Defense strategies used by two sympatric vineyard moth pests

Inter- and Intraspecific Trait Compensation of Behavioural and Morphological Defences in a Damselfly Genus

Predation is a key driver of phenotypic diversification with prey having evolved sets of correlated anti-predator traits. Changes in anti-predator traits can be studied on an evolutionary as well as on a developmental timescale. Using a common garden setup, we studied inter- and intraspecific correlations of behavioural and morphological defences in four damselfly species that either occur in habitats dominated by predatory fish (fish habitats) or fishless habitats by raising larvae either with predatory fish or in a control treatment. We found inter- as well as intraspecific trait compensation (negative correlations) between behavioural and morphological defences. Compared to fishless habitat species, fish habitat species invested more in behavioural defences and less in morphological defences. This was mirrored by fish habitat species investing more in behavioural defences and less in morphological defences when reared with predatory fish whereas fishless habitat species invested less in morphological defences only. Our results emphasise the role of context-specific combinations of defensive traits to avoid predation. We suggest, considering changes in multiple correlated traits on different timescales when studying the evolution of anti-predator traits.

Sublethal concentration of insecticide amplifies interference competition in a tortrix moth

Insecticides are extensively used worldwide to kill insect pests, yet organisms are most often exposed to insecticides at sublethal concentrations. Our understanding of sublethal effects on life histories is needed to predict the impact of insecticides on population dynamics and improve insecticide use and pest control. Sublethal concentrations can impact life histories directly and indirectly through changes in the intraspecific competition. Yet, few studies have evaluated the sublethal effects on intraspecific competition and these do not disentangle the insecticide effects on interference competition versus exploitative competition. As such, sublethal effects on the relative contribution of each pathways in shaping life histories are largely unknown, despite the fact that this can impact population dynamics. In this study, we focused on the neurotoxic insecticide spinosad and investigated its sublethal effects on interference among the aggressive larvae of the tortrix moth Adoxophyes honmai and the consequences for life histories. We conducted a set of paired experiments to disentangle the insecticide effects on interference from the ones on exploitation. Spinosad was found to amplify interference with most effects on mortality which lets us suggest that the insecticide likely increases the level of aggressive interactions resulting in more conspecific killings (e.g. cannibalism). Spinosad exposure was found to impair movement ability. Less movements may increase susceptibility to conspecific attacks and or increase aggresivity for better defence, two plausible mechanisms that could explain the increase in interference with insecticide. This study shows that insecticide at sublethal concentration can impact life histories by altering the strength of interference competition. Many organisms (pest and non-target species) compete through interference and theory predicts that a change in interference can substantially change dynamics. Our finding therefore reveals the importance of assessing the effect of insecticides on the mechanisms of competition when predicting their impact on populations.

How to stand the heat? Post-stress nutrition and developmental stage determine insect response to a heat wave

Organisms are increasingly confronted with intense and long-lasting heat waves. In insects, the effects of heat waves on individual performance can vary in magnitude both within (e.g. from one larval instar to another) and between life stages. However, the reasons underlying these stage-dependent effects are not fully understood. There are several lines of evidence suggesting that individual ability to withstand a heat stress depends on mechanisms based on nutrition and supporting energetically physiological stress responses. Hence, we tested the hypothesis that the efficiency of these food-based buffering mechanisms may vary between different larval instars of a phytophagous insect. Using larvae of the moth Lobesia botrana, we examined the importance of post-stress food quality in insect response to a non-lethal heat wave at two distinct larval instars. Three major conclusions were drawn from this work. First, heat waves induced an overall decline in larval performance (delayed development, depressed immunity). Second, food quality primarily mediated the insect's ability to respond to the heat stress: the reduction in performance following heat wave application was mostly restricted to individuals with access to low-quality food after the heat stress. Third, larval instars differed in their susceptibility to this combination of thermal and food stressors, but conclusions about the instar being the most vulnerable differed in a trait-specific manner. In a global warming context, this study may shed additional light on the combination of direct and indirect (through alteration of plant nutritional value) effects of rising temperatures on the ecology and the evolution of phytophagous insects.

