POTENTIAL TO DEVELOP TOLERANCE TO DELTAENDOTOXINS OF Bacillus thuringiensis subsp. Kurstaki IN Plutella xylostella (Lepidoptera: Plutellidae)

Abstract

This study demonstrates the potential of Plutella xylostella L. to develop tolerance to entomotoxins from Bacillus thuringiensis Subs. kurstaki (Btk.) One susceptible population and one subjected to selection pressure were studied. The first was reared for eight generations without insecticide treatment; for the ninth generation a medium lethal concentration was estimated at 65. Selection of the second population began with Btk treatment. Four continuous generations of the resistant population were exposed to the LC65 concentration estimated for the susceptible population of P. xylostella. The entomotoxin used in the experiment was a 6.4 % formula of Btk (Javelin® WG, Novartis México). In the bioassays, 90 mm diameter discs of cabbage (Brassica oleracea L.) leaves were cut, submerged for 10 seconds in different concentrations of entomotoxins, and placed in Petri dishes where they were infested with third instar larvae, ten larvae per concentration with three replications. The percentage of mortality was quantified 48 hours later. Data analysis was performed using the Probit model. With linear regression in the PcProbit software LC50, LC65, and LC95 were estimated, and relative resistance was calculated. The results indicate that the percentage of mortality of P. xylostella decreases more than 50 % after being subjected to selection pressure and the tolerance factor obtained was approximately 14 times greater.