Phenology of blueberry gall wasp and the effect of insecticides on infestation and biological control

The blueberry stem gall wasp (Hemadas nubilipennis) is a pest of blueberry, with the larvae stimulating gall formation on new shoots. These galls can reduce crop productivity, yet the activity of adults around the time of bloom makes control of these hymenopteran pests very challenging. Over two growing seasons, the phenology of this insect was recorded and related to degree days revealing consistent timing of wasp emergence. Laboratory and single bush comparisons of treatments for prevention of gall formation indicated that addition of oil to registered insecticides can provide heightened levels of control. Field trials over two seasons revealed reduced gall wasp infestation on bushes treated in fall and spring, but these treatments also had complete loss of the natural enemies Ormyrus vacciniicola and Eurytoma solenozopheriae. Our findings highlight the challenges inherent in chemical approaches to control of this pest and emphasize the importance of resistant varieties and selective pruning for reducing blueberry gall wasp infestation.