A model is derived for calculating the percent surface damage to Valencia oranges by populations ofPhyllocoptruta oleivora(Ashmead), taking account of a seasonally-varying damage rate. Accumulated area under mite population curves (mite-days) was linearly related to accumulated damage on fruit surfaces. Linear correlations for all times of the growing season were very high (0.927–0.996). The slope of the regression of mite-days on cumulative damage increased gradually over the season from 0.001%/mite/day in spring to 0.01%/mite/day in winter. Attempts to include temperature effects by putting mites on a degree-day time scale did not produce a more constant damage rate.