Insect species diversity and population abundance of selected natural enemies in Bt corn and non-Bt corn agroecosystems

Insect species diversity and population abundance of the predatory beetles and lacewings and the parasitoid Trichogramma sp. on Yieldguard (Bt corn) were assessed in two sites in Luzon: Villaluna in Cauayan, Isabela and Panagan in Tigaon, Camarines Sur [Philippines]. The study had four treatments comparing the transgenic Yieldguard C-818 (Bt) corn hybrid with its isohybrid (ISO818), treated or untreated with insecticides, and a reference hybrid, P30D44. Species richness was measured using the Shannon index. It was found that indices among treatments were not significantly different for all the samples from the two sites, except Tigaon site at the silk stage of the crop which registered lower insect species diversity. The low diversity index was attributed to the high population density of the corn silk beetle, Monolepta bifasciata (Burm.). Yieldguard showed lesser infestation of the beetles on average compared with 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 also not significantly different for all treatments at the two sites. Likewise, the Asian corn borer (ACB) feeding damage on YG C818 was very low and for the isohybrid moderately low. It appeared that the very low ACB populations on YG C818 did not have detrimental effect on community structure and species diversity. On average, higher population of the predatory beetle (Micraspis sp.) and brown lacewing (Pseudomicromus sp.) were recorded for Yieldguard compared to its isohybrid and the commercial hybrid. Based on corn crop phemology, natural enemies were generally significantly higher at tasseling, silking and maturity. Although our results showed higher densities of some important natural enemies of insect pest on Bt-corn, long-term studies are necessary to further characterized subtle population changes in the field