateurs, 6 ème Journées Normandes de la Consommation[END_REF] but they do not feel directly involved. In this case, we speak of collective nostalgia or how the product can be considered as nostalgic by all but without any personal memory to refer to.

Generation of items

The first generation of items allowed us to suggest three facto :

-Collective nostalgia dealing with the product (4 items): this factor intends to measure how a product can produce nostalgia on the basis of its history or of its design.

-Collective nostalgia with the brand (3 items): this factor measures the nostalgic evocations brought about by the brand.

-Personal nostalgia (6 items): this factor measures the capacity of a product to arouse nostalgia through consumers'experiences and personal recollections.

The validity of this first proposition was analyzed with the collaboration of four marketing researchers. The 13 items are taken into account. The scale has then been tested wih a new sample of 200 people aged between 21 and 54 (males and females in equal proportions). In order to fill in the measurement scale, the initial question was:« Have you already bought a product because it aroused a feeling of nostalgia ? if yes, what is the product and its brand ? .

Then the interviewees had to answer the items acccording to a 7 point Likert scale

Out of the 200 interviewees, 21 gave a negative answer (i.e 10.5%). This proportion is higher than in [START_REF] Sierra | Attitudes and emotions as determinants of nostalgia purchases: an application of social identity theory[END_REF] study on the determinants of nostalgic purchases. In their study 3% of the individuals answered that they had never bought a product entailing a feeling of nostalgia. In our study, the share of food nostalgic purchases amounts to 52%.The Likert scale is not intended to only measure the nostalgic perception of food products; yet, it seems that the latter are predominant in the answers obtained.

A principal components analysis was then performed (Table 2). The data can be factorized since KMO is equal to 717 and Chi2 equal to 42357 (p<0,001). The items which did not comply with the following criteria were discarded: absolute value lower than 0.3, value of communities lower than 0.5 or variable present in at least two dimensions with a standard deviation below 0.4.

The three-dimensional structure has been selected with a total of 9 items distributed as follows (cf. table3) :

-product collective nostalgia: 3 items -brand collective nostalgia: 2 items -personal nostalgia: 4 items Orthogonal rotation (varimax) has been chosen because the correlation between the factors is under 0.3 (Nunally and Berstein, 1994). The total variance explained is 67.283% as shown in the table below: The reliability of the scale was then tested with Cronbach alpha. Table 3 lists the items and alphas corresponding to each factor. Alphas are all above 0 , which is in favour of a high level of reliablility5 .

Confirmatory factorial analyis

The internal reliability of the scale can be calculated thanks to the exploratory factorial analysis and Cronbach alpha and also thanks to the confirmatory analysis with Jöreskog rho.

In order to check the factorial structure of the solution obtained with the principal components analysis, a confirmatory analysis has been performed according to the maximum de vraisemblance method. The Kurtosis multavariate coefficient should be lower than 3 to assert the absence of multinormality [START_REF] Bentler | EQS Structural Equations Program Manual[END_REF]. In our case,it is 9.33 in which case the remedial robust method is recommended [START_REF] Satorra | Corrections of test statistics and standard errors in covariance structure analysis[END_REF]. So, we have only selected the corrected statistics.

The overall results presented in table 4 are well matched with the scale. The GFI, AGFI, NNFI and CFI indices are above 0.9 ; RMSEA and RMSR are lower than 0.05. These results confirm the structure of the scale derived from the principal components exploratory analysis. The trait validity of measurement tools is determined thanks to the converging and discriminatory validity. The converging validity is verified thanks to rhö values which must be above 0.5. In our study, convergence is confirmed as shown in table 5 for product collective nostalgia (0,536) and for personal nostalgia (0,610) but it is rather low for brand collective nostalgia (0,310).

We assessed the discriminatory value by checking that the extracted mean variance (converging validity rhô) of each construct was higher than the square value of the correlations shared by the other constructs.We can conclude that the nostalgic perception measurement scale has a valuable discriminatory validity. The indices of converging and discriminating validity in the confirmatory analysis are rather low for product and brand related collective nostalgia. The same is true for internal validity.

