Field Applications, Leaf-Dip, and Diet Incorporated Diagnostic Assays Used Against Bacillus thuringiensis-Susceptible and Resistant Diamondback Moth (Lepidoptera: Plutellidae)

Abstract

The concentration-mortality responses of 6-8 populations of Plutella xylostella (L.) from leaf-dip bioassays were compared with the mortality of larvae caused by residues of field applications of Bacillus thuringiensis subsp. kurstaki Berliner (Javelin Wettable Granules 6.4% [AI]), and mortality of larvae in 3 diagnostic concentrations of Javelin (48.5, 32, and 20.5 mg [AI]/liter of diet) incorporated in an artificial diet. LC50s of Javelin in leaf-dip assays ranged from 0.07-78.8 mg (Al)/liter of water. Populations with LC50S of 0.07-0.15 mg (AI)/liter showed mortalities usually >80% in field applications at the recommended field rate of 1.12 kg/ha, and >90% at 2.24 kg/ha. Four populations with LC50s of 0.6-1.8 mg (AI)/liter showed mortalities <60% when exposed to 1.12 kg/ha, but >75% when exposed to 2.24 kg/ha. In 3 other populations with LC50s of 4.8-78.8 mg (AI)/liter mortality at 1.12 kg/ha was <7%, and <32% when exposed to 2.24 kg/ha. Mortalities of diamondback moth populations at the diagnostic concentrations of 32 and 20.5 mg (AI)/liter of diet were significantly correlated with mortalities at 1.12 kg/ha, but correlations between 1.12 kg/ha and the 20.5 mg (AI) concentration were higher. We concluded that LC50s of Javelin >0.6 mg (AI)/liter in leaf dip bioassays can be associated with low levels of mortality in field applications, and that the diagnostic concentration of 20.5 mg can be used as an on-farm assay for monitoring P. xylostella resistance to B. thuringiensis subsp. kurstaki.