The ecological consequences of variation in plants and prey for an omnivorous insect

Abstract

We conducted a series of laboratory and field experiments to determine the effects of variation in plant quality and prey species on the survival, dispersal, and population size of a common, omnivorous insect. We also tested the hypothesis that plant feeding allows omnivorous ''predators'' to survive periods when prey are scarce. In addition, we compared the response of omnivores and strict predators to variation in plants and prey. We found that variation in plant parts (pods and leaves) and prey species (moth eggs and aphids) affected the survival, dispersal, and population size of big-eyed bugs, Geocoris punctipes, in Maryland lima beans. The survival of big-eyed bugs fed moth eggs was high and unaffected by the availability of lima bean pods or leaves as supplemental food. The survival of big-eyed bugs fed aphids, however, was relatively low and dramatically improved by the availability of pods. Big-eyed bugs fed only pods developed normally through the first and second instar, but development was arrested in the third instar. Only bugs fed high quality prey or mixtures of low quality prey and high quality plant food developed beyond the third instar. Plant feeding, especially pod feeding, allowed big-eyed bug nymphs to survive prey-free periods, but plant feeding was not as important for adult survival when prey were unavailable.

The dispersal of adult big-eyed bugs from lima bean plants with pods was significantly lower than the dispersal of big-eyed bugs from plants without pods. The presence of aphids on lima bean plants also reduced the dispersal of big-eyed bugs. Unexpectedly, the presence of moth eggs, high quality prey as measured by big-eyed bug survival in previous experiments, did not reduce the dispersal of big-eyed bugs.

Populations of big-eyed bugs were larger in plots of beans with pods than in plots of pod-free beans. Furthermore, the most important predictor of the density of big-eyed bugs was the number of pods per plant. Populations of other omnivorous insects (minute pirate bugs, damsel bugs, and ladybird beetles) were also larger in plots of beans with pods than in plots of pod-free beans. However, other omnivores responded to changes in the numbers of flowers per plant and not to changes in the number of pods per plant or potential prey. Conversely, the most important predictor of the density of strict, non-plant-feeding predators was the density of potential prey (herbivores).

Our results indicate that omnivory provides ecological flexibility for big-eyed bugs. Feeding at more than one trophic level furnishes big-eyed bugs with complimentary resources that allow these omnivores to survive when resources at one trophic level are of low quality (e.g., aphids) or when resources at one trophic level are totally unavailable (e.g., prey). Furthermore, the dynamics of omnivorous insects such as big-eyed bugs are intimately associated with variation in their host plants and not with changes in the density of their prey. Omnivorous insects appear to track resources at the lowest trophic level at which they feed, in this case plants, and not resources farther up the food chain (e.g., prey).