Large-scale cultivation of Bt cotton places high selection pressure on target insects and, consequently, may prompt resistance evolution in pest populations. To better monitor Bt-resistance allele frequency in field populations, a modification of the F1 screen was developed to screen F1 progeny derived from single pair mating between field-collected males and laboratory resistant females (designated as F1 screen). This method was used to survey a field population of Heliocoverpa armigera (Hübner) for resistance alleles at the same loci in the resistant strain. After treatment of the F1 progeny derived from more than 260 single pair mating lines with Bt cotton, there was no clear separation of resistant genotypes from susceptible genotypes based on survival rates which demonstrated a linear distribution within a certain range. We further analyzed larval growth data of the F1 progeny and found a correlation between larval body weight and survival rate. The maximum correlation was obtained when F1 larval body weight reached ≥0.7 mg. To avoid underestimation, body weight ≥0.6 mg was used to initially separate potential positive lines (resistant genotype). After that, a corrected survival rate of >21.3% (the minimal theoretical rate of >25% for F0 males to carry sr and rr genotypes) was used as the criterion to examine F2 progeny derived from single sib-mating of F1 adults and to verify whether the potential positive lines carried resistant alleles.

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Author notes

2Yu Cheng Zhu and Fengyi Liu share senior authorship.

4Department of Pesticide Science, College of Plant Protection, Nanjing Agricultural University, Nanjing 210095, China.