Niche partitioning and coexistence of parasitoids of the same feeding guild introduced for biological control of an invasive forest pest.
When multiple species of host-specific natural enemies from the same feeding guild are introduced to areas against a target pest, strong interspecific competition is likely and may compromise biocontrol unless the agents effectively partition available resources. Here, we evaluate if two parasitoids (Spathius galinae and Tetrastichus planipennisi), introduced for biocontrol of the invasive emerald ash borer (EAB), Agrilus planipennis, into North America have established niche-partitioning, co-existing populations following their sequential or simultaneous field releases to 12 hard-wood forests located in Midwest and Northeast regions of the United States. We found the two recently introduced EAB larval parasitoids, S. galinae and T. planipennisi, established niche-partitioning, co-existing populations in all release areas. Presence, abundance, and/or host attack (parasitism) rates of the two parasitoid species differed significantly among ash tree-size classes, with S. galinae parasitism more abundant in larger diameter and pole-size trees, while T. planipennisi dominated in saplings. We also found the abundance of EAB larvae declined significantly with height in both ash saplings and pole-size trees, whereas the abundance of S. galinae and T. planipennisi broods was unaffected. However, the abundance of both larval parasitoid species was strongly and positively correlated with EAB larval abundance in sampled sections of their preferred tree-size classes. Our findings suggest that the two introduced specialist parasitoids, S. galinae and T. planipennisi, complement one another in protecting trees of different size classes against EAB.