Evolution of pest resistance to al proteins produced by Baciuw thw'mgwensis (Bt) would dera our ability to control agricltural pests with genetically engineered crops designe to express genes coding for these proteins. One of the few microbes that has been used successfully in agricultural insect pest control is Bacillus thuringiensis (Bt). Liquid and powder formulations of this bacterium hold a small but growing share of the pest-control-agent market (1). Genes from Bt that code for production ofa set ofinsecticidal proteins are being cloned and transferred to a number of crop plants (2)(3)(4). Expression ofBt genes in crop plants is appealing because the toxicity of the proteins coded for by these genes is restricted to specific groups of insects (5) (9)(10)(11).Excitement over the potential of engineered plants with Bt genes has been tempered by laboratory and field work which indicates that pest insects have the capacity to adapt to Bt and its toxic proteins (34). However, studies of insect resistance to Bt toxins have found that resistance is toxin-specific.Indian meal moths that were >100-fold resistant to the Bt toxin CryIA(b) were not at all resistant to CryIC (12). Diamondback moths that were >200-fold resistant to Cry-IA(b) were not resistant to CryIB or CryIC (13). A strain of Heliothis virescens selected with CryIA(b) and a mixture of CryIA and CryIIA proteins in an HD-1 strain of Bt was significantly resistant to only some strains of Bt (14). Restriction of resistance to a single or highly related group of toxins may be explained by studies indicating that the biochemical basis for resistance includes changes in receptors in the insect midgut (12,13,15).While high levels of resistance are cause for concern, the specificity of resistance has led to a belief that as insects become resistant to one Bt toxin, that toxin could be successfully replaced by a different Bt toxin (16). Additionally, genetic analyses of Bt resistance have demonstrated that the resistance is usually inherited as a mostly recessive trait (14,15,(17)(18)(19)(20). The rate at which such recessive traits become established in a population can be significantly decreased by the proper use ofBt-toxin-producing plants, so these genetic results have prompted the development of specific "resistance management" strategies (21,22).We report here on Bt-toxin resistance in a strain of H. virescens, a pest of cotton, soybean, tobacco, tomato, and other agricultural crops. The resistance in our strain of H. virescens is not toxin-specific, does not appear to be related to changes in midgut receptors, and is not inherited as a recessive trait when larvae are exposed to high doses of Bt toxin. EXPERIMENTAL PROCEDURESDesign of Selection Experiment. As part of a larger study on insect resistance to Bt toxins, a strain of H. virescens was selected for survival on an artificial diet containing CryIA(c). The selected strain and the control strain were initiated from a sample of a field population collected in July 1988. Precautions, which are...

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Bacillus thuringiensis strains isolated from Latin American soil samples that showed toxicity against three Spodoptera frugiperda populations from different geographical areas (Mexico, Colombia, and Brazil) were characterized on the basis of their insecticidal activity, crystal morphology, sodium dodecyl sulfate-polyacrylamide gel electrophoresis of parasporal crystals, plasmid profiles, and cry gene content. We found that the different S. frugiperda populations display different susceptibilities to the selected B. thuringiensis strains and also to pure preparations of Cry1B, Cry1C, and Cry1D toxins. Binding assays performed with pure toxin demonstrated that the differences in the toxin binding capacities of these insect populations correlated with the observed differences in susceptibility to the three Cry toxins analyzed. Finally, the genetic variability of the three insect populations was analyzed by random amplification of polymorphic DNA-PCR, which showed significant genetic diversity among the three S. frugiperda populations analyzed. The data presented here show that the genetic variability of S. frugiperda populations should be carefully considered in the development of insect pest control strategies, including the deployment of genetically modified maize in different geographical regions.Fall armyworm, Spodoptera frugiperda (J. E. Smith), is a worldwide pest of economic importance for different crops. This species has a highly polyphagous feeding behavior, which includes the consumption of different cultivated plants, such as maize (Zea mays L.), cotton (Gossypium hirsutum L.), and rice (Oryza sativa L.). To date, the most common method for controlling this pest relies on the use of synthetic insecticides such as methomyl, carbaryl, and cypermethrin (10), in spite of the damage they cause to the environment and to nontarget organisms. Two distinct strains of S. frugiperda, one associated with maize and the other with rice, have been already identified in the United States (17, 19). The detection and characterization of genetic diversity among insect populations is a critical issue for the improvement of pest management strategies, since the evolution of resistance to insecticides among insect populations is dependent on the frequency of resistant alleles, the inheritance of resistance, the relative fitness cost, and the gene flow.Bioinsecticides are viable alternatives for insect control in agriculture, and among them, Bacillus thuringiensis is the most widely used. B. thuringiensis is compatible with sustainable and environmentally friendly agricultural practices. This bacterium produces insecticidal proteins (Cry protoxins) during sporulation as parasporal crystals, which are highly specific to their target insects; safe for humans, other vertebrates, and plants; and biodegradable (12). Moreover, recombinant DNA technology using cry genes has developed insect-resistant transgenic plants that are used extensively for cotton, corn, and rice production, among others (23).Information regarding the suscept...

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