Effects of Host Plant Patch Size on Herbivore Density: Patterns

Abstract

The influence of host plant patch size on the population densities of three herbivorous insect species was investigated in seven sizes of experimental patches of squash plants (from 1 to 144 plants). Responses to patch size differed significantly for the three herbivore species, which are closely related but differ in the degree to which they require cucurbits as hosts. The number of individuals of Acalymma vittatum (the striped cucumber beetle) per plant was significantly greater in small patches (1—4 plants) than in larger patches (16—144 plants). The spotted cucumber beetle or southern corn rootworm (Diabrotica undecimpunctata howardi) was most abundant in intermediate—sized patches (64 or 100 plants). Population densities of Diabrotica virgifera (the western corn rootworm) were more variable, but tended to increase with increasing patch size. Relationships between herbivore density and patch size were nonlinear for all three species. The effects of patch size on herbivore population densities also differed between generations of the same species. Second generation Acalymma vittatum were most abundant in large patches, which was opposite to the pattern shown by first generation individuals. Greater numbers of second generation adults emerged in large patches than in small patches, indicating an effect of patch size on larval survivorship and/or oviposition rates. Densities of the spotted cucumber beetle were also differentially affected by patch size at different times throughout the season. Patch size significantly influenced plant growth, as measured by number of leaves and biomass, but did not influence parameters of plant reproduction (numbers of flowers of yield). Plant longevity was significantly greater in larger patches (16 plants or greater) than in small patches (1 or 4 plants). To examine the role of differences in plant size associated with patch size in influencing beetle densities, analyses of the number of beetles per leaf were carried out. Even when differences in plant size were taken into account, patch size still significantly affected densities of all three herbivore species. Thus, beetles responded to patch size per se, not just to associated changes in plant size. Relationships between plant parameters and herbivore parameters were analyzed in detail for the most specialized of the herbivore species, Acalymma vittatum. Beetle densities were positively correlated with numbers of flowers early in the season, but more strongly correlated with numbers of leaves late in the season. Plant longevity was negatively correlated with numbers of early colonizing beetles, indicating a potential effect of herbivory on plant survivorship. Variable herbivore responses to patch size are discussed with emphasis on mechanisms, including plant size/quality and insect movement.