Larval Age Affects Resistance to Bacillus thuringiensis in Diamondback Moth (Lepidoptera: Plutellidae)

We evaluated responses of neonates and 3rd instars from susceptible and resistant colonies of diamondback moth, Plutella xylostella (L.), to leaf residues of formulated Bacillus thuringiensis Berliner. In both susceptible and resistant colonies, neonates were significantly less susceptible than 3rd instars. This difference may be caused by the leaf-mining habit of neonates, greater physiological tolerance of neonates to B. thuringiensis, or both. Stronger expression of resistance in 3rd instars than in neonates was indicated by a significant colony X instar interaction in mortality and by higher resistmlce ratios for 3rd instars than for neonates. For both neonates and 3rd instars, mortality between 2 and 5 days after treatment was significantly lower for resistant larvae than for susceptible larvae. Susceptible and resistant neonates had similar feeding behavior.