Plant Drought Stress and Outbreaks of Spider Mites: A Field Test

I conducted a field experiment to clarify the relationship between drought stress of bush beans, Phaseolus vulgaris, and outbreaks of the twospotted spider mite, Tetranychus urticae, in the presence or absence of an important spider mite predator, Phytoseiulus persimilis. Spider mite populations were highest on well—watered and on severely stressed bean plants and lowest on slight—to—moderately stressed plants, indicating a nonlinear relationship. The predator exaggerated this pattern, reducing mite populations the most on moderatley stressed plants and having the least impact on mites from severely stressed plants. Number of mites per unit leaf mass (a measure of mite injury to plants) was highest on severely stressed plants, lowest on slightly stressed plants, and intermediate on well—watered plants. Bean yield was negatively influenced by drought stress, with those plants that experienced high numbers of mites per unit leaf mass appearing to be more sensitive to drought than plants with low numbers of mites. The implications of these data for understanding the causes of pest outbreaks are discussed.