Cotton aphid response to irrigation and crop chemicals
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The development of cotton aphid, Aphis gossypii Glover, populations in relation to irrigation management and application of crop chemicals was investigated during 1997-1999. Three water management treatments included no irrigation (dryland), irrigation termination by mid-August, and irrigation termination in late August. The four chemical treatments in 1997 and 1998 were the 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 aphicide thiamethoxam, while the other three treatments were the same as those in 1997 and 1998. The 1997 growing season had relatively mild temperatures and high rainfall, while temperatures were high with little rainfall received 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 cyhalothrin combined with late irrigation in August resulted in very high aphid population levels during September. Pymetrozine effectively reduced aphid numbers following the cyhalothrin treatment in dryland and irrigated, early termination treatments, but pymetrozine was less effective in irrigated, late termination treatments. Ethephon did not affect aphid numbers in 1997, but in 1998 the ethephon treatment resulted in higher aphid numbers in relation to the untreated check. In 1999, thiamethoxam suppressed aphid populations in all three irrigation treatments, relative to numbers in the untreated check, for 11 days post-treatment. In relation to yields in dryland plots, lint yields were significantly higher in irrigated plots in 1998 and 1999, but not in 1997. The high aphid populations in September, resulting from application of cyhalothrin, did not affect yield.
