Quantity and specialization matter: Effects of quantitative and qualitative variation in willow chemistry on resource specialization of leaf-chewing insects

Jing V. Leong, Leonardo R. Jorge, Carlo L. Siefert and Martin Volf, Institute of Entomology, Biology Center, Czech Academy of Sciences, Ceske Budejovice, Czech Republic, Jing V. Leong, Carlo L. Siefert and Martin Volf, Zoology, University of South Bohemia, Ceske Budejovice, Czech Republic

Background/Question/Methods
Plants produce a plethora of specialized metabolites to defend themselves against insect herbivores. However, the functional role of phytochemical α and β-diversity on plant-insect networks are poorly understood. Interspecific chemical β-diversity represents variation in presence/absence of species-specific metabolites or quantitative variations of metabolites common to several plant species. Conceivably, the effects of qualitative and quantitative variation in plant chemistry should have differential effects on insect herbivores depending on their level of specialization (diet-breadth or evolutionary adaptations). Using community assemblage data, we linked phytochemical variation in willow salicylates (willow-specific defensive compounds) and flavonoids (widespread defense) to resource specialization in three orders of leaf-chewing herbivores. Using insect orders as a proxy for levels of specialization, we focused on sawflies (Hymenoptera, mostly specialists), beetles (Coleoptera, highly adapted sequestering specialists and generalists), and caterpillars (Lepidoptera, mostly generalists). We expect specialists to respond to quantitative differences in willow-specific metabolites, because they are employed as feeding cues or sequestered to produce anti-predator defenses. Conversely, we hypothesize that generalists are affected by qualitative differences in willow-specific metabolites that probably deter them in small concentrations.
Results/Conclusions
Our results confirmed pronounced differences in specialization between the three orders. Sawflies were the most specialized order regardless of measured resource specialization in indices accounting for simple number of hosts or their chemical similarity. However, specialization of beetles and caterpillars depended on the specialization index used. Beetles were more specialized than caterpillars when accounting for the simple number of hosts, but not when accounting for host chemistry. Interestingly, the level of chemical specialization did not differ between beetle specialists and generalists. This suggests specialist and generalist beetles can overcome willow chemistry although the behavioral and physiological mechanisms most likely differ. Sawflies and caterpillars responded to willow chemistry, being affected mainly by quantitative differences in willow metabolites. Sawfly specialization correlated almost equally to salicylates and flavonoids, suggesting that willow-specific and common metabolites affected their host-choices. Specialization of generalist caterpillars correlated more strongly to salicylates than to flavonoids, but the difference is insignificant. Overall, our results suggest that qualitative and quantitative variations in plant chemistry have differential effects on insect herbivores depending on order and level of specialization. Our results emphasize the importance of quantitative variation in plant chemical composition, suggesting that it does not always have to be rare or species-specific metabolites that drive host-choice of leaf-chewing insects.