Ground cover improves efficacy of spider mite predator releases in raspberry high tunnels

Growing fruit under high tunnels extends the growing season and protects crops from unfavorable weather. Unfortunately, hot arid conditions under the tunnels during peak summer months support outbreaks of spider mites (Tetranychidae) that pose a major pest management challenge. In this experiment we investigated the efficacy of ground cover and Amblyseius californicus (Phytoseiidae) predatory mite introduction for spider mite management in high tunnel grown raspberries. Our experiment consisted of six blocks of raspberry rows divided into four 1.3 x 2.5 m plots separated by buffers. Four treatments: 1) ground cover (GC) with mites, 2) no GC with mites, 3) GC without mites, and 4) no GC without mites, were randomly assigned to plots. A clover ground cover was grown on both sides of the raspberry rows of GC plots. After GC established, initial densities of pest and predator mites on raspberry leaves were measured. Next, A. californicus were released onto raspberries in “with mites” plots. Mite abundances on raspberries in plots were measured one week after mite introduction and for six weeks thereafter. Relative humidity and temperature in the raspberry canopy in GC and no GC plots were recorded throughout the experiment. We found significantly fewer spider mites in GC with mites plots. Daylight temperatures were lower and relative humidity was higher in raspberries adjacent to ground cover than bare ground. Hence, maintaining ground cover and introducing A. californicus lessened spider mites pressure in high tunnel grown raspberries.