Postrelease dispersal of Trichogramma brassicae Bezdenko in corn fields

During the years 1992-93 a study on the dispersal of Trichogramma blassicae Bezd., an egg parasitoid of the European corn borer, Ostrinia nubilalis (Hbn.), was carried out in north-eastern Italy. In 1992 an Italian strain of trichogrammatid wasp and in 1993 a French strain were used. During August in two different plots of corn, about 4 000 and 8 000 T. brassicae were released three times at one point per plot. Four checks of the O. nubilalis egg masses on corn plants were made weekly in August; six distances from the point of release, ranging from 0.75 to 10.6 m, were considered. The rate of parasitization was higher in 1993 when T. brassicae at different developmental stages (larvae and pupae) were released. The percentage of O. nubilalis egg masses parasitized by T. brassicae decreased with the distance from the point of release. In both years, wind seemed to have an important effect on the dispersal of the parasitoid. The dispersal of T. brassicae was lower in the upwind sectors whereas in the downwind sectors increasing the distance from the point of release no significant decrease of the rates of parasitization was observed; the performances of the Italian and the French strains of trichogramma appeared to be the same. The results obtained in this experiment indicate that there should be a high number of release points per hectare to reduce the effects of both distance and wind, and parasitoids at different developmental stages should be released to increase the rate of parasitization. The number of parasitoids released do not seem to effect the rate of parasitization.