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Title: Using an F2 screen to monitor frequency of resistance alleles to Bt cotton in field populations of Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae)

item XU, ZHIPING - Nanjing Agricultural University
item LIU, FENGYI - Nanjing Agricultural University
item CHEN, JIN - Nanjing Agricultural University
item HUANG, FANGNENG - Louisana State University
item ANDOW, DAVID - University Of Minnesota
item WANG, YANHUA - Nanjing Agricultural University
item Zhu, Yu Cheng
item SHEN, JINLIANG - Nanjing Agricultural University

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/5/2008
Publication Date: 1/22/2009
Citation: Xu, Z., Liu, F., Chen, J., Huang, F., Andow, D., Wang, Y., Zhu, Y., Shen, J. 2009. Using an F2 screen to monitor frequency of resistance alleles to Bt cotton in field populations of Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae). Pest Management Science. 65(4):391-397.

Interpretive Summary: F2 screening involves collection of a large number of gravid females to establish isofemale lines. F1 adults from each isoline are allowed to sib-mate to produce F2 progenies for screening resistant genotypes. Theoretically, if a field-collected mated female carries a resistance gene allele, 6.25 percent of its F2 progenies should be homozygous for Bt resistance and able to survive on Bt plant. Through back-calculation of the frequency of resistance-allele carrying family lines, the frequency of the resistance allele in the sampled population can be estimated. Because of its high sensitivity for detecting rare resistance alleles, the F2 screen has been used to estimate the frequency of Bt resistance alleles in several insect species targeted by Bt crops. Three-year (2003-2005) F2 screenings, conducted in this study, indicated that Bt-cotton resistance gene allele frequency in field populations of the cotton bollworm in Northern China increased to 0.0146, which was 2.5-fold higher than the level of 0.0058 detected in 1999. Fast Bt resistance in the insect may attribute to long history of using Bt sprays since 1991 and widely adoption of single-toxin-producing Bt cotton variety since 1998 without conventional cotton allocations in recent years for maintaining the susceptibility or for delaying resistance development in the target insect.

Technical Abstract: Transgenic Bacillus thuringiensis (Bt) cotton producing Cry1Ac protein has been widely planted in northern China. The rapid and extensive commercialization of Bt cotton could increase the risk of resistance development in target insects. In Qiuxian County (Hebei, China), Bt cotton has been commercially planted since 1998. Previous study using F2 screen in 1999 found that the frequency of resistance alleles to Bt cotton in field populations of Helicoverpa armigera was 0.0058. Considering the high selection pressures in this region, we conducted another F2 screen to monitor the Bt resistance in the field populations of H. armigera from 2003 to 2005 as Bt cotton acreage continued to increase over the years. The results from the F2 screen showed that 15 out of 278 isofemale lines collected from cotton fields during the 3 years were identified to carry resistance alleles to Bt cotton. The resistance allele frequency in field populations of H. armigera ranged from 0.0119 to 0.0297 with an overall frequency of 0.0146 and a 95% confidence interval of 0.0084-0.0225 for the 3-year period. The F2 screen had an overall detection power of 95%. Our results suggest that significant shift in resistance allele frequency had occurred in the field populations of H. armigera in Qiuxian County. To prevent further increase in Bt resistance allele frequency in H. armigera in the region, it is necessary to introduce Bt cotton that expressing multi-Bt toxins and integrate the Bt cotton technology with biological, chemical, and cultural practices for management of this key cotton pest.