ABSTRACT     Environmentally benign insecticides derived from the soil bacterium Bacillus thuringiensis are the most widely used biopesticides, but their success will be short-lived if pests quickly adapt to them. The risk of evolution of resistance by pests has increased, because transgenic crops producing insecticidal proteins from Bt are being grown commercially. Efforts to delay resistance with two or more Bt toxins assume that independent mutations are required to counter each toxin. Moreover, it generally is assumed that resistance alleles are rare in susceptible populations. We tested these assumptions by conducting single-pair crosses with diamondback moth (Plutella xylostella), the first insect known to have evolved resistance to Bt in open field popualtions. An autosomal recessive gene conferred extremely high resistance to four Bt toxins (Cry1Aa, Cry1Ab, Cry1Ac, and Cry1F). The finding that 21% of the individuals from a susceptible strain were heterozygous for the multiple-toxin resistance gene implies that the resistance allele frequency was 10 times higher than the most widely cited estimates of the upper limits for the initial frequency of resistance alleles in susceptible populations. These findings suggest that pests may evolve resistance to some groups of toxins faster than previously expected.

Insecticidal crystal proteins from the cosmopolitan soil bacterium Bacillus thuringiensis (Bt) are becoming a cornerstone of insect pest management (1). These toxins kill insects by binding to and creating pores in midgut membranes (2). Unlike many conventional insecticides, Bt toxins do not harm people, arthropod natural enemies, or most other nontarget organisms (1)...The risk of resistance has increased, because sprays of Bt are being used more often to control pests of crops and forests as well as insect vectors of human disease (1). Also, 1996 was the first year that transgenic crops producing insecticidal proteins from Bt were grown commercially in the United States...

Extended maintainence of a resistance allele frequency close to 0.10 without exposure to Bt implies that in the absence of Bt, heterozygotes have little or no fitness disadvantage relative to susceptibles. Like the resistance to Bt conferred by the multiple-toxin resistance gene, fitness costs associated with this resistance (20,41) appear to be recessive.

If selection against heterozygotes in the absence of insecticide is weak, initial frequencies of resistance alleles may be much higher than previously thought (38), even if mutation rates are low....

8/19/97