Polyphagy by omnivory: scavenging improves performance of a polyphagous caterpillar on marginal hosts

Few species of insect herbivores are highly polyphagous, but those few species are disproportionately ecologically and economically important and include many of the most destructive crop pests. Common correlates of extreme polyphagy across insects include the related behaviors of cannibalism and omnivory, though any functional consequences of these behaviors on the host range are unknown. I hypothesized that omnivory may allow these insects to exploit marginal hosts successfully (an expansion of realized niche). Using the polyphagous pest caterpillar, Heliothis virescens, I tested the polyphagy by omnivory hypothesis using ten host plants of varying suitability and small quantities of insect carrion. Caterpillars which were allowed omnivory had increased performance on lower-quality hosts; this treatment raised survival, growth rate, and pupal mass over controls on a strictly plant diet. Omnivory allowed successful development on two plants that caterpillars could not exploit alone a potential niche expansion. This effect was limited, however: (1) on high-quality hosts, omnivory did not improve performance, and (2) omnivory on poor hosts did not increase growth rate or pupal mass to levels matching the most suitable hosts and it could not permit exploitation of a completely unpalatable plant. Omnivory may therefore be an important (and overlooked) factor in determining the success of generalist insect herbivores in a variety of ecological settings.