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United States Department of Agriculture

Agricultural Research Service

Title: Foraging behavior of field populations of Diadegma spp. (Hymenoptera: Ichneumonidae): testing for density-dependence at two spatial scales

Authors
item Legaspi, JR., Benjamin - STATE OF FLORIDA
item Legaspi, Jesusa

Submitted to: Journal of Entomological Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 1, 2005
Publication Date: July 20, 2005
Citation: Legaspi, Jr., B.C., Legaspi, J.C. 2005. Foraging behavior of field populations of Diadegma spp. (Hymenoptera: Ichneumonidae): testing for density-dependence at two spatial scales. Journal of Entomological Science. 40(3):295-306.

Interpretive Summary: Diamondback moth, Plutella xylostella, is a major insect pest of vegetables worldwide. One method to control this pest is to enhance the use of naturally-occurring beneficial insect parasites in the field crop. Scientists with the Center for Medical, Agricultural and Veterinary Entomology, Gainesville, FL, and the State of Florida studied how effectively an insect parasite, Diadegma sp. attacked the diamondback moth in brussel sprout plants. Using binoculars, we recorded the numbers of parasites found in or near host plants infested with different numbers of moth larvae. We found more parasite sightings at higher levels of moth larvae early in the experiment. Later, the parasites were found in equal numbers irrespective of host numbers. These findings are consistent with theoretical predictions that more profitable host ‘patches’ are exploited first, after which all patches are exploited equally. In a separate experiment, we infested field plants with moth larvae to detect differences in parasitism due to numbers of host larvae on individual plants or small groups of plants. Contrary to theory, we found no clear and consistent effects at the individual plant or cluster of plants. These results suggest a more complex interaction of the pest-parasite system in a field crop. These findings will help predict when and where different control methods are most effective.

Technical Abstract: The foraging behavior of populations of Diadegma sp. (Hymenoptera: Ichneumonidae) attacking the diamondback moth, Plutella xylostella L. (Lepidoptera: Pyralidae) was studied in the field. The effect of host density on percentage parasitism was investigated at two spatial scales: that of the individual plant, as well as a “cluster of plants” at low, medium and high densities. Using binoculars, parasitoid searching was observed on host plants at different host densities over an 8-hr period. Behavior was compared to that predicted under Patch Selection Theory. Percentage parasitism was independent of host density at both spatial scales. However, the behavioral studies showed parasitoid aggregation at higher host densities. The population displayed an imperfect preference for higher host densities in the first four hours of foragjng. In the second 4-hr period, no preference was observed. The behavior in terms of biological mechanisms, implications in the evaluation and use of biological control agents are discussed.

Last Modified: 4/17/2014
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