Infochemical use and dietary specialization in parasitoids: a meta-analysis

Many parasitoid species use olfactory cues to locate their hosts. In tritrophic systems, parasitoids of herbivores can exploit the chemical blends emitted by plants in reaction to herbivore-induced damage, known as herbivore-induced plant volatiles (HIPVs). In this study, we explored the specificity and innateness of parasitoid responses to HIPVs using a meta-analysis of data from the literature. Based on the concept of dietary specialization and infochemical use, we hypothesized that (i) specialist parasitoids (i.e., with narrow host ranges) should be attracted to specific HIPV signals, whereas gener-alist parasitoids (i.e., with broad host ranges) should be attracted to more generic HIPV signals and (ii) specialist parasitoids should innately respond to HIPVs, whereas gener-alist parasitoids should have to learn to associate HIPVs with host presence. We characterized the responses of 66 parasitoid species based on published studies of parasitoid behavior. Our meta-analysis showed that (i) as predicted, specialist parasi-toids were attracted to more specific signals than were generalist parasitoids but, (ii) contrary to expectations, response innateness depended on a parasitoid's target host life stage rather than on its degree of host specialization: parasitoids of larvae were more likely to show an innate response to HIPVs than were parasitoids of adults. This result changes our understanding of dietary specialization and highlights the need for further theoretical research that will help clarify infochemical use by parasitoids. K E Y W O R D S dietary breadth, foraging behavior, herbivore-induced plant volatiles, information use, parasitoid, tritrophic interactions 1 | INTRODUCTION In plant–herbivore–carnivore tritrophic systems, different chemical cues might be used by carnivores to locate their herbivore prey/hosts. These infochemicals can be produced by the plant (i.e., via constitutive or herbivore-induced expression) or by the herbivores themselves. Vet and Dicke (1992) hypothesized that the use of infochemicals by carni-vores should evolve in response to dietary specialization. They placed carnivores in discrete classes based on their degree of prey/host specialization (Figure 1) and then generated the following predictions: (i) specialists at the plant level (A–B) should be innately attracted to info-chemicals produced by the plant; (ii) specialists at the herbivore level and generalists at the plant level (C) might not display innate attraction but rather learn to associate infochemicals with their target prey/ hosts; (iii) more specialized species should rely more on infochemicals to locate their prey/hosts; and (iv) extreme generalists (D) should not use infochemicals at all. Steidle and van Loon (2003) tested Vet and Dicke (1992)'s predictions using data from the literature on infochemi-cal use by carnivorous arthropods. Their results supported hypotheses (ii) and (iii): associative learning was more common in generalists than