Emerald ash borer biocontrol in ash saplings: The potential for early stage recovery of North American ash trees

In many parts of North America, ash (Fraxinus) stands have been reduced by the emerald ash borer (Agrilus planipennis) invasion to a few surviving mature trees, saplings, basal sprouts, and seedlings. Without a soil seed bank for Fraxinus spp., tree recovery will require survival and maturation of these younger cohorts to reproductive age. Here we report and analyze the population dynamics of emerald ash borer and its associated natural enemies in ash saplings (2.5-5.8 cm DBH) in six deciduous forest stands in southern Michigan. At these sites, the outbreak population of the pest collapsed during the study, and a biocontrol agent introduced from China, the larval parasitoid Tetrastichus planipennisi, became widely established and increased in rates of parasitism. To assess the potential for ash recovery in these stands, we also quantified the abundance and crown condition of the ash saplings and surviving ash trees at the study sites. We found that T. planipennisi was the dominant biotic mortality factor in saplings, killing 36-85% of the late instar borer larvae. Neither woodpecker predation nor native parasitoids caused more than minor levels (<20%) of borer mortality in saplings. Life table analyses of these data further showed that the net population growth rate of the pest in saplings was near or under replacement levels, and that the introduced biocontrol agent reduced the pest's net population growth rate in saplings at our study sites by over 50%. In addition, stand inventories found that healthy ash saplings (4-16 per 100 m(2)) and smaller (pole size) trees (2-9 per 100 m(2)) remained in the six study sites, despite an early high density population of the pest at the sites. These findings indicate that the introduced biocontrol agent T. planipennisi is providing significant biocontrol services, enhancing ash survival and promoting recovery of the ash in southern Michigan. Published by Elsevier B.V.