Assessing chorion permeability of healthy and parasitized (by Telenomus podisi, Hymenoptera: Scelionidae) brown stink bug (Euschistus servus, Heteroptera: Pentatomidae) eggs to insecticides in three classes

Preliminary bioassays and field investigations of the efficacy of insecticides against healthy eggs and parasitoids developing in eggs of the brown stink bug indicated that there was almost no mortality to healthy eggs exposed to acephate, lambda-cyhalothrin, thiamethoxam, or spinosad. However, there was almost 100% mortality to the developing parasitoids. These higher rates of parasitoid mortality could be attributed to either increased sensitivity to insecticides, or increased exposure due to differences in chorion permeability. Scanning electron microscopy of parasitized eggs showed that oviposition wounds were sealed by a ‘scab’ but it was not clear whether these wounds allow for increased insecticide movement. To further investigate possible differences in chorion permeability, healthy and parasitized eggs (N=128-160) were immersed in solutions containing each of the 14C-ammended (0.32 µCi of radiolabel) insecticides at field application rates for 0, 30, 120 or 240 minutes. After exposure, eggs were rinsed 3 times with water, homogenized, and then centrifuged. Rinsates and supernatants were analyzed for radiolabeled content which was used to compare differences in insecticide movement into eggs. Using ANOVA, we found that insecticide movement into the egg increased significantly with immersion time for both acephate (P=0.0005) and lambda-cyhalothrin (P=0.0084), but there were no significant differences between healthy and parasitized eggs. Neither immersion time nor egg health was significant for thiamethoxam. When immersion time, egg health, and insecticides were analyzed together, a model (P=0.0012) was constructed that predicts amount of insecticide entering the egg at any given time.