Predation in diversified agroecosystems: relations between a coccinellid predator Coleomegilla maculata and its food.
A common prediction based on ecological theory is that natural populations of insect predators will be more abundant and effective in diverse plant assemblages. Contrary to expectation, the abundance of the predaceous coccinellid Coleomegilla maculata was higher on maize in monocultures than on maize in 2 different polycultures in New York State. In addition, the predation rate by the beetle on egg masses of the pyralid Ostrinia nubilalis was higher in monocultures. Important alternative resources for the beetle in these systems were aphids and pollen. Temporal dispersion of pollen and aphids was more even and species richness of the aphids was greater in the polyculture, while overall density of maize pollen and aphids was greater and their spatial distribution more even in the monoculture. The higher density of evenly spaced food rewards in the maize monoculture had the greatest impact on beetle behaviour, resulting in decreased emigration and therefore greater abundance. Plant diversification can have important effects on the density, species richness and temporal and spatial dispersion of alternative resources of generalist predators. Depending on the particular effects and how a predator responds to them, the predator may be more or less abundant in the diverse system. If diversification results in a greater abundance of food resources, or a more even spatial dispersion of food, then predators can be made more abundant and effective in diversified agroecosystems.