Host utilization and fitness of the larval parasitoid Tetrastichus planipennisi are influenced by emerald ash borer’s food plants: Implications for biological control

Abstract

Emerald ash borer (Agrilus planipennis) is one of the most destructive, invasive forest pests in North America and is responsible for the death of hundreds of millions of ash trees (Fraxinus spp.) in its introduced host range. Recently, this invasive beetle was also discovered attacking the white fringetree (Chionanthus virginicus) in North America. In response to emerald ash borer’s lack of evolutionary history with this host plant, we hypothesize that this host range expansion will result in enemy free space from the introduced larval parasitoid Tetrastichus planipennisi. Here we conducted both No Choice and Choice assays to evaluate the parasitism response of T. planipennisi to the emerald ash borer larvae reared or inserted in white fringetree sticks vs sticks of the favored host plant, green ash (F. pennsylvanica). Additionally, we observed the host-finding behavior of T. planipennisi when given a choice with the two host plants infested with emerald ash borer larvae. The No Choice assay demonstrates that T. planipennisi is able to parasitize and successfully develop on emerald ash borer larvae reared in white fringetree sticks. Choice assays further show that T. planipennisi is capable of parasitizing suitable stages of emerald ash borer larvae inserted in white fringetree even with the presence of emerald ash borer infested green ash in the same assay arena. However, rates of the host larval parasitism by T. planipennisi were significantly lower in white fringetree sticks than in green ash sticks and T. planipennisi spent less time on white fringetree sticks than green ash. These findings suggest that emerald ash borer may experience partial enemy free space through the utilization of white fringetree as an alternative host to ash, and thus biological control of emerald ash borer in white fringetrees with T. planipennisi may be less effective than in ash trees.