Effects of insecticides and defoliants applied alone and in combination for control of overwintering boll weevil (Anthonomus grandis; Coleoptera: Curculionidae)--laboratory and field studies

In laboratory, greenhouse and field tests, we determined the effects of combining full rates of the defoliants tribufos and thidiazuron and the herbicide thifensulfuron-methyl with half rates of the insecticides lambda-cyhalothrin or azinphos-methyl, and the combination of tribufos and thidiazuron, both in half rates, on mortality of the boll weevil, Anthonomus grandis grandis Boheman and on the quality of defoliation. Tribufos, 0.47 kg ha(-1) and tribufos, 0.235 kg ha(-1) + thidiazuron, 0.125 kg ha(-1) exhibited a slightly toxic effect to boll weevil, while tribufos, 0.47 kg ha(-1) + lambda-cyhalothrin, 0.019 kg ha(-1), tribufos, 0.47 kg ha(-1) + azinphos-methyl, 0.14 kg ha(-1), and tribufos, 0.235 kg ha(-1) + thidiazuron, 0.125 kg ha(-1) + azinphos-methyl, 0.14 kg ha(-l), provided control of boll weevil as good as or better than full-rate azinphos-methyl or lambda-cyhalothrin alone owing to synergistic effects. Thidiazuron or thifensulfuron-methyl alone or in combination with insecticides did not affect boll weevil mortality. Treatment with tribufos + thidiazuron, both at half rate, significantly increased defoliation compared to full rates of tribufos or thidiazuron alone, and provided adequate defoliation for approximately the same cost per hectare.