Effect of Three Adjuvants on Coverage, Persistence, and Efficacy of Ground-Applied Chlorpyrifos for Suppression of Cotton Aphid (Homoptera: Aphididae)

Abstract

Effects of adjuvants on chlorpyrifos for suppression of cotton aphid, Aphis gossypii (Glover), in cotton, Gossypium hirsutum (L.), were evaluated using four parameters: percentage cotton aphid mortality, spray droplet coverage (percentage area covered) on water-sensitive cards, spray droplet penetration (average spot diameter frequency, distributions) into the cotton canopy, and the rate of disappearance (decay) of chlorpyrifos residues from cotton leaf surfaces. All factors were measured on cotton averaging 171.4 cm in height at three canopy levels (from terminal): upper, 0 cm; middle, 45.7 cm; lower, 76.2 cm. Mortality, of aphids in the upper canopy level (89%) was higher than middle (66%) and lower (57%) levels. Spray coverage, measured as mean percentage area of water-sensitive cards covered by spots, decreased at each lower canopy level as did mean total number of spots per square centimeter. Chlorpyrifos alone generally had a higher proportion of spots per square centimeter in the middle and lower canopy levels relative to the upper level, compared with the adjuvant combinations. No differences in spot diameter frequency distributions were detected among treatments within a canopy level or within a treatment among canopy levels. Initial mean chlorpyrifos deposits on cotton leaf surfaces were highest at the upper canopy level (678 ng/cm2) and decreased by approximately 50% at each descending canopy level. Rate of disappearance of chlorpyrifos from cotton leaf surfaces was significantly reduced at the middle and lower canopy levels compared with the upper level. Adjuvants did not significantly affect chlorpyrifos efficacy, deposition characteristics, or rate of disappearance of chlorpyrifos. Canopy level was important for all three measured criteria.