Gas Exchange of Orange (Citrus sinensis) Leaves in Response to Feeding Injury by the Citrus Red Mite (Acari: Tetranychidae)

Historically, treatment thresholds for the citrus red mite, Panonychus citri McGregor, on citrus were designed to minimize the extent of visible injury to mite-damaged leaves because there was no quantitative information relating mite damage to reductions in leaf physiological function or fruit yield and quality. To obtain better information on the impact of feeding by the citrus red mite on leaf function, individual trees in a 13-yr-old commercial orange grove in the San Joaquin Valley, Calif., were differentially treated with acaricides to create a range of mite densities during the spring of 1986. Gas exchange of selected trees was measured at 28-d intervals during the summer. Peak mite densities on untreated trees were more than 7-fold greater than the conventional treatment thresholds. Stomatal conductance was always independent of mite densities, however, and photosynthesis was independent of mite densities on three of the four sampling dates. Data suggest that the conventional “visible injury” threshold may be conservative in preserving the physiological capacity of leaves injured by the citrus red mite.