Sustainable mite management: Effects of regulated deficit irrigation and the overwintering biology of mites in vineyards
We evaluated the effects of regulated deficit irrigation on spider mite (Acari: Tetranychidae) and leafhopper (Homoptera: Cicadellidae) outbreaks in a commercial vineyard in Madera, CA. We established three irrigation treatments corresponding to 50, 70 and 100%of crop evapotranspiration (ETc) and monitored leaf water potential, leaf temperature and spider mite (Acari: Tetranychidae) and leafhopper (Homoptera: Cicadellidae) densities through the growing season. Leaf water potential of vines in the three irrigation treatments was significantly different on one sampling date in September only, when vines in the 50%ETc treatment were significantly more water stressed than vines in the 70 and 100%ETc treatments. Similarly, on the same sampling date leaf temperature was significantly higher on vines in the 50%ETc treatment than vines in the 70 and 100%ETc treatments. The lack of differences in leaf water potential between the different deficit irrigation treatments in this vineyard suggest a problem with the calculation of the ETc requirements of vines and/or the amount of irrigation water needed to cover ETc. Densities of Pacific spider mite, Tetranychus pacificus, and Willamette spider mite, Eotetranychus willamettei, begun to increase towards the end of June. A miticide application in mid August due to concerns of potential mite damage reduced mite densities to zero within three weeks. Spider mite densities did not differ significantly among the three irrigation treatments. Leafhopper densities remained at very low levels. A glasshouse study was carried out to complement the field study and provide information on the effects of regulated deficit irrigation and temperature on T. pacificus outbreaks. Vines were assigned into two glasshouse rooms representing cool (22.4°C) and hot (27.4°C) environmental conditions and were kept under five irrigation treatments corresponding to 100, 80, 60, 40 and 20%of ETc. Vines under hot environmental conditions were significantly more water stressed than vines under cool conditions. In addition, vines in the 20%Etc treatment were significantly more water stressed than vines in the other four irrigation treatments. However, we detected no significant differences in T. pacificus densities among irrigation treatments or between vines kept under cool or hot environmental conditions. Our results suggest that different irrigation amounts may not result in differences in plant-based measures of water stress. Grape growers need to estimate crop evapotranspirational needs and corresponding irrigation amounts precisely in order to successfully apply regulated deficit irrigation. Further studies are required to understand the effects of regulated deficit irrigation on spider mite outbreaks in California vineyards.