Delaying the development of resistance to Bt corn in the population of Asian corn borer, Ostrinia furnacalis (Guenee)

This paper reports on the determination of the baseline and discriminating concentrations of Cry 1Ab toxin for the neonates of Asian corn borer (ACB) and its use in the monitoring of the development of resistance of ACB populations to Bt corn. While sourcing for Cry 1Ab protein, ACB larvae reared in the laboratory for several generations were tested on lyophilized Bt corn leaf, exposed on a detached Bt corn leaf in the laboratory and on the Bt-corn potted plants in the screen house. The use of powdered lyophilized Bt corn leaves, although relatively more laborious, may offer an alternative method in the estimation of LC values in the event that purified Cry 1Ab protein is unavailable. The Cry 1Ab content in the powdered lyophilized Bt-corn leaves was estimated to be 1.25 mg/mg leaves by ELISA. Purified Cry 1Ab protein was obtained from Dr. Willian Moar, Professor, Auburn University, Alabama, USA. Susceptibility to Cry 1Ab toxin was determined on the neonates of six-field-collected ACB populations from non-Bt corn growing areas. ACB neonates were exposed to artificial diet treated with increasing Cry 1Ab concentrations then mortality and growth inhibition were evaluated after seven days. All populations of ACB neonates showed comparable susceptibility against Cry 1Ab toxin except Tranca population. No significant differences in the percent weight reduction were found on Batangas, Tranca, Calauan and CES [Central Experiment Station] UPLB population while Calamba and Biotech UPLB populations were comparable. Among the six ACB populations tested, the highest baseline concentration (LC sub50) of 0.356 mg/sq cm was obtained from Tranca population while the lowest of 0.216 mg/sq cm was obtained from Calauan and Calamba populations. The highest discriminating concentration (LC sub99) values of 41.33 ng/sq cm was also obtained from Tranca population while the lowest, 6.191 ng/sq cm were obtained from Batangas and Biotech, UPLB populations. The lowest discriminating dose was used in monitoring resistance of ACB populations collected from Bt corn growing areas. Based on resistance ratio, all populations collected from three sites in Pangasinan are still susceptible to Cry 1Ab toxin. The LT sub50 of the third instar ACB larvae fed with the whole Bt corn leaf is 85.08 hours. However, when the larvae were allowed to imbibe the extract of the lower, middle and tip portions of Bt corn leaf the LT50 are 80.35, 63.42 and 23.12 hours, respectively. This suggests that the tip portion of Bt corn leaf had the highest Cry 1Ab content. ACB larvae can survive in potted Bt corn plants than in a detached Bt corn leaf placed inside an acrylic pan because they can settle in the inner leaf sheath and in the stalk where Bt content is probably negligible. Pollen and decaying Bt corn leaves were not detrimental to the earwigs after 15 days of feeding. Among the six ACB populations tested, the third instar ACB larvae from Sto. Tomas, Calauan, Laguna [Philippines] was the most susceptible to Cry 1Ab protein. It has an LDsub25, LDsub50 and LDsub99 of 297.10, 597.82 and 6,660.20 ng, respectively, followed by Tranca, and Calamba [Laguna, Philippines] populations that had almost the same level of susceptibility. The order of their decreasing susceptibility to Bt toxin is as follows: Sto. Tomas, Calauan = Tranca, Bay = Bungo, Calamba = San Felix, Sto. Tomas, Batangas = Biotech, UPLB CES, Pili Drive UPLB. Based on probit analysis and on the fiducial limits of the LDsub25, LDsub50, all populations except that from Pili, CES, UPLB were susceptible to Cry 1Ab protein. The nymphs and adults of the predatory earwig, Euborellia annulipes prefer to prey only early instar ACB than the late instar larvae. The earwig whether 3rd of 4th instar nymphs or adults (male and female) had almost the same predatory consumption of about 145 neonates in 24 hrs. No differences in the earwigs survival, fecundity and lifecycle was observed on those earwig fed with Cry 1Ab-treated and -untreated ACB larvae. The earwig population in the field was monitored by trapping from pre-vegetative to tassel stage. More earwigs were collected during pre-vegetative stage and decreases at whorl and tassel stage. However, earwig population after harvest was not determined because the corn plants was washed out by the 2 meters high flood caused by typhoon 'Milenyo'. ACB egg masses and scrapped leaves by the neonates were observed on the Bt corn (Mon 818) in the field but no larva were collected. ACbB infestation was high on supersweet than in IPB 911 variety. The other dominant natural enemies observed in the field were the Tachinid sp. and Sarcophadidae parasites, lady beetle, Cheilomenes seymaculata. There is indeed a need to monitor the development of ACB resistance to Bt corn in the field at least once a year since results of the project suggested that there is an ACB population which showed some level of resistance to Bt corn.