The invasive cactus moth, Cactoblastis cactorum, poses a threat to opunitoid cacti species of North America. The following work contains four separate studies investigating C. cactorum host plant preference and performance, predation and parastitism of C. cactorum, effects of C. cactorum on local Opuntia populations, and associational effects of host and non-host plants on C. cactorum and native Opuntia-feeding herbivores. We found that, among southwestern and Mexican opuntioid taxa, moths preferred O. engelmannii var. linguiformis and var. engelmannii for oviposition, while Consolea rubescens and O. streptacantha were superior larval hosts. Oviposition was best predicted by number of cladodes and degree of spininess; epidermal toughness was a significant predictor of most larval fitness parameters. In general, oviposition preference was not correlated with larval performance. A lack of co-evolutionary history between C. cactorum and North American opuntioid species may help explain this disconnect. We placed irradiated C. cactorum eggsticks and pupae on Opuntia plants in the field to test for predation. We found evidence of predation, most likely from ants, on ~16% of eggsticks and ~18% of pupae. Predation rates, ant abundance, and cladode growth were higher, and C. cactorum damage lower, on Opuntia located near the extrafloral nectar-producing legume Chamaecrista fasciculata. We attribute these associational effects to the ability of C. fasciculata to attract ants to its extrafloral nectar. Over the course of six years, ~78% of nearly 600 tagged Opuntia were attacked by C. cactorum at least once and ~76% of the plants survived. Two separate studies found that Opuntia stricta was more likely to be attacked by C. cactorum than O. humifusa; we also found that O. stricta was more likely to die following an attack. A plant's odds of survival decreased as C. cactorum attack frequency increased, but plants that did survive had positive growth rates, regardless of attack status. We did not find significant evidence of associational effects for O. humifusa and O. stricta, either for C. cactorum or native Opuntia-feeding herbivores. It could be that present herbivore densities are low enough, and host plants plentiful enough, to avoid mechanisms that usually lead to associational resistance or susceptibility. Overall, our results suggest that the presence and spread of C. cactorum should be taken seriously, especially for rare opuntioids and the Opuntia-rich deserts of North America. However, for more common opuntioid host species, there may be enough resistant or tolerant individuals, and sufficient top down control through ant predation, for populations to persist at current C. cactorum densities. We acknowledge that information on Opuntia reproduction and recruitment rates are needed to confirm this suggestion, and see this as an excellent opportunity for future research

There is much evidence that herbivory is a key innovation for the tremendous success of insect. In this thesis I have investigated different aspects of host plant utilization and phenotypic plasticity using the polyphagous comma butterfly, Polygonia c-album. Even though external conditions affect a phenotypic plastic response, the outcome is often influenced by a genetic background which may differ among populations. In Paper I we suspected the genetic background to seasonal polymorphism to be X-linked. However, results from interspecific hybridization between two populations suggested that diapause response is instead inherited in a mainly autosomally additive fashion, with a possible influence of sexual antagonism on males. In Paper II we showed that female oviposition preference is not a plastic response influenced by larval experience, but has a genetic background coupled to host plant suitability. Further, there is a strong individual correlation between larval host plant acceptance and female host plant specificity (Paper III). We believe this to be a larval feed-back genetically linked to female host specificity: offspring to ‘choosy’ specialist mothers benefit by remaining on the original host while offspring to less discriminating generalist mothers should risk inspecting the surroundings, thus compensating for potential poor female choice. In the larval mid-gut, genes are differentially expressed depending on host plant diet (Paper IV). Therefore, we expected to find fitness consequences of host plant switch. However, although growth rate was affected in a few treatments, larvae were generally surprisingly good at adjusting to new diets (Paper V). To conclude, host plant choice in both female and larval life stage is connected to performance. Combined with increased understanding about the plastic response to diet intake and seasonal polymorphism we have gained further insights into the processes of local adaptations and speciation in the Lepidoptera. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted Manuscript; Paper 5: Manuscript

