Family line study: quantifying genetic variability of ACB [Asian corn borer] susceptibility to Bt traits for the prediction of product durability

The family line method measured the frequency of rare resistance alleles of Asian corn borer by examining the survival rate of the offspring of single-pair mating. This was done to predict the genotypic proportions of each family line of ACB populations from Alicaocao, Banga and General Santos City to the new pyramided Bt corn product. The family line study also determined the larval movement of ACB in 100% non-Bt (GT) and seed blend refuge configurations. In turn, this demonstrated the effect of structured and seed blend refuge in the development of resistance and durability of the stacked Bt corn product. Field studies were conducted through artificial infestation in 30-38 DAP corn plants of F1 generation 'family lines' resulting from assortative mating of single-paired feral population of ACB moths. The surviving artificially infested larvae were recovered through destructive sampling, 20 days after infestation. Larval movement was also assessed by taking the coordinates of the plants where the insects were recovered from the whole plant assay. Overall results from the field and leaf disc assays showed that the three population from GenSan, Banga Alicaocao population were observed to survive on Bt plants. This gradual process among some individuals may be a precursor to tolerating the Bt toxin and more exposure in the future may lead to resistance development to Bt corn. Development of resistance is more likely to occur in Alicaocao as indicated by the significant number of insects recovered and there were also individuals that were collected from 100% Bt plants. The ability to tolerate the toxin during different instar development is likely an initial episode to gradually complete their growth and development until they can fully develop on the Bt corn, Larval movement showed that most of the insects surviving in Bt corn were adjacent to refuge corn thus plant movement from a non-Bt corn to a Bt corn may present a higher chance of tolerance to the Bt toxin in later stages of the ACB development. Results also showed that there were fewer larvae recovered in a seed blend and may not yield as many susceptible individuals as the structured refuge. Thus, seed-blend refuge strategies may exacerbate the development of resistance to Bt corn of the ACB through the introduction of low-dose Bt toxin. For the durability of the Bt product, results showed that if the occurrence of resistant alleles in the field was intermediate between rare and common. resistance could develop from as few as 5 generations of 67 generations. While most simulation models start with q=.0001 (1 resistant allele in 1000 individuals), a higher initial resistant frequency was suspected. From the calculations made, it appeared the resistance frequency was suspected. From the calculations made, it appeared the resistance could develop rapidly once the initial frequency reaches 5% resistant alleles at 10% refuge size.