Biological control of Pacific mites and Willamette mites in San Joaquin Valley vineyards: I. Role ofMetaseiulus occidentalis

Abstract

Population studies in southern San Joaquin Valley vineyards showed that the distributional patterns of Willamette mite, Eotetranychus willamettei Ewing, and Pacific mite, Tetranychus pacificus, differ in the Valley, in the vineyard, and on the vine itself. Pacific mite generally does best under hot and dry conditions while Willamette mite prefers cooler and more humid conditions, although there is considerable overlapping of the two species. Detailed populations studies showed that untreated vineyards with no histories of pesticide treatment often exhibit more efficient levels of predation than do vineyards which have been treated. Also, contrary to the views of some investigators, these studies indicate that Metaseiulus occidentalis (Nesbitt) has no self-limiting aspects in its response to low or high prey densities. In undisturbed vineyards, a mild predator-prey interaction appears to help perpetuate effective control of low prey densities by M. occidentalis. In disturbed vineyards, widely fluctuating predator and prey populations may lead to population crashes and continuation of imbalance. Moreover, when regulating low, but potentially serious, Pacific mite densities in undisturbed vineyards, M. occidentalis appears to benefit from the presence of the less seasonally restricted Willamette mite. Tonnage and fruit-quality data showed that Willamette mite is often not a significant pest. Little differentiation has been made in the past between Pacific mite and Willamette mite, and treatments have been applied as if both were serious pests.