Only personal nostalgia meets the criteria of reliability and validity.

Discussion on the resulting structure

The three-dimensional structure is used for the confirmatory factorial analysis. Yet, the two dimensions of collective nostalgia (product and brand) only partially meet the reliability and validity criteria.

-Collective nostalgic perception of the product : with these items, we intend to analyse how a nostalgic « repackaging » (" this product recalls another product which has long been available ", « this product has a design which recalls the past » and "this product is inspired from another product »is part of the nostalgic perception. A posteriori, these items look relevant for products belonging to the retro-marketing wave [START_REF] Brown | Marketing the Retro Revolution[END_REF]al, 2003, Brown, 2007). These products are inspired from products of the past. They have got a retro design but are equipped with the latest technologies, as for example the Mini BMW, the Vespa scooters. This dimension, however, is less appropriate for the measurement of the products which have hardly changed over time.

-Collective nostalgic perception of the brand: the items «The references to the history of the brand are obvious » and « the brand of this model or product recalls the past » seem to better fit retro-innovative products or already existing brands which have gained legitimacy and are known by all. It seems that respondents have had some problems in answering these items either because they did not know the brand well enough or because the brand did not evoke the past.

-Personal nostalgic perception : only this dimension meets the criteria of reliability and validity.The four items facilitate the measurement of the nostalgic perception of a product experienced by a consumer or the way a product can recall one's memories.

We have not measured the personal nostalgic perception of a brand because some brands overlap several classes of products and items are too vague.

CONTRIBUTIONS, LIMITATIONS OF THE NOSTALGIC PERCEPTION SCALE AND AVENUES FOR RESEARCH

Undoubtedly, the analysis of nostalgia in the food sector is of great interest to managers and researchers. The scale designed in the framework of this study helps to better apprehend a concept with a measurement tool which, to our knowledge, did not exist before. Its psychometric value is rather low for two dimensions out of the three studied but its use is relevant for retro-marketing products. It is, however, less valuable for food products.

As already observed in qualitative exploratory studies, products -food product or not-do not necessarily include nostalgic elements which can be interpreted by everybody (design, graphics...). Yet, they belong to the private sphere of each individual. Retro-innovative products use elements which are part of collective memory. This is why this scale is more appropriate for the latter category of products.

Future research work should focus on the construction of a measurement scale specific to food products integrating notions inherent to the object studied, such as taste, smell or the other sensory dimensions of food. It would be unfortunate to ignore the sensorial aspects of food when measuring nostalgic perception. Two avenues for research are worth mentioning: integrating nostalgic perception into a wider scope model in order to test its role in consumer's behaviour and studying the differences between generational cohorts in relationship with the nostalgic perception of products and brands.

From a managerial

  perspective, this scale should facilitate consumer segmentation and should propose different communication schemes, making the distinction between the consumers who have really been familiar with the original product and those who only know the current copy. Can the new Beetle, for instance, recreate the same nostalgic experience for consumers who have known the original model and for younger ones who have not known the original model ?. Should we communicate differently with these two groups or not? The nostalgic perception scale can help managers in designing their offer of nostalgic or retro-innovative products. It can also be helpful in brand management positioning. More generally speaking, using the concept of nostalgia in the agrifood industry seems to be highly relevant. Since 2008, « Bonne Maman » has proposed a new range of 22 recipes of desserts and yoghurts while maintaining its nostalgic perception strategy. The limitations of this research are linked to the statistical constraints of our measurement scale. The results on reliability and validity are disappointing. Moreover, we have not analysed the discriminating value of the scale with other measurement tools.

