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Title: RESPONSE BY COCCINELLIDS TO SPATIAL VARIATION IN CEREAL APHID DENSITY (REVISE TITLE, JOURNAL NAME, AND ADD ACC DATE)

Author
item Elliott, Norman - Norm
item KIECKHEFER, ROBERT - RETIRED USDA-ARS

Submitted to: Population Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: Factors determining the effectiveness of lady beetles in cereal aphid biological control in wheat are not well understood. Further knowledge on this topic is needed in order to predict when lady beetles will control aphid infestations, and to determine ways to improve their performance as biocontrol agents. The purpose of this study was to determine whether coccinellids adjust their distribution in wheat fields in response to the patchy distribution of cereal aphids, and the patterns of cereal aphid population growth that result. Field experiments were completed in which the area of aphid-infested patches, aphid density in the patches, and access to patches by coccinellids, were varied. Some lady beetle species aggregated in aphid-infested patches, while others did not. Aggregation by lady beetles and the rate of aphid population growth were not related to the physical dimensions of aphid-infested patches. Aphid populations decreased faster in high aphid density patches than in low aphid density patches. Cereal aphid population change could not be explained solely by the action of lady beetles, and emigration of aphids from patches and variation in the quality of wheat plants as hosts played a role in aphid population decline. The results of this research will contribute toward developing predictive tools for assessing whether lady beetles will effectively control cereal aphid infestations in wheat fields, thereby precluding the need for insecticides to control the aphids.

Technical Abstract: The objectives of this study were to determine whether coccinellids adjusted their spatial distribution in response to spatial variation cereal aphid density in wheat fields, and patterns of cereal aphid population growth that resulted. Field experiments were completed in which the dimensions of patches of aphids, aphid density in patches, and access to patches by coccinellids, were varied. Densities of adult Hippodamia convergens and Coccinella septempunctata were correlated with aphid density in patches, while density of Coleomegilla maculata was not correlated with aphid density. The aggregative numerical response by larval coccinellids was slower than that of adults. The numerical response by coccinellids and the rate of aphid population growth, r, were independent of patch area. r was lower in supplemented than naturally infested coccinellid inclusion patches in all trials, but was significantly lower in only one trial. r was ssignificantly greater in coccinellid exclusion patches than in inclusion patches. The aphid to coccinellid ratio was always greater in supplemented than naturally infested patches. Cereal aphid population change could not be explained solely by the numerical and functional response of coccinellids, and emigration of aphids from patches and effects of variation in the host plant quality to aphids probably also were important.