INTERACTION OF THE 2-SPOTTED SPIDER-MITE (ACARI, TETRANYCHIDAE) WITH YIELD OF DAY-NEUTRAL STRAWBERRIES IN CALIFORNIA

Yield of day-neutral strawberries, Fragaria X ananassa Duch ('Selva'), grown in California were evaluated for;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 esta blished each fall in Watsonville, Santa Cruz County, CA. T. urticae po pulation densities were calculated on a per midtier leaflet basis ever y 2 wk as the average number of motile T. urticae counted on 10 midtie r leaflets sampled from each plot. Mite-days were calculated from thes e counts and summed for monthly intervals by using 15 March and 30 Aug ust as starting and ending dates, respectively. These calculations pro vide a standard index of T. urticae feeding pressure on strawberry pla nts among and within years. Yield loss resulting from T. urticae feedi ng 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. M aximum T. urticae abundance was negatively associated with yield, and greater mite-day accumulations were associated with yield decline. Reg ression analysis indicated that the 0% yield reduction threshold avera ged 30 cumulative mite days during an), monthly period. This implies t hat detectible yield reduction caused by mite feeding occurs at popula tion densities >1 T. urticae per leaflet. However, late winter and ear ly 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 relative ly low population levels throughout spring and damage resulting from m ite feeding plateaus during summer.