Response by coccinellids to spatial variation in cereal aphid density

The objectives of this study were to determine if coccinellids adjusted the ir distribution within spring wheat fields in response to spatial variation cereal aphid density in the fields and to describe the patterns of cereal aphid population growth that resulted. Field experiments were completed in which the physical dimensions of patches infested with cereal aphids, cerea l aphid density, and access to patches by coccinellids were varied. Aphid i nfestations consisted of naturally occurring densities (natural patches) an d much greater densities created by supplementing patches with aphids (supp lemented patches). Coccinellids were denied access to some supplemented pat ches (exclusion patches) but allowed unlimited access to others. Densities of adult Hippodamia convergens and Coccinella septem-punctata were correlat ed with aphid density in patches whereas density of Coleomegilla maculata w as not. Aggregation by coccinellids was independent of patch area. The real ized aphid population growth rate (r) was lower in supplemented than natura l patches in all four trials but was significantly lower in only one trial. The lower r in supplemented patches was not exclusively caused by coccinel lid predation, and emigration of aphids from patches probably also contribu ted, r was significantly greater in exclusion patches than supplemented and natural patches, indicating that coccinellids markedly reduced aphid numbe rs in patches even when aphid density was extremely high.