We compared responses to six insecticidal crystal proteins fromBacillus thuringiensisby a Cry1A-resistant strain (NO-QA) and a susceptible strain (LAB-P) of the diamondback moth,Plutella xylostella. The resistant strain showed >100-fold cross-resistance to Cry1J and to H04, a hybrid with domains I and II of Cry1Ab and domain III of Cry1C. Cross-resistance was sixfold to Cry1Bb and threefold to Cry1D. The potency of Cry1I did notdiffersignificantlybetweentheresistantandsusceptiblestrains.Cry2Bdidnotkillresistantorsusceptible larvae. By combining these new data with previously published results, we classified responses to 14 insecti- cidal crystal proteins by strains NO-QA and LAB-P. NO-QA showed high levels of resistance to Cry1Aa, Cry1Ab, and Cry1Ac and high levels of cross-resistance to Cry1F, Cry1J, and H04. Cross-resistance was low or nil to Cry1Ba, Cry1Bb, Cry1C, Cry1D, Cry1I, and Cry2A. Cry1E and Cry2B showed little or no toxicity to susceptible or resistant larvae. In dendrograms based on levels of amino acid sequence similarity among proteins, Cry1F and Cry1J clustered together with Cry1A proteins for domain II, but not for domain I or III. High levels of cross-resistance to Cry1Ab-Cry1C hybrid H04 show that although Cry1C is toxic to NO-QA, domain III of Cry1C is not sufficient to restore toxicity when it is combined with domains I and II of Cry1Ab. Thus, diamondback moth strain NO-QA cross-resistance extends beyond the Cry1A family of proteins to at least two other families that exhibit high levels of amino sequence similarity with Cry1A in domain II (Cry1F andCry1J)andtoaproteinthatisidenticaltoCry1AbindomainII(H04).Theresultsofthisstudyimplythat resistance to Cry1A alters interactions between the insect and domain II.

Insecticidal crystal proteins from Bacillus thuringiensis (Bt) offer safe, specific, and effective insect control. Evolution of resistance by pests is the greatest threat to the long-term effectiveness of Bt. The first insect to evolve resistance to Bt in open field populations is the diamondback moth (Plutella xylostella), a global pest of vegetables. Resistance to Bt has been found in some populations of diamondback moth from Hawaii, the continental United States, and several Asian nations. Resistance to Bt in diamondback moth is inherited as an autosomal, recessive trait and is associated with reduced binding of toxin to sites on the larval midgut membrane. Resistance to CryIA toxins from Bt subsp. kurstaki caused cross-resistance to CryIF, but not to CryIB or CryIC. In laboratory populations containing susceptible and resistant individuals, fitness costs associated with resistance promoted reversal of resistance in the absence of exposure to Bt. In an isofemale line derived from an extensively selected strain, >5,000-fold resistance persisted for >20 generations without selection, which shows that at least one resistant genotype is not inherently unstable. Field-evolved resistance to Bt in diamondback moth has provided a warning and an opportunity to gain knowledge essential for sustainable use of Bt.

Neither toxins from Berliner nor conspecific eggs deterred oviposition by the diamondback moth, (L.) (Lepidoptera: Plutellidae), in laboratory choice tests. The finding that toxins did not deter oviposition by moths from a susceptible line shows that oviposition preference and larval survival were not associated in this line. Selection for larval resistance to toxins did not significantly alter oviposition preference, which rules out a strong genetic correlation between larval performance and oviposition preference. Failure of conspecific eggs to deter oviposition may not represent lack of association of preference and performance because other evidence suggests that larval performance may not be greatly affected by larval density. These results suggest that the ability of refuges to slow evolution of physiological resistance to toxins will not be magnified by an oviposition preference for untreated foliage.

Neither toxins fromBacillus thuringiensis Berliner nor conspecific eggs deterred oviposition by the diamondback moth,Plutella xylostella (L.) (Lepidoptera: Plutellidae), in laboratory choice tests. The finding that toxins did not deter oviposition by moths from a susceptible line shows that oviposition preference and larval survival were not associated in this line. Selection for larval resistance to toxins did not significantly alter oviposition preference, which rules out a strong genetic correlation between larval performance and oviposition preference. Failure of conspecific eggs to deter oviposition may not represent lack of association of preference and performance because other evidence suggests that larval performance may not be greatly affected by larval density. These results suggest that the ability of refuges to slow evolution of physiological resistance toB. thuringiensis toxins will not be magnified by an oviposition preference for untreated foliage.