Niche partitioning and coexistence of two larval parasitoids introduced for biocontrol of the invasive emerald ash borer: The importance to North American ash recovery

When multiple species of parasitoids are introduced to an area against a target pest, strong interspecific competition may compromise biocontrol unless they can effectively partition available resources. Following the invasion of Agrilus planipennis (Coleoptera: Buprestidae), a serious pest of ash (Fraxinus spp.), the larval parasitoids Tetrastichus planipennisi (Hymenoptera: Eulophidae) and Spathius galinae (Hymenoptera: Braconidae) were introduced from Asia to the United States for biocontrol. To determine if these introduced parasitoids can coexist, we measured the larval parasitism rates by debarking infested ash trees of different sizes from nine forests in several northern states, where these parasitoids had been released for biocontrol of A. planipennis over five years ago. We found that T. planipennisi and S. galinae have become the dominant parasitoids attacking A. planipennis larvae in these regions, accounting for 79% – 85% of A. planipennis larval parasitism. Parasitism rates by S. galinae (range 2 – 49%) in large (DBH = 23–37 cm) and pole size (DBH = 8–20 cm) trees were several times greater than those (0 – 12%) in saplings (DBH = 2.8 – 6.3 cm), whereas parasitism rates by T. planipennisi, in saplings (range 28 – 50%) were over three times greater than in large and pole size trees (0 – 16%). Our findings strongly indicate that these two introduced larval parasitoids complement one another, protecting trees of different ages against A. planipennis. Thus, both are important to North American ash recovery in the aftermath of A. planipennis invasion.