Abstract Larval mortality and growth inhibition of a Cry1Ab-resistant and a Cry1Ab–susceptible strains of the sugarcane borer, Diatraea saccharalis (F.) (Lepidoptera: Crambidae) were evaluated on a meridic diet containing trypsin-activated Cry1Aa or Cry1Ac toxin in laboratory bioassays. Neonates of sugarcane borer were fed on the meridic diet incorporating Cry1Aa or Cry1Ac toxin at a concentration range of 0.015625 to 64 ug/ml. Larval mortality, body weight and number of survival larvae that did not gain significant weight (<0.1 mg per larva) were recorded on the 7th day after inoculation. Using a measure of practical mortality (larvae either died or no significant weight gain), the median lethal concentration (LC₅₀) of the-resistant strain was 80 and >45 folds greater than that of the susceptible larvae to the Cry1Aa and Cry1Ac toxins, respectively. Larval growth inhibition of both the resistant and susceptible strains were observed on Cry1Aa or Cry1Ac treated diet, but the inhibition of the resistant strain was significantly less than that of the susceptible insects. The results suggested that the Cry1Ab-resistant sugarcane borer strain is also resistant to both Cry1Aa and Cry1Ac toxins.

Key Words Diatraea saccharalis, Bacillus thuringiensis toxicity, Bt resistance management