Diversity, community structure and wet season population abundance of insect on Bt-corn agroecosystem in two sites on Luzon Island, Philippines

Insect diversity and wet season population abundance of predatory beetles and lacewings as well as the parasitoid Trichogramma sp. on yieldgard (Bt corn) were assessed for two sites in Luzon: Villaluna, Cauayan, Isabela and Panagan, Tigaon, Camarines Sur [Philippines]. The transgenic yieldgard corn hybrid DK818YG was compared in four treatments with its isohybrid (DK818), treated and untreated with insecticide and a reference hybrid, P30D44. Diversity indices among treatments were not significantly different for all the samples from the two sites except for Tigaon site at silking stage that registered lower insect diversity. The low diversity index for Tigaon at silking stage was due to the high population density of the corn silk beetle, Monolepta bifasciata (Burm.). On the average, yieldgard showed less infestation of beetles than its isohybrid and the commercial hybrid. Different guilds such as phytophagous, neutrals, predators and parasitoids were also not significantly different among treatments for each sampling date and site. Guild structures for the different treatments were not also significantly different among all treatments in the two site. Likewise, feeding damage on the leaves of DK818YG by the Asian corn borer (ACB) was very low while that on the isohybrid was moderate. It appears that very low ACB population on DK818YG does not have detrimental effect on community structure and diversity. On the average, higher number of the predatory beetle, Micraspis discolor (Fabr.) and the brown lacewing, Pseudomicromus igorotus (Banks) were recorded on yieldgard than on its isohybrid and the commercial hybrid. Based on corn crop phenology, natural enemies were generally significantly higher at tasseling, silking and maturity. Although the results showed higher densities of some important natural enemies of insect pests on yieldgard, long-term studies are necessary to further characterize subtle population changes in the field.