Insecticide-Induced Population Changes in Four Mite Species on Alfalfa1

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Abstract
Three basic population effects were found resulting from spray treatments on alfalfa: (1) DDT treatments resulted in increased populations of the two-spotted spider mite Tetranychus telarius (L.), and reduced populations of Typhlodrornus cucumeris Oudms. and Tydeus sp.; (2) Dieldrin and endrin increased populations of Typhlodromus but decreased Tetranychus and Tydeus; (3) Schradan and other organic phosphates increased populations of Tydeus and reduced Typhlodromus and Tetranychus. Combinations of materials caused intermediate reactions. A fourth species of mite, Tarsonemus confuses Ewing, abundant only during 1 year of the study, was increased in number by demeton and reduced by dieldrin The population changes occurred over a period of 4 to (J weeks after treatment and the changes arc the result of a combination of direct insecticide effects on individual species and of biotic interactions. Since large populations of Typhlodromus and Tydeus may develop in the near absence of two-spotted spider mites, these predators are not dependent on the two-spotted spider mite as a source of food.

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