SELECTION FOR HOST MODIFICATION BY INSECT PARASITOIDS.

Abstract

The ability of parasites to influence the behavior of their hosts is an important aspect of parasite-host biology (Price, 1980). Holmes and Bethel (1972) review an extensive literature that documents the influence of parasites on the behavior of their intermediate hosts. They argue that these effects by parasites on their hosts are adaptations of the parasites to increase transmission to subsequent hosts. Holmes and Bethel suggest that parasites affect intermediate host behavior by reducing stamina, increasing conspicuousness, causing disorientation, and altering responses so that hosts are more susceptible to predation by the definitive host of their parasites. Modification of insect hosts by their parasitoids, however, has received little attention. Insect parasitoids, unlike parasites, are of relatively large size, they possess a freeliving stage, and their development kills the hosts; therefore their effect on host population dynamics resembles that of predators (Doutt, 1959). Parasitoids, other than insects, and parasitic castrators show similar characteristics (Kuris, 1974; Trail, 1980). Most insect species are attacked by one or more parasitoid species during their development. The growth, development, and survival of parasitized hosts are of direct consequence to the parasitoids; if the host dies before the parasitoid emerges, so does the parasitoid. Vinson (1975) developed the concept that host regulation is necessary for the parasitoid's success. He stated that growth and development of parasitoids were enhanced by physiological regulation of the host through venoms, hormones and parasitoid feeding. Parasitized hosts may behave differently from unparasitized hosts in predator avoidance responses, microhabitat selection, and developmental and growth rates. Some of these modifications may be adaptively significant, and promote the coevolution between the host and its parasitoid. Here I propose a hypothesis that extends Vinson's host regulation concept to explain the evolution of host modification by insect parasitoids. This hypothesis leads to predictions of the characteristics of hosts that should influence how parasitoids should modify their hosts.