The preliminary results of two years of field studies in Lodi and Madera vineyards link Pacific spider mite (Tetranychus pacificus – PSM) outbreaks to water stress and the frequency of high temperatures on south-facing leaves. We found a positive relationship between water stress and leaf temperature, with water stressed plants having warmer leaves with higher PSM populations. However, we found no relationship between high leaf temperatures and Willamette spider mite (Eotetranychus willamettei – WSM). Grape growers should use deficit irrigation with caution, especially in vineyards with significant risk of PSM outbreaks.
We also carried out a study on the effects of temperature on development of PSM, WSM and western predatory mite (Galendromus occidentalis – WPM). Preliminary results suggest that 15° C slightly favored WSM over PSM, although PSM laid more eggs. At 22° C development time of the two mites did not differ. However, at 28° C PSM developed faster, lived longer and laid more eggs than WSM, which experienced higher immature mortality. At both 15 and 28° C WPM developed faster than PSM or WSM. Nevertheless, PSM laid 2.5 times as many eggs as WPM at 28° C suggesting that PSM may escape control as temperature rises. Prey type (WSM or PSM) at 28° C did not affect development or longevity of WPM. However, a greater number of WPM immatures escaped from grape leaf disks with WSM as prey compared to PSM as prey suggesting that WPM may be retained more effectively on PSM infested leaves in vineyards.
In addition, we ran a study on the effects of three pesticides on biological control of PSM using grape plants in the laboratory in a fully-crossed, two factor design: Presence and absence of WPM crossed with presence and absence of pesticide. The insecticide imidacloprid (Provado) did not affect PSM population growth, but drove WPM close to extinction, such that PSM densities were 70%lower on grape plants treated with WPM alone than with WPM and imidacloprid. In contrast, the fungicide wettable sulfur (Thiolux Jet) significantly decreased PSM populations by about 50%, and had no effects on WPM, such that PSM densities were about 40% less in the WPM and sulfur treatment than in the WPM alone treatment. The insecticide buprofezin (Applaud) had no effect on PSM, but it reduced WPM populations by 70%}, indicating its potential for long term disruptive effects. Our study demonstrates that use of pesticides such as imidacloprid and buprofezin may facilitate the development of PSM outbreaks, while other pesticides such as wettable sulfur may help WPM to suppress PSM.
Furthermore we tested the resistance of PSM populations to the miticides bifenazate (Acramite) and pyridaben (Nexter). We found that PSM from vineyards that reported miticide failures in south Salinas Valley and Lodi showed 10 times higher resistance to pyridaben than a susceptible population. In addition, PSM from Sonoma and Lodi demonstrated seven- and fourfold resistance to bifenazate respectively. Our results underline the dynamic nature of resistance evolution in PSM and point to the need for alternating miticides from different chemical groups to minimize resistance problems.