Les insectes phytophages forment un groupe d’organismes très diversifié et la plupart sont considérés comme spécialistes. Les patrons de spécialisation des insectes vis à vis de leurs plantes hôtes dépendent en partie de leur capacité à interagir avec les plantes (niche fondamentale) et aux facteurs environnementaux modulant ces interactions et aboutissant aux observations en milieu naturel (niche réalisée). La spécialisation fondamentale est déterminée par l'évolution conjointe de deux traits : la performance des larves et la préférence des femelles. Pour comprendre cette spécialisation, nous avons étudié une communauté de huit espèces de mouches des fruits (Diptera : Tephritidae) présentes à La Réunion. Dans un premier temps, nous avons déterminé la niche réalisée de chaque espèce et montré que ces niches étaient structurées par la phylogénie des plantes avec D. demmerezi, D. ciliatus et Z. cucurbitae spécialistes des Cucurbitaceae, N. cyanescensspécialiste des Solanaceae et C. catoirii, C. capitata et C. quilicii généralistes attaquant des plantes de différentes familles. Après l'invasion de B. zonata en 2000, C. capitata et C. quilicii ont subi une réduction de leur gamme d'hôtes. Dans un deuxième temps, nous avons déterminé la niche fondamentale de ces espèces (sauf D. ciliatus). Nous avons évalué les préférences des femelles en mesurant la fécondité de chacune d’entre elles sur une gamme de 29 fruits, puis nous avons testé l'existence d'une corrélation entre la préférence des femelles et la performance des larves (mother knows best hypothesis). Nous avons montré une corrélation positive chez spécialistes des Cucurbitaceae qui pondent sur les plantes pour lesquelles les larves survivent le mieux contrairement aux généralistes pondant et survivant sur une large gamme d'hôtes mais sans corrélation entre ces deux traits.Enfin, la sélection de l'hôte par les femelles se faisant principalement grâce aux composés organiques volatils (COVs) émis par les fruits, nous avons montré que les fruits infestés par les généralistes ont pour point commun l'émission de COVs responsables de la maturation des fruits. Au contraire, les fruits de plusieurs Solanaceae émettent des COVs spécifiques suggérant la détection de ceux-ci par les femelles de N. cyanescens. Les Cucurbitaceae émettent des COVs abondants peu présents dans les autres familles suggérant une détection d'un mélange spécifique de ces COVs par les Tephritidae spécialistes des Cucurbitaceae. / Phytophagous insects are a very diverse group of organisms and most of them are considered as specialized. Patterns of specialization regarding their host plants depend on their ability to interact with their hosts (fundamental niche) and on environmental factors which modulate these interactions leading to observed patterns in the field (realized niche). Fundamental specialization is determined by the joint evolution of two traits: larval performance and female preference. To understand this specialization, we studied a community of eight fruit fly species (Diptera: Tephritidae) present in La Réunion.First, we determined the realized niche of each species and showed that they were structured by plant phylogeny with D. demmerezi, D. ciliatus and Z. cucurbitae as Cucurbitaceae specialists, N. cyanescens as Solanaceae specialist and C. catoirii, C. capitata et C. quilicii as generalists feeding on plants belonging to different families. After the invasion of B. zonata in 2000, C. capitata et C. quilicii were subjected to a decrease of their host range.Then, we determined the fundamental niche of these species (except D. ciliatus). We assessed female preferences by measuring their fecundity on 29 fruits, then we tested the presence of a correlation between female preference and larval performance (mother knows best hypothesis). We showed a positive correlation for Cucurbitaceae specialists laying eggs on plants where larvae survive the best, at the opposite of generalist species laying eggs and surviving on many hosts without any correlation between these two traits.Finally, host selection by females being mostly done thanks to volatile organic compounds (VOCs) emitted by fruits, we showed that fruits infested by generalist species have common VOCs responsible for fruit maturation. On the contrary, the fruits of several Solanaceae emit specific VOCs suggesting their detection by N. cyanescens females. Cucurbitaceae species emit abundant VOCs rarely present in other families suggesting a detection of a specific blend of these VOCS by Cucurbitaceae specialists.