Natural Mortality Factors Acting on Potato Aphid (Macrosiphum euphorbiae) Populations in Processing-Tomato Fields in Ohio

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Abstract
The populations of potato aphids and their natural enemies on processing-tomatoes were monitored in 1980 and 1981 in Wooster and Fremont, Ohio. Rainfall in combination with high winds appeared to be the major factor in mortality in 1980. In the dry 1981 season, large populations developed. Natural enemies did not control aphid populations in Wooster in 1981, or in either location in 1980. Aphidius nigripes was the dominant parasitoid, but its effectiveness against aphids was limited by high hyperparasitization by Asaphes lucens and other hyperparasitoids. Predators (coccinellids, neuropterans, and syrphids) exhibited slow numerical responses and were for the most part ineffective in controlling aphid populations. High population levels in Wooster in 1981 caused massive emigration of alates. In Fremont in 1981, the population levels were much lower. Slower aphid colonization, syrphid predation early in the season, and density-dependent parasitization throughout the season appeared to have been responsible for the lower aphid population in Fremont. Large numbers of natural enemies apparently immigrated to the Fremont site from a heavily infested, adjacent tomato field.

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