Host finding and parasitism efficiency of obliquebanded leafroller egg masses by Trichogramma minutum after inundative releases in apple orchards

A local strain of Trichogramma minutum Riley (Hymenoptera: Trichogrammatidae) was mass produced and released weekly in 0.25-ha plots of four commercial apple orchards to evaluate the level of parasitism in sentinel egg masses of the obliquebanded leafroller, Choristoneura rosaceana Harris (Lepidoptera: Tortricidae). Sentinel eggs were attached to foliage, either on release trees or at mid-distance between two release trees, and were renewed every 3-4 days. Other sentinel eggs were placed in adjacent plots (approx. 20 m from the release plots). The incidence of parasitism and number of eggs parasitized within each mass were evaluated. In dwarf tree orchards, parasitism was lower on sentinel egg masses placed at mid-distance between two release trees than on release trees; however, parasitism was more uniform in semi-dwarf tree orchards. The detection of parasitoid activity in the adjacent plots, even upwind, showed that under repetitive releases at a high release rate, many parasitoids travelled or were transported more than 20 m from their release site. The occurrence of parasitism and the number of parasitized eggs per mass were higher in release plots than in adjacent plots, but partial parasitism of host egg masses generally occurred, even in close proximity to release sites. In addition, even if host finding efficiency was higher, parasitized egg masses in release plots were not more heavily parasitized than those in adjacent plots. These results suggest that interference between females or detection of previously parasitized hosts may prevent additional oviposition in the same egg mass by conspecifics, and that the level of this response is less pronounced for females that travelled further from their emerging sites before encountering a host.