We conducted laboratory and field experiments to determine the effects of plant quality and prey abundance on the intensity of interactions involving an omnivorous insect, its two herbivorous prey, and their shared host plant. We found that variation in plant quality, prey abundance, and presence of alternative prey altered the functional response of the omnivorous big-eyed bug, Geocoris punctipes (Heteroptera: Geocoridae). The presence of high-quality plant parts (lima bean pods) reduced the number of prey (pea aphids and corn earworm eggs) consumed by big-eyed bugs. The results of our caged, field experiments mirrored those of the functional-response experiment. Populations of pea aphids were larger when caged with big-eyed bugs on bean plants with pods than on plants without pods. Pods, therefore, had an indirect, positive effect on the survivorship of herbivorous insects that feed on lima beans. Because pods reduced the number of prey consumed by big-eyed bugs, and caged prey populations were larger on plants with pods, we hypothesized that herbivore populations would be larger in fields of beans with many pods than in fields of beans with few pods. To test this hypothesis, we established 20 x 30 m field plots of lima beans with many pods and with few pods. The results of this experiment forced us to reject our hypothesis: populations of herbivores were much smaller in plots of beans with many pods than in plots of beans with few pods. In an earlier study, we found that pods had a powerful, direct, positive effect on big-eyed bugs. Big-eyed bug populations were larger in plots of beans with many pods in the present study. It appears that the positive direct effects of pods on big-eyed bugs overwhelm the positive indirect effects of pods on herbivore population size, and that pod feeding by big-eyed bugs ultimately results in smaller herbivore populations. Plant quality, therefore, mediates the effect of this omnivore on prey suppression.