Where you come from matters: temperature influences host-parasitoid interaction through parental effects

Temperature alters host suitability for parasitoid development through direct and indirect pathways. Direct effects depend on ambient temperatures experienced by a single host individual during its lifetime. Indirect effects (or parental effects) occur when thermal conditions met by a host parental generation affect the way its offspring will interact with parasitoids. Using the complex involving eggs of the moth Lobesia botrana as hosts for the parasitoid Trichogramma cacoeciae, we developed an experimental design to disentangle the effects of (1) host parental temperature (temperature at which the host parental generation developed and laid host eggs) and (2) host offspring temperature (temperature at which host eggs were incubated following parasitism, i.e. direct thermal effects) on this interaction. The host parental generation was impacted by temperature experienced during its development: L. botrana females exposed to warmer conditions displayed a lower pupal mass but laid more host eggs over a 12-h period. Host parental temperature also affected the outcomes of the interaction. Trichogramma cacoeciae exhibited lower emergence rates but higher hind tibia length on emergence from eggs laid under warm conditions, even if they were themselves exposed to cooler temperatures. Such indirect thermal effects might arise from a low nutritional quality and/or a high immunity of host eggs laid in warm conditions. By contrast with host parental temperature, offspring temperature (direct thermal effects) did not significantly affect the outcomes of the interaction. This work emphasises the importance of accounting for parental thermal effects to predict the future of trophic dynamics under global warming scenarios.

Are life-history traits equally affected by global warming? A case study combining a multi-trait approach with fine-grain climate modeling

Predicting species responses to climate change requires tracking the variation in individual performance following exposure to warming conditions. One ecologically relevant approach consists of examining the thermal responses of a large number of traits, both related with population dynamics and trophic interactions (i.e. a multi-trait approach). Based on in situ climatic data and projections from climate models, we here designed two daily fluctuating thermal regimes realistically reflecting current and future conditions in Eastern France. These models detected an increase in mean temperature and in the range of daily thermal fluctuations as two local facets of global warming likely to occur in our study area by the end of this century. We then examined the responses of several fitness-related traits in caterpillars of the moth Lobesia botrana - including development, pupal mass, survival rates, energetic reserves, behavioral and immune traits expressed against parasitoids - to this experimental imitation of global warming. Increasing temperatures positively affected development (leading to a 31% reduction in the time needed to complete larval stage), survival rates (+19%), and movement speed as a surrogate for larval escape ability to natural enemies (+60%). Conversely, warming elicited detrimental effects on lipid reserves (-26%) and immunity (total phenoloxidase activity: -34%). These findings confirm that traits should differ in their sensitivity to global warming, underlying complex consequences for population dynamics and trophic interactions. Our study strengthens the importance of combining a multi-trait approach with the use of realistic fluctuating regimes to forecast the consequences of global warming for individuals, species and species assemblages.

No evidence of an immune adjustment in response to a parasitoid threat in Lobesia botrana larvae

Immune function is a key determinant of an organism's fitness, and natural insect populations are highly variable for this trait, mainly due to environmental heterogeneity and pathogen diversity. We previously reported a positive correlation between infection prevalence by parasitoids and host immunity in natural populations of the vineyard pest Lobesia botrana. Here, we tested whether this correlation reflects a plastic adjustment of host immunity in response to the local presence of parasites. To this end, we measured immunity of non-parasitized L. botrana larvae exposed, respectively, to one of the two most common species of parasitoids in vineyards, over 6days. Larvae were able to sense the parasitoid through visual, chemical, or mechanical cues, but contact larvae-parasitoid were excluded. Contrary to our hypothesis, we found that L. botrana larvae did not increase their immune defenses in the presence of parasitoids, despite their ability to sense a potential threat. Our results therefore suggest that the positive correlation between infection prevalence by parasitoids and L. botrana immunity among natural populations may result from micro-evolutionary changes resulting from long-term local selection pressures imposed by parasitoids in wild populations rather than plastic adjustments of immunity.

The relative abundance of hemocyte types in a polyphagous moth larva depends on diet

Hemocytes are crucial cells of the insect immune system because of their involvement in multiple immune responses including coagulation, phagocytosis and encapsulation. There are various types of hemocytes, each having a particular role in immunity, such that variation in their relative abundance affects the outcome of the immune response. This study aims to characterize these various types of hemocytes in larvae of the grapevine pest insect Eupoecilia ambiguella, and to assess variation in their concentration as a function of larval diet and immune challenge. Four types of hemocytes were found in the hemolymph of 5th instar larvae: granulocytes, oenocytoids, plasmatocytes and spherulocytes. We found that the total concentration of hemocytes and the concentration of each hemocyte type varied among diets and in response to the immune challenge. Irrespective of the diet, the concentration of granulocytes increased following a bacterial immune challenge, while the concentration of plasmatocytes and spherulocytes differentially varied between larval diets. The concentration of oenocytoids did not vary among diets before the immune challenge but varied between larval diets in response to the challenge. These results suggest that the resistance of insect larvae to different natural enemies critically depends on the effect of larval diet on the larvae's investment into the different types of hemocytes.