Table 1 : Main definitions of nostalgia (adapted fromPerrusson, 2003 ; Divard and Robert-Demontrond, 1997)

 1

	Authors	Affects	Definition
	Belk (1990)	Mood	Melancholia created by an object, an image, a smell or a melody
			Preference (positive link, positive attitude or
	Holbrook and Schindler (1991)	Preference	affect) for objects people, sights (fashionable or readily available at a younger age -childhood, adolescence, or even before
			birth )
			Two-sided emotions. Emotion linked to
	Belleli (1991)	Ambivalent emotion 3	longing and regrets , hence awareness of the unattainability of a loved object ; emotion related to memory : what is lost can be
			retrieved thankd to recollections.
	Stern (1992)	Emotional state	Emotional state in which an individual longs for an idealized, unspoilt period of time
			Nostalgia is a bittersweet emotion often
			associated with a cognitive activity ; what is
	Divard and Robert-Demontrond (1997)	Ambivalent emotional reaction	felt by an individual when an internal or external stimulus brings him/her back into a time or recalls an event derived from an
			idealized past, part or not of his/her own
			experience.

Graph 1 : The different types of nostalgia adapted from Havlena and Holak (1996)

	Personal
	Personal nostalgia	Interpersonal
		nostalgia
	Granny's jams, afternoon	Recipes transmitted from generation to
	snacks, goûters	generation and any type of food educationI
	Direct	Indirect
	Banania chocolate	Hard Rock Café, peanut
	breakfast mix,	butter, Vegemite…
	bouillon//stock cubes,
	Nestlé mini cartons
	Cultural nostalgia	Virtual nostalgia
	Collective

Table 2 : Principal components exploratory analysis (n=179)

 2

			KMO = 0,727
	Principal components analysis		Bartlett ² = 479,797
	Varimax rotation		p<0,001
		Com.	F1	F2	F3
	Pers3 0,686	0,821
	Pers1 0,599	0,772
	Pers6 0,580	0,742
	Pers5 0,582	0,709
	Colp2 0,711		0,843
	Colp4 0,660		0,751
	Colp3 0,600		0,721
	Colm7 0,837			0,896
	Colm5 0,800			0,866
	Cronbach alpha		0,761	0,718	0,816
	Eigen values		2,352	1,923	1,781
	Explained variance (%)		26,134	21,362	19,787
	Total variance explained (%)		67,283
	Matrix of the correlations between the components	F1 F2 F3	1,000 0,133 0,250	0,133 1,000 0,124	0,250 0,124 1,000

Table 3 : Principal components factorial analysis (n=179)

 3

	Dimension	Items	without the item	de la dimension
	Personal nostalgia	This product reminds me of my own past. This product reminds me of my youth. This product recalls good times. It helps to revive pleasant memories.	0,705 0,710 0,666 0,737	0,761
	Collective nostalgia (product)	This product recalls another product which has long been on the market. The product is inspired from a past model. The design of the product recreates the past.	0,583 0,661 0,643	0,718
	Collective nostalgia (brand)	References to the brand history are obvious. The brand reminds me of the past.	0,444 0,508	0,816

Table 4 : Confirmatory factorial analysis (n=179)

 4

	Indices	Collection1 n=179
	²	25,374
	Ddl	24
	GFI/AGFI	0,966/0,936
	SRMR	0,147
	RMSEA	0,021 (confidence interval
		0,000 et 0,066)
	NNFI / CFI	0,991/0,994
	²/ddl	1,057
	AIC	-22,093

Table 5 : Confirmatory factorial analysis, converging and discriminating validity (n=179)

 5

		F1	F2	F3
		Product	Brand	Personal
		collective	collective	nostalgia
		nostalgia	nostalgia
	Internal validity rhô	0,775	0,473	0,861
	F1	0,536 1
	F2	0,272 2 (0,522) 3	0,310
	F3	0,058 (-0,242)	0,023 (-0,154)	0,610
	1 Values set in bold correspond to the converging validity rhô of the corresponding
	dimension.
	2 Values in plain characters are coefficients representing the correlations between square
	factors
	3 Values in ( ) indicate the coefficients representing simple correlations

Source : L'Expansion 30/06/2006.

« La santé de plus en plus prégnante dans l'alimentation des français» by Gaignier C. and Hebel P. -september 2005 -CREDOC

Belleli indicates that the word « emotion » is used generically and means emotional reaction.

The detailed results on nostalgia and nostalgic situations won't be presented in this paper.

Factor 5 only counts two items ; yet, the correlation between both items is statistically sufficient to keep this factor with a correlation index equal to 0, 7