Contrasting trophic cascades generated by a community of generalist predators

We examined whether intraguild predation (IGP) weakens the ability of two t axa of generalist predators, carabid beetles and lycosid spiders, to contro l herbivore densities in gardens of cucumber and squash. We also measured f ruit production to determine whether IGP weakens the ability of the predato rs to impact plants indirectly through trophic cascades. These predators di d impact herbivore densities, but their impact on different herbivores vari ed dramatically. As the herbivore community changed through the season, so too did the predators' indirect effect on fruit production. In spring cucumber gardens, lycosids caused fruit production to increase by reducing densities of striped cucumber beetles, which are important early- season herbivores. Carabids also weakly reduced cucumber beetle densities, and the entire carabid-lycosid guild enhanced cucumber yield. However, the carabid-lycosid guild's impact changed dramatically later in the season. In summer squash gardens, carabids increased fruit production by reducing den sities of the squash bug, the major late-season herbivore. In marked contra st, lycosid spiders strongly reduced squash yield, possibly by IGP on impor tant hemipteran predators of the squash bug. The late-season effects of car abids and lycosids counteracted one another, so that the complete carabid b eetle-lycosid spider guild had no impact on squash fruit production. Thus, the complex trophic connections between generalist predators and the rest o f the community made the predators' impact relatively fluid, changing even within a single season. Results of pairing predators with herbivores or oth er predators in laboratory microcosms supported hypothesized trophic intera ctions in both spring and summer food webs.