Effects of Plant Moisture Stress and Rainfall on Population Dynamics of the Twospotted Spider Mite (Acari: Tetranychidae)

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Abstract
A rainout shelter was used to control soil moisture and rainfall conditions in soybean plots artificially infested with twospotted spider mites, Tetranychus urticae Koch, in 1987 and 1988. Leaf disks were collected to determine the separate and interactive effects of plant water stress and impact energy of rainfall on spider mite population dynamics. In both seasons, spider mites initially colonized lower leaves and dispersed into the upper canopy as populations peaked 3–4 wk later. Precipitation applied with a rainfall simulator was not found to significantly affect mite intensity (mites per leaf area). Significantly greater mite intensities were found in stressed plots on two of six sampling dates in 1987 and in the pooled data of 1988. However, the differences in intensity were slight and may be accounted for, in part, by less total leaf area on the stressed plants. Results did not support the traditional belief that plant stress and the occurrence of heavy, driving rains are important factors in spider mite population dynamics. Rather, other factors associated with drought conditions require consideration to explain spider mite population dynamics in soybean.