Predation on egg parasitoids Trichogramma chilonis: Effect of timing and placement of irradiated host eggs on effectiveness of augmentative releases in sugarcane fields

The egg parasitoid Trichogramma chilonis Ishii (Hymenoptera: Trichogrammatidae) is used worldwide to manage lepidopterous pests. Predators such as ants and spiders can hamper the parasitism achieved by released bio-control agents, hence reducing the efficacy of augmentative biological control. The present study was conducted to assess the best timing and placement position of "Tricho" cards (containing 25 Gy irradiated Sitotrogacerealella (Olivier) (Lepidoptera: Gelechiidae) host eggs with mature pupae of T. chilonis) in sugarcane field to minimize predation by ants and others. The cards were placed fortnightly at three different positions, i.e. top, middle and bottom in the sugarcane crop canopy and the experiment was repeated at morning, afternoon and evening times. The experiment continued throughout the 2010 sugarcane growing season, recording the percent parasitism by T. chilonis in fresh S. cerealella irradiated eggs glued on paper cards having approximately 100 eggs per card at ten cards per acre and placed in all plots for 24 hrs. on sugarcane leaves on the day after release. Results show the presence of predators throughout the sugarcane growing season, but maximum damage to parasitoid pupae on cards was observed during August-September. Although release time had no significant impact on predation, there was a significant difference in terms of the location of parasitoid release: least damage to parasitoids was observed when the parasitoids were placed at the top position within the plant canopy (morning, 3.69%, afternoon, 2.16% and evening, 2.71%) followed by the middle position (morning, 6.22%, afternoon, 5.96% and evening, 5.47%). Maximum predation occurred when the parasitoids were placed at the bottom position (morning, 14.49%, afternoon, 14.31% and evening, 14.27%). Microhabitat therefore can play an important role in the percent parasitism achieved by released parasitoids. The lowest infestation by the borers in sugarcane fields and the highest percent parasitism averages on the Tricho cards were observed when parasitoids were placed in the middle canopy and when this was done in the evenings.