ABSTRACT
A field strain of the tobacco budworm, Heliothis virescens, was selected for eleven successive generations by topical application with technical spinosad (a mixture of spinosyns A and D), the active ingredient in the insecticide Tracer®. The first indication of resistance was noted in the sixth generation. By the ninth generation a maximum dose of 0.8 ug of spinosad per third instar produced no mortality at 12 d after application, and in the eleventh generation, 60 ug per larva produced less than 40% mortality. Compared to the LD50 of the parental strain, the selected budworms in the eleventh generation demonstrated a resistance ratio >355-fold. The selected insects are also resistant when fed spinosad formulated as Tracer. The topical LD50's for three tobacco budworm field strains collected from NC, LA and MS in 1998 were similar to the unselected parental strain and to LD50's previously reported for field strains (Leonard et al., 1996).
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