Cotton aphid (Homoptera : Aphididae) response to irrigation and crop chemicals

The development of cotton aphid, Aphis gossypii Glover, populations in rela tion to irrigation management and application of crop chemicals was investi gated during 1997-1999. Three water management treatments included no irrig ation (dryland), irrigation termination by mid-August, and irrigation termi nation in late August. The four chemical treatments in 1997 and 1998 were t he untreated check. application of cyhalothrin which is known to stimulate aphid population increase, cyhalothrin followed by the aphicide pymetrozine . and the harvest-aid ethephon. In 1999 the ethephon treatment was replaced with an application of cyhalothrin followed by an application of the aphic ide thiamethoxam, while the other three treatments were the same as those i n 1997 and 1998. The 1997 growing season had relatively mild temperatures a nd high rainfall. while temperatures were high with little rainfall receive d in 1998 and 1999. Two population peaks, one in mid-August and a second in mid-September, occurred in 1997. but there was only one population peak in early September, 1998 and one in late September, 1999. An application of c yhalothrin combined with late irrigation in August resulted in very high ap hid population levels during September. Pymetrozine effectively reduced aph id numbers following the cyhalothrin treatment in dryland and irrigated, ea rly termination treatments, but pymetrozine was less effective in irrigated . late termination treatments. Ethephon did not affect aphid numbers in 199 7, but in 1998 the ethephon treatment resulted in higher aphid numbers in r elation to the untreated check. In 1999, thiamethoxam suppressed aphid popu lations in all three irrigation treatments. relative to numbers in the untr eated check, for 11 days post-treatment. In relation to yields in dryland p lots, lint yields were significantly higher in irrigated plots in 1998 and 1999. but not in 1997. The high aphid populations in September, resulting f rom application of cyhalothrin, did not affect yield.