Can Spathius galinae attack emerald ash borer larvae feeding in large ash trees?

Abstract

Agrilus planipennis,the emerald ash borer (EAB), is an invasive forest pest decimating North American ash trees. Population-wide management of EAB is focused on biological control through the introduction of four parasitic wasps, including the recently approved larval parasitoid Spathius galinae. This species was approved for release in the north-central and northeastern US in 2015 and its long ovipositor (4–5.3mm) is expected to allow it to reach EAB larvae in larger ash trees with thicker bark, than the only other successfully established larval parasitoid, Tetrastichus planipennisi. Using experimentally infested logs of varying diameters in the laboratory, we measured the effect of bark thickness on oviposition of S. galinae to understand its potential value for controlling EAB in trees of differing diameter and bark thickness. Parasitism by S. galinae was highest when bark was thin (<4mm) and dropped significantly as valley bark thickness increased beyond 6.5mm. We also found that EAB larval feeding on inner bark, reduced the bark thickness directly over the larval galleries by 0.4mm on average. Our results suggest that S. galinaewill be able to reach EAB larvae in >95% of all ash in the northeastern United States. S. galinae will likely play a vital role in providing additional control and in supporting ash regeneration in aftermath areas of EAB invasions.