The effect of bark thickness on host partitioning between Tetrastichus planipennisi (Hymen: Eulophidae) and Atanycolus spp. (Hymen: Braconidae), two parasitoids of emerald ash borer (Coleop: Buprestidae)

Parasitoids have recently been introduced from Asia to aid in biological control in the United States of the invasive, highly damaging, emerald ash borer, Agrilus planipennis. Three introduced parasitoids have established and field biological studies are underway to improve our understanding of niche partitioning among them. Here we report one such investigation, a field experiment conducted to determine how outer bark thickness of ash trees might affect parasitism by one introduced (Tetrastichus planipennisi) and one native parasitoid (Atanycolus spp.). We found that T. planipennisi was unable to parasitize EAB larvae in trees with outer bark thicker than 3.2 mm (>11.2-cm DBH) whereas Atanycolus spp. parasitized EAB larvae in ash trees with outer bark up to 8.8 mm thick (>57.4-cm DBH). These results suggest that establishment of, and control by T. planipennisi at release sites with only large diameter trees is less likely, and that T. planipennisi will be more effective in stands with younger trees (<12-cm DBH). Releasing T. planipennisi near the leading edge of EAB invasion may have little impact on EAB populations if many ash trees are too large. We recommend releasing T. planipennisi in stands dominated by small, early successional or regenerating ash trees. This may maximize the establishment and effectiveness of this species. This limitation of T. planipennisi for biological control of emerald ash borer suggests that other EAB parasitoids from its native range with longer ovipositors, such as Spathius galinae (Hymenoptera: Braconidae), should be sought and evaluated for possible use as EAB biocontrol agents in the US. The results of this study also suggest the importance of parasitoid guild introduction for biological control in general, and hint at possible broader implications relating to resource partitioning among native and introduced parasitoids.