BEMISIA-ARGENTIFOLII (HOMOPTERA, ALEYRODIDAE) ACTION THRESHOLDS AND COTTON PHOTOSYNTHESIS

Action thresholds for whiteflies, Bemisia argentifolii Bellows & Perri ng on cotton, Gossypium hirsutum L., are being developed to determine the whitefly densities at which cotton should be treated with insectic ides to prevent lint yield losses. However, the effects action thresho ld. treatments have on cotton photosynthesis have not been determined. Changes in the photosynthetic rate of cotton call be used to assess d amage to cotton early in tile season before lint is harvested. In this study photosynthetic rates and stomatal conductance of cotton treated at 4 different action thresholds for whiteflies were measured 12 time s during July and August 1995 in the Imperial Valley of California usi ng a portable photosynthesis system. Action thresholds were 2.5, 5, 10 , and 20 adults per leaf. Untreated control plots were also used for c omparison in a 5 x 5 Latin square design. Adult, egg, and nymphal whit efly densities on untreated cotton were significantly higher than thos e on all other treatments. In general, no differences in densities of any stage were seen among the 2.5, 5, and 10 treatments. Densities fro m the 20 treatment were variable. Photosynthetic rates of untreated co tton leaves were usually significantly lower compared with those of 2. 5, 5, and 10 treatments. In general, no significant differences in pho tosynthetic rates were seen among the 2.5, 5, and 10 treatments. Photo synthetic rates of leaves from the 20 treatment were significantly low er and higher than leaves from the other treatments and the untreated control, respectively, on some dates. In general, no significant diffe rences were detected in stomatal conductance among tile 2.5, 5, and 10 treatments, and. results were similar to those seen with photosynthes is. Based on photosynthetic rates and stomatal conductance, measurable physiological damage to cotton leaves occurs between action threshold s of 10-20 adult whiteflies per leaf.