Interaction of the Twospotted Spider Mite (Acari: Tetranychidae) with Yield of Day-Neutral Strawberries in California

Yield of day-neutral strawberries, Fragaria × ananassa Duch (‘Selva’), grown in California were evaluated for 7 yr over a range of twospotted spider mite, Tetranchus urticae Koch, population densities. From 1988 through 1994, acaricide treated and untreated plots of Selva were established each fall in Watsonville, Santa Cruz County, CA. T. urticae population densities were calculated on a per midtier leaflet basis every 2 wk as the average number of motile T. urticae counted on 10 midtier leaflets sampled from each plot. Mite-days were calculated from these counts and summed for monthly intervals by using 15 March and 30 August as starting and ending dates, respectively. These calculations provide a standard index of T. urticae feeding pressure on strawberry plants among and within years. Yield loss resulting from T. urticae feeding in untreated plots versus plots treated with acaricides averaged 25%. T. urticae feeding had little or no effect on fruit size but caused a significant reduction in the number of berries produced per plant. The later T. urticae were first detected in winter or spring, the less yield was reduced during the peak fruiting months of June and July. Maximum T. urticae abundance was negatively associated with yield, and greater mite-day accumulations were associated with yield decline. Regression analysis indicated that the 0% yield reduction threshold averaged 30 cumulative mite days during any monthly period. This implies that detectible yield reduction caused by mite feeding occurs at population densities > 1 T. urticae per leaflet. However, late winter and early spring mite feeding resulted in greater yield reductions than mite feeding at equivalent levels of infestation in late spring and summer. Yield reduction resulting from mite feeding is cumulative at relatively low population levels throughout spring and damage resulting from mite feeding plateaus during summer.