F₂ screening for rare resistance alleles of asian corn borer, ostrinia furnacalis guenee (Lepidoptera: Crambidae) in the Philippines

Ostrinia furnacalis Guenee commonly known as Asian Corn Borer (ACB) is the major pest of corn in the Philippines. This study attempted to determine the frequency of Cry1Ab resistance alleles in ACB that will serve as quantitative baseline data for monitoring ACB resistance in Bt corn in the Philippines. The initial frequency of Cry1Ab resistance alleles in ACB collected from corn farms in Lubao, Pampanga, Philippines was studied using the F₂ screen method from March 2006 to December 2008. A total of 2,606 females of ACB were used to establish 800 isofemale lines. From these isofemale lines, 37,702 larvae from the F2 screen were susceptible to the toxin, Cry1Ab protein. The mortality rate for all the isofemale lines peaked during the first seven days of exposure to Cry1Ab. Two hundred fifty three (253) isofemale lines or 10% were observed to be partially positive to the toxin wherein four (4) isofemale lines (15, 19, 116, and 121) have survivors and successfully reached adult stage. However, due to the low survival of ACB larvae of each isolines, unsynchronized adult emergence and high percentage of male moth emergence, the re-testing of resistance to the isofemale lines were unsuccessful. Furthermore, an equal number of ACB larvae (37,702) were used for the control. A total of one thousand twenty seven (1,027) or 2.88% mortality was observed. This small percentage mortality from the control can be attributed to mechanical injury from handling. The calculated frequency of resistance alleles (E [pR]) was 3.12×10^-4. This shows that the resistance allele is rare.