Frequency of Alleles Conferring Resistance to the Bacillus thuringiensis Toxins Cry1Ac and Cry2Ab in Australian Populations of Helicoverpa punctigera (Lepidoptera: Noctuidae) From 2002 to 2006

Helicoverpa punctigera and Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) are important pests of field and horticultural crops in Australia. The former is endemic to the continent, whereas the latter is also distributed in Africa and Asia, Although H. armigera rapidly developed resistance to virtually every group of insecticide used against it, there is only one report of resistance to an insecticide in H. punctigera. In 1996 the Australian cotton industry adopted Ingard, which expresses the Bacillus thuringiensis (Bt) toxin gene cry1Ac. In 2004/2005, Bollgard II (which expresses Cry1Ac and Cry2Ab) replaced Ingard and has subsequently been grown on 80% of the area planted to cotton, Gossypium hirsutum L. From 2002/2003 to 2006/2007, F-2 screens were used to detect resistance to Cry1Ac or Cry2Ab. We detected no alleles conferring resistance to Cry1Ac; the frequency was <0.0005 (n = 2,180 alleles), with a 95% credibility interval between 0 and 0.0014. However, during the same period, we detected alleles that confer resistance to Cry2Ab at a frequency of 0.0018 (n = 2,192 alleles), with a 95% credibility interval between 0.0005 and 0.0040. For both toxins, the experiment-wise detection probability was 94%, i.e., if there actually was a resistance allele in any tested lines, we would have detected it 94% of the time. The first isolation of Cry2Ab resistance in H. punctigera was before the widespread deployment of Bollgard II. This finding supports our published notion for H. armigera that alleles conferring resistance to Cry2Ab may be present at detectable frequencies in populations before selection by transgenic crops.