Frequency of Alleles Conferring Resistance to the Bt Toxins Cry1Ac and Cry2Ab in Australian Populations of <I>Helicoverpa armigera</I> (Lepidoptera: Noctuidae)

Abstract

Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) is an important lepidopteran pest of cotton (Gossypium spp.) in Australia and the Old World. From 2002, F2 screens were used to examine the frequency of resistance alleles in Australian populations of H. armigera to Bacillus thuringiensis (Bt) CrylAc and Cry2Ab, the two insecticidal proteins present in the transgenic cotton Bollgard II. At that time, Ingard (expressing Cry1Ac) cotton had been grown in Australia for seven seasons, and Bollgard II was about to be commercially released. The principal objective of our study was to determine whether sustained exposure caused an elevated frequency of alleles conferring resistance to Cry1Ac in a species with a track record of evolving resistance to conventional insecticides. No major alleles conferring resistance to Cry1Ac were found. The frequency of resistance alleles for Cry1Ac was <0.0003, with a 95% credibility interval between 0 and 0.0009. In contrast, alleles conferring resistance to Cry2Ab were found at a frequency of 0.0033 (0.0017, 0.0055). The first isolation of this allele was found before the widespread deployment of Bollgard II. For both toxins the experiment-wise detection probability was 94.4%. Our results suggest that alleles conferring resistance to Cry1Ac are rare and that a relatively high baseline frequency of alleles conferring resistance to Cry2Ab existed before the introduction of Bt cotton containing this toxin.

Resistance Bioassays and Allele Characterization Inform Analysis of Spodoptera frugiperda (Lepidoptera: Noctuidae) Introduction Pathways in Asia and Australia

“…The surface overlay of Bt toxin assays followed the approach outlined in Mahon et al (2007b , 2008 , 2010 , 2012 ) and Walsh et al (2014) . They were conducted in 96-well trays where each well contained approximately 300 µl of rearing diet with a surface area of around 0.567 cm 2 per well.…”

Frequency of Cry1Ac and Cry2Ab resistance alleles in pink bollworm, Pectinophora gossypiella Saunders from Andhra Pradesh, India

“…Two different diets were prepared with Cry1Ac toxin and Cry2Ab toxin for screening. Discriminating dose of Cry1Ac and Cry2Ab (Fabrick et al, 2014(Fabrick et al, , 2015Mohan et al, 2016;Reynolds, 2006;Tabashnik et al, 2005Tabashnik et al, , 2006Tabashnik et al, , 2012Tabashnik et al, , 2013 were used to screen the F 2 population for resistance alleles Blanco et al, 2008;Kukanur et al, 2018;Mahon et al, 2007). An artificial diet was prepared and allowed to cool.…”

“…One of these assumptions requires Bt resistance frequencies in target insect populations to be rare (e.g., < 0.001) . Several studies have been conducted across the globe that measured the frequency of resistance alleles to Cry toxins in target pests Huang et al, 2007;Mahon et al, 2007). Estimating the frequency of resistance alleles to Cry toxins expressed in GM crops in target pests has helped to detect, measure, and monitor the evolution of resistance in field populations.…”