Bemisia argentifolii (Homoptera:Aleyrodidae) Action Thresholds and Cotton Photosynthesis

Action thresholds for whiteflies, Bemisia argentifolii Bellows & Perring on cotton, Gossypium hirsutum L., are being developed to determine the whitefly densities at which cotton should be treated with insecticides to prevent lint yield losses. However, the effects action threshold treatments have on cotton photosynthesis have not been determined. Changes in the photosynthetic rate of cotton can be used to assess damage to cotton early in the 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 times during July and August 1995 in the Imperial Valley of California using a portable photosynthesis system. Action thresholds were 2.5, 5, 10, and 20 adults per leaf. Untreated control plots were also used for comparison in a 5 × 5 Latin square design. Adult, egg, and nymphal whitefly densities on untreated cotton were significantly higher than those on all other treatments. In general, no differences in densities of any stage were seen among the 2.5, 5, and 10 treatments. Densities from the 20 treatment were variable. Photosynthetic rates of untreated cotton leaves were usually significantly lower compared with those of 2.5, 5, and 10 treatments. In general, no significant differences in photosynthetic rates were seen among the 2.5, 5, and 10 treatments. Photosynthetic rates of leaves from the 20 treatment were significantly lower and higher than leaves from the other treatments and the untreated control, respectively, on some dates. In general, no significant differences were detected in stomatal conductance among the 2.5, 5, and 10 treatments, and results were similar to those seen with photosynthesis. Based on photosynthetic rates and stomatal conductance, measurable physiological damage to cotton leaves occurs between action thresholds of 10–20 adult whiteflies per leaf.