Managing Resistance to Bacillus thuringiensis: Lessons from the Diamondback Moth (Lepidoptera: Plutellidae

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Abstract
Laboratory selection increased resistance to B. thuringiensis in three strains established from a moderately resistant field population of diamondback moth, Plutella xylostella (L.). Five generations of laboratory selection caused 5- to 7-fold increases in LC50, resulting in 150- to 190-fold resistance compared with a susceptible laboratory colony. Nine generations of selection produced 430- to 820-fold resistance. In contrast, five foliar applications of B. thuringiensis in the field did not increase the LC50 of a moderately resistant population. Field-selected resistance to B. thuringiensis declined slowly in the absence of treatments. The rapid response to laboratory selection shows intrapopulation genetic variation in susceptibility to B. thuringiensis and suggests that intense selection may produce much higher levels of resistance to B. thuringiensis than those previously reported from the field. We hypothesize that resistance increased faster in the laboratory than in the field because selection intensity was lower in the field. Because susceptibility was not restored quickly when treatments were discontinued, rotations may not be especially effective for managing resistance to B. thuringiensis in diamondback moth. Field populations of diamondback moth developed resistance to a commercial formulation containing a mixture of B. thuringiensis toxins, an event that raises doubts about the ability of mixtures to retard resistance development. Extensive and intensive exposure of pests to B. thuringiensis toxins through transgenic crop plants or other tactics may cause widespread pest resistance. We urge judicious use of B. thuringiensis to conserve its efficacy.

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