Mechanism of resistance alleviation in Helicoverpa armigera (Lepidoptera: Noctuidae) to pyrethroid caused by Bacillus thuringiensis pretreatment

The mechanism of resistance alleviation in cotton bollworm Helicoverpa armigera (Hubner) to pyrethroid after Bacillus thuringiensis (Berliner) treatment was measured by bioassay and biochemical methods. The results showed that the sensitivities to fenvalerate in 2nd instars of I susceptible (YZ-S) and 2 resistant (HN-R and DE-R) strains were all increased when the larvae were treated by B. thuringiensis for 24 h. The LD50 values to fenvalerate were 12.9-times in the YZ-S, 34.4-times in the HN-R, and 67.5-times in DE-R lower when the strains were exposed to B. thuringiensis than when not exposed to B. thuringiensis. Biochemical analyses indicated that in the 2 resistant strains, the acetylcholinesterase (AChE) kinetics and half inhibition concentrations of inhibitors to AChE were lower in larvae pretreated with B. thuringiensis than those of untreated larvae, whereas higher values of the bimolecular rate constant were detected in larvae pretreated with B. thuringiensis. Inhibition by specific inhibitors of AChE in larvae pretreatment with B. thuringiensis was more rapid compared with those without B. thuringiensis pretreatment. Variations in quantity and quality of the isozyme profiles between the larvae with and without B, thuringiensis treatment by polyacrylamide gel electrophoresis (PAGE) test also were found. A change in structure or affinity of AChE may be 1 factor causing the increase in sensitivity of H. armigera to fenvalerate after B. thuringiensis treatment.