Predator interference and the establishment of generalist predator populations for biocontrol

We conducted a field experiment to determine the extent to which interferen ce among generalist predators Limits their effectiveness as biocontrol agen ts. We manipulated immigration of a guild of actively hunting generalist gr ound predators, carabid beetles and lycosid spiders, by intercepting them a s they attempted to enter fenced 50-m(2) vegetable gardens, Immigration was blocked, allowed at the mean rate measured at our field site, or doubled, Altered immigration rates were maintained through a spring garden of cabbag e, bean, eggplant, and cucumber, followed by a summer garden of squash. We monitored densities of carabids and lycosids to discover if altering their immigration rate changed their densities in the plots. We also measured den sities of other predators on the ground and in plant foliage, pest numbers, and vegetable yields. Doubling the immigration rate of carabids and lycosi ds approximately doubled the densities of carabids inside the plots, but di d not increase lycosid densities. Increasing the rate of immigration of car abids and lycosids depressed densities of nonlycosid ground spiders. In the spring gardens, manipulation of carabid and lycosid immigration did not in fluence numbers of predators or herbivores in the foliage and did not affec t vegetable productivity. In contrast, in the summer gardens, foliage-dwell ing predators were lower, pest densities were marginally lower, and squash productivity was higher in the carabid and lycosid immigration plots compar ed to the no-immigration treatment, Doubling carabid and lycosid immigratio n rate never increased the magnitude of their effects on other predators, p ests, or plant productivity. Predator interference limited lycosid establis hment, reduced densities of other predator taxa, and apparently prevented a doubling of carabid densities from having an increased impact on pest numb ers. Nevertheless, despite widespread effects of predator interference, all owing immigration of lycosids and carabids increased squash productivity. ( C) 1999 Academic Press.