Ecotypic variation of the Asian corn borer, Ostrinia furnacalis (Guenee), populations in the Philippines

Six corn varieties (three hybrids, three OP's) were evaluated for differential responses to the Asian corn borer, Ostrinia furnacalis (Guenee). This three-year project involved several studies on the ecotypic variation of the Asian corn borer, O. furnacalis, in the Philippines. Study 1 evaluated the differential responses of ACB to six varieties of corn conducted at UPLB and in other regional sites in Isabela, Camarines Sur, Leyte, Bukidnon and South Cotabato. Study 2 consisted of studies to determine the possible variation of the local population of the ACB collected from the six regional sites through life history, morphology, cytogenetics and isozyme analyses. The objective of the project is to determine the occurrence of variable populations or different strain of the ACB in the Philippines. Specifically for study 1, the objectives are to determine the field infestation cycle by the ACB in different corn growing areas and evaluate and quantify damage level of the different ACB population on the different test corn varieties. For Study 2, the objectives are to determine the variability of local ACB population using life history studies, cytogenetics and isozyme analysis. Results of the studies revealed the following: 1. The infestation cycle of the ACB start as early as 30 DAP with eggmasses already oviposited on the leaves and lasted until 60 DAP. Larval infestation was observed to start at 20 DAP and peaked at 60 DAP. Starting at 50 DAP, an overlapping presence of eggmasses and larvae in the field was observed. Based on the above data, there is only one infestation cycle of the Asian corn borer per growing season in all study sites. 2. ACB populations was observed highest during the wet season and the highest eggmass density was recorded between 45 and 50 DAP. The ACB populations from Laguna, Bukidnon and Camarines Sur preferred to oviposit on the hybrids while the Leyte population preferred the open pollinated varieties. Preference was also a mixture of either hybrids or OP's on ACB populations from South Cotabato and also from other local populations like Camarines Sur and Laguna. 3. ACB larval and pupal populations were highest during the wet season, peaking at 60 DAP and declining at 80 DAP. Results showed consistent trend where hybrids were least preferred and the OP's were highly preferred for feeding. This is demonstrated in almost all the project sites in Camarines Sur, Isabela, Leyte, Bukidnon and South Cotabato except Laguna for the hybrid C818 which had more borer holes but had the least tunnel length showing tolerance to the ACB. 4. Statistical analysis of the yield data revealed high significance of the mean yield of six varieties of corn planted in three seasons across sites or locations. The two yellow corn hybrids, C818 and NK8840, were superior among the test varieties having a yield advantage of more 250% computed from the lowest yielder Philippine supersweet. Across site, the highest yield was obtained by NK8840 in Isabela with yield as high as 9546 kg/ha. Life history revealed significant difference in larval duration, pupal duration and total duration among the ACB local populations when reared in artificial diet. ACB from Laguna and Leyte underwent six larval instars while the rest underwent five larval instars. This result may suggest that there could be mixed populations of the ACB. 6. Life history using ACB from Bukidnon revealed significant difference in larval duration, pupal duration and total duration when reared in fresh corn plants. It also confirmed that the ACB from Bukidnon underwent six larval instars which contradict the earlier results where ACB only underwent five larval instars when grown under artificial diet. 7. The meiotic indices were highest on Leyte and lowest on Bukidnon. Based on the behavior of the meiotic chromosomes, ACB possesses small and holokinetic chromosome. There are some discrepancies in the haploid numbers observed among the populations of O. furnacalis from Bukidnon, Leyte and South Cotabato which could imply a mild speciation process occurring among populations as well as within population of ACB in the Philippines. The Leyte population showed three modal chromosome numbers. 8. Differences in esterase banding patterns were observed by the presence of both low and fast moving bands in the first zone of activity. For acid phosphatase, the two samples from Camarines Sur and Bukidnon also showed a fast moving band and a slow moving band and 9. Genetic variation within and among the three ACB population revealed that Isabela population of O. furnacalis was most diverse while the Laguna population was the least diverse. Camarines Sur and Laguna populations were the most distant ones and the most related populations were Camarines Sur and Isabela. The Laguna population, on the other hand, which was found distantly related from Isabela and Camarines Sur based on genetic identity, formed a distinct cluster on the dendogram.