Herbivory, plant traits and nectar chemistry interact to affect the community of insect visitors and pollination in common milkweed, Asclepias syriaca

Abstract

Herbivory can alter plant fitness directly through changing reproductive allocation and indirectly through changing pollinator identity or behavior. Common milkweed is a plant of conservation concern with an inducible chemical defense that is also an important nectar resource. In this study, we aim to understand how herbivory severity and plant traits, including morphology and nectar chemistry, interact to affect insect visitation and pod production in common milkweed. We conducted pollinator watches on plants with experimentally varied herbivory severity and quantified insect frequency and visit length as a response to nectar chemistry, ramet height, number of inflorescences, number of flowers per inflorescence and percent tissue removed. We also quantified pollinator effectiveness and importance. Increased herbivory severity reduced floral displays, including fewer inflorescences and fewer flowers per inflorescence. A reduced floral display was correlated with reduced sucrose, fructose and glucose and resulted in a reduced number and species richness of insect visitors. Fewer flowers per inflorescence reduced the frequency of bumble bee and fly visitors, which were two important pollinators. Although honeybees, flies, small bees, soldier beetles and bumble bees were equally effective pollinators, only bumble bee frequency was positively correlated with pod production. The differences in pollinator visitation have the potential to create diversifying selection on plant floral traits, many of which are also affected by herbivores. This research demonstrates potentially conflicting selection pressures between native and non-native pollinators as well as non-native herbivores.

Keywords: Apis mellifera; Bombus griseocollis; Chauliognathus pensylvanicus; Sugar ratio; Tetraopes tetrophthalmus.