Abstract
After an initial application of chemical insecticides, all field plots of fall lettuce treated with nuclear polyhedrosis viruses isolated from Trichoplusia ni (Huibner) and Autographa californica Speyer, or with a commercial preparation of strain HD-l, Bacillus thuringiensis var. Kurstaki (3A,3B) DeBarjac, demonstrated adequate control of lepidopterous larvae. However, damage by thrips was excessive and decreased the quality of the crop. Of T. ni larvae collected from untreated plots, 28% were parasitized; no more than 1.1% of the T. ni and Spodoptera exigua (Hubner) larvae collected from lettuce treated with chemical insecticides were parasitized; and 5.2–15.4% of those collected from plots treated with microbial insecticides were parasitized. Survival to pupation of larvae of both species was highest among those collected from the chemically-treated fields and from plots treated with HD-l and lowest among those collected from plots treated with the nuclear polyhedrosis viruses. The microbial insecticides therefore have a potential as control agents for lepidopterous pests, chemical pesticides have a severe effect on parasites, and the microbial insecticides have a deleterious effect on parasitism that may limit control by a combination of pathogens and parasites.
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