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

Authors: Duan, Jian; BAUER, LEAH - Us Forest Service (FS); VAN DRIESCHE, ROY - University Of Massachusetts

Submitted to: Forest Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/24/2017
Publication Date: 3/29/2017
Citation: Duan, J.J., Bauer, L.S., Van Driesche, R.G. 2017. Emerald ash borer biocontrol in ash saplings: the potential for early stage recovery of North American ash. Forest Ecology and Management. 394:64–72.

Interpretive Summary: The emerald ash borer (EAB) is a serious invasive forest pest that has killed tens of millions of ash trees in the United States and Canada. In many parts of North America, ash stands have been reduced by this invasive beetle to a few surviving mature trees and young basal sprouts, saplings, and seedlings. Without a seed bank, ash recovery will require survival and maturation of these younger trees to reproductive age. Scientists from the ARS Beneficial Insects Introduction Research Unit in Newark DE, the US Forest Service, and the University of Massachusetts surveyed populations of EAB and its natural enemies on ash saplings over a three year period in six stands of deciduous forest in southern Michigan. At these sites, the outbreak population of EAB collapsed during the study and an introduced parasitic wasp (Tetrastichus planipennisi) became widely established, killing 36–85% of the EAB larvae in ash saplings. Neither woodpecker predation nor native parasitoids caused more than minor levels of EAB mortality. There were abundant healthy ash saplings and small ash trees remaining in the six study sites. These results indicate that the introduced biocontrol agent is providing significant biocontrol and enhancing ash survival and promoting its recovery in Michigan.

Technical Abstract: In many parts of North America, ash stands have been reduced by the emerald ash borer (Agrilus planipennis) invasion to a few surviving mature trees and young basal sprouts, saplings, and seedlings. Without a seed bank, ash 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 indicated that the net population growth rate of pest populations are near or under-replacement levels, and that the introduced biocontrol agent reduced over 50% of the pest’s net population growth rates in our study sites. In addition, the results of stand inventories found that healthy ash saplings (4–16 per 100 m2) and smaller (pole size) trees (2–9 per 100 m2) remained in the six study sites, despite an early high density population of the pest at the sites. These findings strongly indicate that the introduced biocontrol agent T. planipennisi is providing significant biocontrol services, enhancing ash survival and promoting recovery of the ash in central Michigan.