Comparison of leaf-dip and diet bioassays for monitoring Bacillus thuringiensis resistance in field populations of diamondback moth (Lepidoptera: Plutellidae)

Seven Plutella xylostella (L.) populations were compared for resistance to Bacillus thuringiensis subsp. kurstaki Berliner in leaf-dip and diet-incorporated bioassays. The persistence of the microbial insecticide incorporated in diet was studied at 2 storage temperatures (26.7 and 5 degrees C). The effect of larval age on toxicity of a discriminating concentration of Javelin incorporated in diet (20.5 mg [Al]/liter) was also studied. in the leaf-dip bioassay, LC(50)S of 6 field populations were 2.6- to approximate to 829-fold higher than the LC(50) of Geneva 88, the susceptible population, whereas LC(50)s in the diet-incorporated bioassay-were 1.7- to approximate to 42-fold higher in the field populations compared with the LC(50) Of Geneva 88. Despite the difference in resistance ratios between bioassay methods, LC(50)S were significantly correlated. Both methods were effective in distinguishing between susceptible and resistant populations. With 1 exception, slopes of the regressions in individual populations were not significantly different. We concluded that both bioassay techniques were suitable for monitoring evolution of resistance to B. thuringiensis in field populations of P. xylostella. Compared with diet prepared the day of the tests, the toxicity of B. thuringiensis did not vary significantly in diet stored for 10 d at 26.7 degrees C, and the LC(50)s of Javelin did not vary significantly in diet stored for 14 d at 5 degrees C. Mortality of 2nd and 3rd inslars was not significantly different when exposed to a discriminating concentration of Javelin incorporated in diet, but mortality was significantly lower in 4th instars. The diet-incorporated B. thuringiensis assay can be used as a resistance test kit for resistance studies.