Responses of Liriomyza trifolii (Diptera: Agromyzidae) to chemical and biorational insecticides

One of the most troublesome pests of snap beans is the American serpentine leafminer, Liriomyza trifolii (Burgess) (Diptera: Agromyzidae). Growers commonly use abamectin, spinosad, and cyromazine to manage L. trifolii populations; however, the biological insecticide azadirachtin and the fungus Isaria fumosorosea Wize offer promising alternatives. We tested the effectiveness of these five insecticides for controlling L. trifolii under field conditions in southern Florida. Abamectin and spinosad were generally the most effective for reducing L. trifolii mines, larvae, and/or pupae. Cyromazine and azadirachtin were less effective than abamectin or spinosad, although better than I. fumosorosea or the untreated control. Isaria fumosorosea was the least promising treatment; however, it still performed better than the control in reducing L. trifolii mines, larvae, and/or pupae. As pesticide effectiveness increased, differences in numbers of mines, larvae, and pupae appeared to be reduced among the 5 dates that pesticides were sprayed. Overall, mines appeared to be more effectively controlled 1 to 2 d after treatment than after 7 d, whereas larvae and pupae were controlled equally throughout the period. The pesticides employed can be classified into 3 general groups based on modes of action: abamectin and spinosad ( disrupters of insect neural and muscular systems), cyromazine and azadirachtin (disrupters of molting), and I. fumosorosea ( invades the insect, produces a toxin, and halts feeding). Pesticides with at least two modes of action were each able to provide effective control. Alternating pesticides may therefore control L. trifolii while limiting the development of resistance in L. trifolii populations.