Intraspecific variation in herbivore infestation on Monarda fistulosa genotypes from resource-divergent origins

Intraspecific variation in plant traits can strongly structure insect herbivore communities. This may often result from variation in secondary metabolites, which are often produced by plants as a means of defense against herbivory and yet are also used by different guilds of arthropods for searching for host plants. Chemical defense may vary with geographic origin due to spatial variation in resource availability. In turn, secondary metabolites should reduce herbivory from generalist insects, yet may increase abundance of specialists that utilize these compounds to locate suitable hosts. Here we examined damage and abundance of insects colonizing plants in a common garden of Monarda fistulosa, a perennial herb defended primarily by terpenoid compounds. Genotypes originated from resource-rich mesic grasslands in southeastern Wisconsin and resource-poor arid grasslands in western Montana. We previously showed that terpenoid concentrations were nearly twice as high in the populations originating from low- vs. high-resource environments. We compared patterns of leaf damage, galling by a specialized cecidomyiid fly and infestation by Aphis monardae aphids on the plant populations originating from low- and high-resource environments.  Leaf damage predominantly inflicted by generalist herbivores was greater on low-resource populations. Galls were more abundant on low-resource populations, and aphid infestation was greater on high-resource populations.  This work shows that variation in resource availability that have important impacts on plant chemical phenotypes can subsequently structure insect communities that damage and colonize their host plants.