Ação de inimigos naturais em ovos de Helicoverpa zea (Boddie, 1850) (Lepidoptera: Noctuidae) não parasitados e parasitados por Trichogramma pretiosum Riley, 1879 (Hymenoptera: Trichogrammatidae)

Abstract

The influence of three natural enemies in the parasitism of the corn earworm Helicoverpa zea by Trichogramma pretiosum was investigated both under laboratory and field conditions inEMBRAPA's Research Center for Corn and Sorghum in Sete Lagoas, and in field conditions in Curvelo, State of Minas Gerais, Brazil. In Sete Lagoas, at the EMBRAPAs field plots only D. luteipes was found in the field due to a dry period, which is unfavorable to both H zea and T pretiosum. In Curvelo a 7.6 % parasitism ofH zea by Trichogramma sp. was found, with an average of1.6 parasitoids per egg; no predators were collected. During the preliminary tests in the laboratory, parasitised eggs ofA. kuehniella by T pretiosum were not predated by both D. luteipes and C. externa, which may be an indication thatparasitism may not be affected bythese predators under field conditions. In the main experiment in the temperatures of20 and 25 °C 89.6 % ofeggs were predated by C. externa (Treatment #3), while only 66.7 % parasitism was found when the eggs were Xl exposed to T pretiosum alone. When this species was introduced along with Telenomus sp. (Treatment # 2) only T pretiosum parisitised the eggs in a rate of63.2 %. Treatment # 4 (D. luteipes) showed an average predation of56.7 %. At 15 and 30 °C the experiments which contained C. externa and D. luteipes did not dififer statistically, with an average predation of 98.9 and 84.6 % respectively. On the other hand, treatments with T pretiosum alone or T pretiosum + Telenomus sp. also did not dififer statistically although the percent of eggs parasitised was 36.4 and 28.2 %, respectively. Eggs ofH. zea parasitised by T. pretiosum were not predated in ali temperatures studied. In the tests with predators, eggs not parasitised by T. pretiosum were predated, showing a complementary action in the control ofH zea as compared to the one obtained by T. pretiosum alone. The temperatures did not afifect the predatory capacity ofthe species studied but parasitism by T pretiosum was significantly reduced at 15 °C. This indicate that predation by C. externa and D. luteipes may be more efifective than parasitism by T pretiosum in áreas ofcorn wherethe climate is moremild.