A “Vaccination” of Willamette Spider Mites (Acari: Tetranychidae) to Prevent Large Populations of Pacific Spider Mites on Grapevines

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Abstract
Grape vineyards, Vitis vtnifera L., in the San Joaquin Valley that support populations of Willamette spider mites, Eotetranychus willamettei (McGregor), tend not to have outbreaks of Pacific spider mites, Tetranychus pacificus McGregor. Pacific spider mites are considered the more economically damaging of the mite species on grapes in this region. Earlier, small-scale experiments indicated strong negative interactions between these two species. We introduced Willamette spider mites into a commercial ‘Zinfandel’ vineyard that had a history of chronic problems with Pacific spider mites. In 1988, populations of Pacific spider mites were reduced significantly on rows to which we had released Willamette spider mites in fall 1987 compared with control rows. The effect was apparent throughout the growing season. The grower treated the vineyard with acaricide once in mid-summer and effectively reduced spider mite populations to undetectable levels. When populations of Pacific spider mites resurged in late summer, they were three times higher on control rows compared with rows to which we had previously released Willamette spider mites. When these rows were censused in 1989, we found no differences in mite populations between the two treatments. A second experiment releasing Willamette spider mites was done in 1989 in a different part of the same vineyard. At peak densities, Pacific spider mites were 3.5 times more numerous on control vines compared with vines that we had infested with Willamette spider mites earlier in the spring. In the first study in 1988, the two treatments did not differ in berry yield or sugar concentration measured at harvest. In 1989, yield for these vines also did not differ between treatments, but sugar concentration was higher for vines that we had inoculated with Willamette spider mites. In the second experiment, yields did not differ between treatments, but sugar concentrations were higher for vines that had received Willamette spider mites.