The microbial insecticide Bacillus thuringiensis (Bt) has become the mainstay of non-chemical control of Lepidopteran pests, either as sprays or through the incorporation of Bt toxins into transgenic crops. Given the wide use of Bt, it is striking that currently only one pest species, Plutella xylostella, has been reported to have developed significant resistance to Bt outside the laboratory. By contrast, we report here the frequent and rapid development of resistance to B. thuringiensis kurstaki (Dipel, Abbott) in populations of cabbage loopers, Trichoplusia ni, in commercial greenhouses. Resistance to Bt appears to be costly and there is a rapid decline of resistance in populations collected from greenhouses and maintained in the laboratory without selection. Management of pests resistant to Bt in vegetable greenhouses will require sporadic use of Bt-based sprays or alternatively use of sprays containing other Bt toxins.

The cabbage looper, Trichoplusia ni, is one of only two insect species that have evolved resistance to Bacillus thuringiensis in agricultural situations. The trait of resistance to B. thuringiensis toxin Cry1Ac from a greenhouseevolved resistant population of T. ni was introgressed into a highly inbred susceptible laboratory strain. The resulting introgression strain, GLEN-Cry1Ac-BCS, and its nearly isogenic susceptible strain were subjected to comparative genetic and biochemical studies to determine the mechanism of resistance. Results showed that midgut proteases, hemolymph melanization activity, and midgut esterase were not altered in the GLEN-Cry1Ac-BCS strain. The pattern of cross-resistance of the GLEN-Cry1Ac-BCS strain to 11 B. thuringiensis Cry toxins showed a correlation of the resistance with the Cry1Ab/Cry1Ac binding site in T. ni. This cross-resistance pattern is different from that found in a previously reported laboratory-selected Cry1Ab-resistant T. ni strain, evidently indicating that the greenhouse-evolved resistance involves a mechanism different from the laboratory-selected resistance. Determination of specific binding of B. thuringiensis toxins Cry1Ab and Cry1Ac to the midgut brush border membranes confirmed the loss of midgut binding to Cry1Ab and Cry1Ac in the resistant larvae. The loss of midgut binding to Cry1Ab/Cry1Ac is inherited as a recessive trait, which is consistent with the recessive inheritance of Cry1Ab/Cry1Ac resistance in this greenhouse-derived T. ni population. Therefore, it is concluded that the mechanism for the greenhouse-evolved Cry1Ac resistance in T. ni is an alteration affecting the binding of Cry1Ab and Cry1Ac to the Cry1Ab/Cry1Ac binding site in the midgut.

Selection for resistance to insecticides, diseases and parasitoids is assumed to be costly and often requires tradeoffs with reproductive fitness. The costs of resistance, however, are often difficult to measure. Cabbage looper, Trichoplusia ni, a generalist Lepidopteran herbivore, has become highly resistant following the extensive use of the microbial insecticide, Bacillus thuringiensis kurstaki (Bt) in vegetable greenhouses. We compared the growth rate, pupal size and survival of resistant, susceptible and hybrid T. ni larvae fed on tomato, bell pepper and cucumber. Performance was best on cucumber and worst on pepper, and the magnitude of fitness costs associated with Bt resistance increased with declining host plant suitability. This supports the hypothesis that in this system, resistance costs are condition dependent and are greatest in the most stressful environment. Management strategies that rely on the presence of fitness costs to reduce the frequency of resistance genes must consider this variation and should be more successful on crops that are less suitable food plants. In general, condition dependence should be considered in studies designed to measure the costs of resistance.

No abstract

A population of cabbage looper, Trichoplusia ni (Hübner), collected from commercial greenhouses in the lower mainland of British Columbia, Canada, in 2001 showed a resistance level of 24-fold to Dipel, a product of Bacillus thuringiensis (Bt) subspecies kurstaki. This population was selected with Cry1Ac, the major Bt Cry toxin in Dipel, to obtain a homogenous population resistant to Cry1Ac. The resulting strain of T. ni, named GLEN-Cry1Ac, was highly resistant to Cry1Ac with a resistance ratio of approximately 1000-fold. The larvae from the GLEN-Cry1Ac strain could survive on Cry1Ac-expressing transgenic broccoli plants that were highly insecticidal to T. ni and diamondback moth, Plutella xylostella (L.). The inheritance of Cry1Ac resistance in this T. ni strain was autosomal and incompletely recessive. The degree of dominance of the resistance was -0.402 and -0.395, respectively, for the neonates in reciprocal crosses between the GLEN-Cry1Ac and a laboratory strain of T. ni. Using chi2 goodness-of-fit test, we demonstrated that the inhibition of larval growth resulting from testing 12 toxin doses in the progeny of the backcross fit the predicted larval responses based on a monogenic inheritance model. Therefore, we conclude that the inheritance of the resistance to Cry1Ac in the T. ni larvae is monogenic.