The Effect of Predator Age and Prey Defense on the Functional Response of Podisus maculiventris Say to the Density of Hyphantria cunea Drury

Abstract

Experiments were designed to find the functional response (response per predator) of the pentatomid Podisus maculiventris to the population density of larvae of the fall webworm, Hyphantria cunea. The response was measured in confined universes, (glass jars), representing webworm nests, and it was found permissible to vary universe size, within certain limits, in order to obtain a wide range of prey densities without handling large numbers of larvae. The “disc equation” proposed by Holling (1959) was found to describe the response curves, except at very high prey densities, and to provide a satisfactory biological explanation for their decreasing slopes. P. maculiventris ingests about 84% of its own weight for each fifth-instar larva of H. cunea attacked successfully, and the time spent in feeding and becoming hungry enough to make another attack reduces very substantially the time devoted to searching activities. Both prey defense (Fig. 1) and the age of the adult predator (Fig. 2) have very important effects on the functional response curves and on the values of the parameters, a and b, of the disc equation. These effects are explained. The “rate of discovery”, a, is contrasted to Nicholson's (1933) “area of discovery” and it is shown why the former is more likely to be independent of prey density than the latter.In view of the population of webworm larvae in the average nest, it is concluded that the functional response of the various predators inside the nest is probably not important, and that attention should be devoted instead to the functional and numerical responses of predators and parasites to the density of nests per unit area. Because of the vigorous defense exhibited by fifth-instar webworm larvae, predation by timid predators like P. maculiventris is selective and larvae whose development is delayed by parasitism are likely to suffer more predation than healthy larvae.