The eco-evolutionary dynamics between host plants and herbivorous insects putatively follow an arms race scenario: plants produce new defenses and herbivores increasingly specialize by adapting to those defenses. In addition, if plants and herbivores have patchy distributions, these dynamics can facilitate local adaptation. Our previous work in the Northeast-US region showed that stem weevils (Rhyssomatus spp.) locally vary their manipulation of common milkweed (Asclepias syriaca) before laying eggs. Northern weevils dig a trench on the stem (i.e. Shaving), while in the south they poke/girdle around the stem. These behaviors, typically associated with oviposition, may optimally avoid plant toxins; therefore, we addressed whether this variation is a plastic response to host plant genotype or a locally adapted trait. We collected common milkweed seeds and weevils from locations at the north and south of the weevil’s distribution. We placed weevils on native and novel host plant genotypes grown from seed in a common garden in 2016. Each day we measured number of weevils that fed from leaves, counted number of pokes on the stem, type and frequency of oviposition behavior, and after 3 weeks we counted presence of larvae in stems to determine survival of eggs laid.
Results/Conclusions
We found that weevils fed equally from the leaves of all plant genotypes. Meanwhile, only one weevil, from the south, showed the girdling oviposition behavior. The number of pokes to the stem and shaving frequency varied significantly depending on weevil origin but not by host plant genotype. However, subsequent larval presence was explained by host plant origin and the interaction between host plant and weevil origin, not by weevil origin alone. Presence of larvae was more frequent in southern plants when southern weevils laid eggs on them; for northern weevils that laid eggs, there was no significant variation in larvae presence for either host plant genotype. Larval frequency was not explained by number of pokes to the stem or shaving frequency, suggesting that amount of stem damage by the adult was not linked to larvae survival. We conclude that Rhyssomatus weevils do not have a plastic oviposition behavior that can be modified according to host plant genotype and larval data suggest local adaptation to host plant genotype, at least in southern populations.