This project examined how and when individual Helicoverpa armigera carrying a resistant allele (BX) were favoured in field-grown cotton, particularly on Ingard®. BX-like and other forms of resistance to Cry1Ac were found to be rare in field populations of H. armigera. It follows that individuals that carry will almost exclusively be heterozygous. We measured the survival and growth of heterozygous larvae and homozygous susceptible larvae on leaf samples from field-grown Ingard® and conventional varieties of cotton. By performing assays throughout the season, we were able to identify occasions that favoured heterozygotes and thus the evolution of resistance. Averaging relative survival rates of heterozygotes and homozygous susceptible genotypes over the 2002 - 03 and 2003 - 04 seasons we found that, over the first season, susceptible individuals would have survived and grown only half as frequently as the heterozygote. In the second season susceptible individuals fared slightly better surviving 0.68 as well as the heterozygote. Computer models were prepared that incorporated this information as well as new information on the frequency of Cry1Ac resistance in the field.

A second aim of the project was to detect and isolate alleles that conferred resistance to Cry1Ac and Cry2Ab in field populations of Helicoverpa armigera and H. punctigera. An F2 screen technique was employed that enabled the calculation of the frequency of alleles that confer resistance, and importantly, enabled the detection of recessive alleles. No instances of resistance were detected among 68 alleles scored for Cry1Ac and Cry2Ab in H. punctigera. Similarly, no instances of resistance to Cry1Ac were detected among 416 alleles scored for H. armigera. For this species, 95% confidence intervals around the zero determine that Cry1Ac resistance alleles would not be more common than a frequency of 0.0088. Thus despite the deployment of Ingard® for seven seasons, Cry1Ac resistance remains rare.

However, a surprising result was that resistance to Cry2Ab, the second toxin present in Bollgard II®, is relatively common. For H. armigera, three instances of resistance to Cry2Ab were isolated from 416 alleles examined. Preliminary analyses suggest that all three isolates represent mutations at the same locus. While the F2 technique was expected to identify ‘resistant alleles’ if a sufficient number were tested, the expectation was that prior to the deployment of Bollgard II®, Cry2Ab resistance would be at a frequency approaching the mutation rate, say 10 -5 to 10 -8.

With the imminent wide-scale deployment of Bollgard II®, it was important to assess the magnitude of the threat posed to Bollgard II® by the previously unsuspected resistance to Cry2Ab. Unlike the situation for Cry1Ac resistance, where resistant alleles are rare, for the more common Cry2Ab resistance it was important to examine the fitness of both heterozygotes and homozygotes. Laboratory analyses performed in CSE108C have shown that Cry2Ab resistant insects are fully susceptible to Cry1Ac. Thus it is not surprising that early in the season all larvae struggled on Bollgard II®. However later in the season when titres of Cry1Ac presumably had declined, homozygous Cry2Ab resistant larvae (but perhaps not heterozygous larvae) exhibited enhanced growth on Bollgard II® relative to susceptible insects. Thus although the resistance appears to be functionally recessive on Bollgard II®, opportunities exist for the homozygote to be favoured. Together these results suggest that this form of resistance may inpact on the longevity of Bollgard II®.