Favored by plant water stress, harmful spider mites displace plant-defense inducing mites from California vineyards

Management practices that influence leaf microclimate can also affect herbivore interactions. In California vineyards, Willamette spider mites (Eotetranychus willamettei) infest grape leaves from April until June, when Pacific spider mites (Tetranychus pacificus) make their first appearance. Researchers have documented that early infestations of Willamette mites lead to lower populations of the more damaging Pacific mites, because Willamette mites induce plant defenses. In recent years, however, outbreaks by Pacific mites have increased, coinciding with the wide application of deficit irrigation (water-stress) in vineyards, a technique that accelerates sugar accumulation in fruit. We hypothesized that water stress provides a competitive advantage to Pacific mites by indirectly elevating leaf temperature. We estimated development rates of Pacific and Willamette mites at temperatures from 10 – 40^oC in the laboratory. Additionally, we measured leaf water potential, leaf temperature and mite densities in eight vineyards in the San Joaquin Valley in 2006 and 2007. Development rates of the two mites were the same between 10 and 22.6^oC, but were faster for Pacific mite at higher temperatures. Our field study demonstrated that water stress caused an increase in leaf temperature that in turn favored population growth of Pacific, but not Willamette mite. There was a positive relationship between Pacific mite densities and the number of hours leaf temperature exceeded 31^oC, the upper limit for Willamette mite development. While many factors influence the outcome of interspecific competition between herbivores in nature, this study is the first to highlight a strong indirect effect of water stress via leaf temperature.