Food-mediated modulation of immunity in a phytophagous insect: An effect of nutrition rather than parasitic contamination

Inherent to the cost of immunity, the immune system itself can exhibit tradeoffs between its arms. Phytophagous insects face a wide range of microbial and eukaryotic parasites, each activating different immune pathways that could compromise the activity of the others. Feeding larvae are primarily exposed to microbes, which growth is controlled by antibiotic secondary metabolites produced by the host plant. The resulting variation in abundance of microbes on plants is expected to differentially stimulate the insect antimicrobial immune defenses. Under the above tradeoff hypothesis, stimulation of the insect antimicrobial defenses is expected to compromise immune activity against eukaryote parasites. In the European grape berry moth, Eupoecilia ambiguella, immune effectors directed towards microbes are negatively correlated to those directed towards eukaryotic parasites among host plants. Here, we hypothesize this relationship is caused by a variable control of the microbial community among host plants by their antibiotic metabolites. To test this hypothesis, we first quantified antimicrobial activity in berries of several grape varieties. We then measured immune defenses of E. ambiguella larvae raised on artificial diets in which we mimicked levels of antimicrobial activity of grape berries using tetracycline to control the abundance of growing microbes. Another group of larvae was raised on artificial diets made of berry extracts only to control for the effect of nutrition. We found that controlling microbe abundance with tetracycline in diets did not explain variation in the immune function whereas the presence of berry extracts did. This suggests that variation in immune defenses of E. ambiguella among grape varieties is caused by nutritional difference among host plants rather than microbe abundance. Further study of the effects of berry compounds on larval immune parameters will be needed to explain the observed tradeoff among immune system components.

Differential water mite parasitism, phenoloxidase activity, and resistance to mites are unrelated across pairs of related damselfly species

Related host species often demonstrate differences in prevalence and/or intensity of infection by particular parasite species, as well as different levels of resistance to those parasites. The mechanisms underlying this interspecific variation in parasitism and resistance expression are not well understood. Surprisingly, few researchers have assessed relations between actual levels of parasitism and resistance to parasites seen in nature across multiple host species. The main goal of this study was to determine whether interspecific variation in resistance against ectoparasitic larval water mites either was predictive of interspecific variation in parasitism for ten closely related species of damselflies (grouped into five “species pairs”), or was predicted by interspecific variation in a commonly used measure of innate immunity (total Phenoloxidase or potential PO activity). Two of five species pairs had interspecific differences in proportions of individuals resisting larval Arrenurus water mites, only one of five species pairs had species differences in prevalence of larval Arrenurus water mites, and another two of five species pairs showed species differences in mean PO activity. Within the two species pairs where species differed in proportion of individuals resisting mites the species with the higher proportion did not have correspondingly higher PO activity levels. Furthermore, the proportion of individuals resisting mites mirrored prevalence of parasitism in only one species pair. There was no interspecific variation in median intensity of mite infestation within any species pair. We conclude that a species’ relative ability to resist particular parasites does not explain interspecific variation in parasitism within species pairs and that neither resistance nor parasitism is reflected by interspecific variation in total PO or potential PO activity.

Ecomorphological variation of the wireworm cephalic capsule: studying the interaction of environment and geometric shape

Studying the association between organismal morphology and environmental conditions has been very useful to test hypothesis regarding the influence of climate on shape. It has been long recognized that different environments produce dissimilar stress levels in insects, which can be reflected on the ability of an individual to overcome these pressures and spread further. Agriotes (Coleoptera: Elateridae) species infest agricultural fields in different parts of Croatia, inhabiting different climatic conditions. Previous biological studies have indicated that there is a relationship between some Agriotes biological parameters such as density and climatic conditions such as soil moisture and temperature. However, it is still unknown how these environmental properties influence the wireworm morphological structure. This is highly relevant because the head of this species is directly involved in the mobility in the soil, thus affecting the invasive capacity of this insect. Therefore the aim of this study was to assess the association between different climatic conditions and the morphological variation of Agriotes cephalic capsule. Advanced multivariate analysis and geometric morphometric tool were applied to study the covariation between shape and environmental variables. Partial Least Squares methods were used in order to analyse the association between the wireworm head shape and three different climatic conditions: soil type, temperature and rainfall. Our results showed that there is a high covariation between the wireworm head shape and the climatic conditions. It was suggested that the observed shape–environment association could be result of the high plasticity of this species in relation to its invasive capacity.