Date of Award


Degree Name

MS in Agriculture - Plant Protection Science


Horticulture and Crop Science


Michael J. Costello


The spider mite family (Tetranychidae) is a well known pest group in agriculture. Within this family, Willamette spider mite (Eotetranychus willamettei) causes physical harm and potential damage to grapevines (Vitis vinifera) along the central and north coast of California as well as Washington and Oregon. Willamette spider mite prefers cooler climates and feeds by puncturing the plant leaf tissue; therefore removing plant nutrients in the early stages of plant growth. Amending soils with fertilizer is a common cultural practice used in commercial vineyards, but no study has documented the interaction between the effects fertilizer concentrations have on Willamette spider mite. This project consisted of a field study (at Cambria Vineyards & Winery, Santa Maria, CA) and a laboratory study (at a California Polytechnic State University, San Luis Obispo campus greenhouse). The field study tested the effects potassium, nitrogen, and control (unfertilized) treatments had on Willamette spider mite on ‘Chardonnay’ grapes. Field results showed no significant difference among the three treatments, but suggest a response, given that mite density peaked highest in the potassium treatment and had a second high peak in the nitrogen treatment. In addition, egg density peaked highest in the potassium treatment. The lab study tested the effect four different nitrogen treatments had on Willamette spider mites. Treatments ranged from High N (1500 ppm N fertilizer), Med-High N (1500 ppm N fertilizer), Med–Low N (500 ppm N fertilizer), and Low N (0-50 ppm N fertilizer). Four parameters were tested: male and female days to maturation, male and female survivorship to adult, adult female longevity and oviposition. Longevity and oviposition lab results indicate that Willamette spider mite has a non-linear response to grape N concentration. Performance was better within the two medium treatments compared to High N and Low N treatments. Survivorship suggests the same as days to maturation although not statistically significant.