Frequency of Bt resistance alleles in Helicoverpa armigera in 2007-2009 in the Henan cotton growing region of China

► Resistance allele frequency in H. armigera to Cry1Ac was assessed. ► Resistance alleles are still rare in Xinxiang of Northern China. ► Resistance gene frequency did not increase significantly from 2007 to 2009. ► No substantial increase in Cry1Ac tolerance was found over the 3-yr period.