A THREE-TROPHIC-LEVEL PERSPECTIVE ON FACTORS INFLUENCING HERBIVORE ABUNDANCE ON COLLARD

The influence of natural enemies and plant apparency in determining the abundance of the cabbage aphid Brevicoryne brassicae L. and the imported cabbageworm Pieris rapae L. was examined on collards grown in a collard monoculture and in a collard-alfalfa biculture. This design was used to examine the factors affecting collard-herbivore abundance from a three-trophic-level perspective.; Colonization rates of B. brassicae, the aphid parasite Diaeretiella rapae M'Intosh and the syrphid Sphaerophoria sulphuripes Thom. onto collards were higher in the monoculture, while P. rapae females laid more eggs on collards grown in the biculture.; The abundance and diversity of foliar predators was greater in the biculture, while the ground predator fauna did not vary as a whole relative to the habitat modification.; Predators most commonly observed on the collard plants were the harvestman Phalangium opilio L., the orb-weaver Tetragnatha laboriosa Hentz and the dwarf spider Erigone dentosa O. P. Cambridge in the biculture and Coccinella trifasciata L. and E. dentosa in the monoculture.; Serological analysis and natural enemy exclusion experiments indicated that D. rapae and S. sulphuripes were associated with significant mortality of B. brassicae in the monoculture while P. opilio was associated with significant mortality of both P. rapae and B. brassicae in the biculture.; These results argue for including plant properties as parameters in models of herbivore-natural enemy relations: (A) Plant apparency may influence the relation between herbivores and their natural enemies if both use the same cues for host location. (B) Changes in plant apparency over time may have substantial effects on local population dynamics of herbivores and natural enemies. (C) When herbivores respond differentially to plant apparency, relative abundance of herbivores on the host plant will change; this may cause switching of generalist predators from one herbivore to another, depending on the location of the host plant. (D) Since plant apparency may often be inversely related to plant species diversity, decreases in plant apparency may be correlated with increased establishment of generalist predators that respond to plant species diversity.; A three-trophic-level approach to theories on plant defense and predator-prey relations would add to our understanding of patterns of herbivore abundance in nature and would offer a theoretical framework for the design of multiple cropping systems.