Submitted to: PLoS One
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 4, 2013
Publication Date: December 9, 2013
Citation: Jennings, D.E., Gould, J.R., Vandenberg, J.D., Duan, J.J., Shrewsbury, P.M. 2013. Quantifying the impact of woodpecker predation on population dynamics of the emerald ash borer (Agrilus planipennis). PLoS One. 8(12):e83491 DOI:10.1371/journal.pone.0083491. Interpretive Summary: Since it was first discovered in Michigan in 2002, the emerald ash borer (EAB) has killed millions of ash trees across North America. The management of this invasive beetle is projected to cost more than $10 billion over the next decade. Woodpeckers are important predators of EAB larvae in their native range in Asia and are known to prey on EAB in North America as well. Understanding the impact of woodpeckers on the population dynamics of EAB in North America will enable us to predict the spread of this pest more accurately and to target certain life stages more effectively to achieve biological control. To determine the effect of woodpeckers on EAB we used cages to exclude woodpeckers from trees with natural and artificially established EAB infestations. We then compared the relative importance of different sources of mortality on EAB populations in the presence and absence of woodpecker predation. We found that woodpecker predation can constitute a large source of mortality for EAB larvae, and that more larvae were fed upon by woodpeckers at sites that had been infested by EAB for longer periods. Additionally we found that fewer larvae were parasitized in the presence of woodpecker predation, which suggests that it might be useful to deploy caging around trees to improve the likelihood of establishment of introduced parasitoids (as biological control agents). Our findings are important because they indicate that woodpecker predation alone will not be sufficient to control EAB population growth.
Technical Abstract: The emerald ash borer (EAB), Agrilus planipennis, is an invasive beetle that has killed millions of ash trees since it was accidentally introduced to North America in the 1990s. Woodpeckers are an important source of mortality for EAB in their native range, and understanding their effect on the population dynamics of EAB in their introduced range has implications for management of this pest. We combined two experimental approaches to elucidate the relative importance of woodpecker predation on EAB populations. First, we examined wild populations of EAB in ash trees in New York in winter of 2012, with each tree having a section screened to exclude woodpeckers. Second, using a life table approach we established experimental cohorts of EAB in ash trees in Maryland in the summer of 2012, and half of these trees were caged to exclude woodpeckers. The following spring these trees were debarked and the fates of the EAB larvae were determined. We found that trees from which woodpeckers were excluded consistently had significantly lower levels of predation, and that woodpecker predation comprised a greater source of mortality at sites with a more established wild infestation of EAB. Furthermore, in our experimental cohorts we observed that trees from which woodpeckers were excluded had significantly higher levels of parasitism. While woodpecker predation did reduce the predicted EAB population growth by a small amount, it appears unlikely to be a substantial factor in their control. The lower levels of parasitism on EAB larvae when exposed to woodpeckers has implications for EAB biological control, suggesting that it might be prudent to exclude woodpeckers from trees when attempting to establish parasitoid populations. Future work should include utilizing EAB larval cohorts with a range of densities to explore the functional response of woodpeckers.