Plant density affects interactions between Solanum carolinense and its insect herbivores: Implications for plant population regulation

Although interest in managing invasive exotic plants has increased consideration of the importance of top down factors (such as insect herbivores) in plant population dynamics, such effects remain controversial. We have argued that better tests of herbivore effects on plant populations are required, and advocate an approach incorporating density dependence (which is necessary for population regulation to occur) across the full life cycle of the plant. One component of this approach involves characterizing mechanisms of density effects on plant-herbivore interactions.  Here we present results testing density effects on interactions between Solanum carolinense and its insect herbivores, including Leptinotarsa juncta and Manduca sexta. To test for changes in resistance to herbivores with plant density, we conducted larval bioassays on potted plants grown at high and low densities in the greenhouse. Larvae grew significantly faster when eating leaves from plants grown at low density. Adult females also responded to plant density when making oviposition decisions. After controlling for plant size, females laid more eggs on plants individually in pots than in plants grown with 1, 3, or 7 neighbors. Density also affected plant-herbivore interactions in the field in northern Florida: plants growing at higher density had greater total herbivore damage levels across both natural and experimentally-manipulated plant densities. These results demonstrate that plant density changes plant quality/resistance to herbivores as well as insect herbivore behavior in this system. These density effects allow for herbivore effects on population dynamics in S. carolinense. To rigorously test this possibility, we will parameterize density-dependent matrix population models using data from on-going experiments that manipulate both plant and herbivore densities.