Comparison of imidacloprid and thiamethoxam for control of the silverleaf whitefly, Bemisia argentifolii and the leafminer, Liriomyza trifolii, on tomato

The silverleaf whitefly, Bemisia argentfolii Bellows & Perring, also known as strain B of the sweetpotato whitefly [B. tabaci (Genn.)] is the dominant pest of tomatoes, Lycopersicon esculentum Mill, in southern Florida. The insect causes damage directly by inducing an irregular ripening disorder of tomato and indirectly by transmitting plant viruses, particularly tomato yellow leaf curl virus (TYLCF). The leafminer, Liriomyza trifolii (Burgess), has long been considered a serious pest of tomatoes in Florida, causing defoliation through mining of leaves by larvae. Imidacloprid and thiamethoxam are members of a new class of systemic, nicotinoid insecticides that can be applied either as soil drenches or as foliar sprays. Two experiments were conducted in the fall seasons of 2000 and 2001 and 10 experiments were conducted during the spring of 2002 to compare the efficacy of imidacloprid and thiamethoxam. When imidacloprid 2F (16 oz/acre) or thiamethoxam 2SC (8 oz/acre) were applied to the soil at or within 3 weeks of transplanting, both provided control of whitefly nymphs for eight to 12 weeks at nine sites on sandy soil but only 3 to 5 weeks at one site on gravelly loam soil. Control with imidacloprid appeared to be slightly greater and to persist slightly longer at three sites, although differences were not significant when the 2002 data were averaged over all experiments. Foliar applications of imidacloprid 1.6F at 3.75 oz/acre and thiamethoxam 25WG at 4 oz/acre at a threshold of five nymphs/10 leaflets also provided significant control of nymphs, although control was not as good as with soil applications, and control appeared to be greater with thiamethoxam. The incidences of plants with symptoms of TYLCV were reduced with soil applications but not foliar applications. Imidacloprid appeared to provide greater and more consistent reductions in the occurrence of TYLCV infected plants at three sites as well as when the data were averaged over six sites in 2002, although differences from thiamethoxam were small and not significant. Soil applications of either insecticide resulted in significant reductions of L. tritolii leafmines, although control was consistently better with thiamethoxam.