Can birds defeat the emerald ash borer?

Possibly the longest running crowdsourcing project, the 113th annual Christmas Bird Count is now in full swing. Originally started as an alternative to Christmas bird shoots, it immediately helped to reduce the death count by recognizing living birds for their beauty. But you don’t have to appreciate them just for their looks. Saving birds may help save Midwest forests from the spread of a destructive invasive pest.

Detected in Michigan in 2002, the emerald ash borer has rapidly spread through much of the Great Lakes region, leaving behind millions of dead ash trees. The prolific beetles tend to attack only weakened trees in their native Asia, yet healthy North American forest stands are also vulnerable. With no effective control measures in sight, and Midwest deciduous forest appearing doomed, researchers questioned whether native insect predators could be part of the solution.

beetle distribution Can birds defeat the emerald ash borer?

An analysis of bird abundance data from the citizen science Project FeederWatch showed that some birds might be benefiting from the growing beetle population. After the introduction, red-bellied woodpeckers and white-breasted nuthatches increased in areas close to the epicenter. The surplus food source may have increased rates of adult survival and reproduction, but the direct link between foraging birds and beetle mortality was still missing.

Another group of biologists working in Ohio may have found the connection. They felled infected ash trees and marked the location of beetle entry and exit holes as well as the holes drilled by woodpeckers. After removing the bark and following galleries chewed through the cambium, they were able to determine how many of the beetles exited alive or via a bird gut. On average, 37 percent were consumed, and in some trees, as many as 85 percent were snapped up. The results, published in Forest Ecology and Management, indicate a beetle population with exponential growth could be limited by foraging birds.

When the bird blitz will take effect is unclear. Woodpeckers may need time to catch up with the beetle population, and since the beetle is a novel prey item, birds may not immediately recognize it as edible. Keep in mind that birds may control the outbreak and save some trees, but not eliminate the problem. The beetles, like any good adversary, will never completely disappear. Miles Becker | 6 January 2014

Sources: Flower, C.E. et al. 2014. Native bark-foraging birds preferentially forage in infected ash (Fraxinus spp.) and prove effective predators of the invasive emerald ash borer (Agrilus planipennis Fairmaire). Forest Ecology and Management doi: 10.1016/j.foreco.2013.11.030

Koenig, W.D. et al. 2013. Effects of the emerald ash borer invasion on four species of birds. Biological Invasions doi: 10.1007/s10530-013-0435-x

Photo © USDA

Figure © Springer Science+Business Media Dordrecht

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1 Comment

  • George (Jeff) Boettner January 9, 2014 at 3:11 pm

    Ash borer can lay 200 eggs per female. Assuming 50/50 sex ratio then you need greater than 99% mortality from all factors to begin to collapse the population. At 99% mortality each female is producing 1 male and 1 female and the population is stable. So even at the 85% mortality from birds, the population would be increasing 15 fold per year!! This is still a great start, and I am hopeful that this might open the door for a bio-control agent, such as the wasp being worked on by Dr. Juli Gould at USDA-APHIS in Massachusetts. This could mean that even if a bio-control agent attacked only 15-20% it might still be worth pursuing. Crossing my fingers as ash borer has now been found in CT, NH and MA in 2013–the map is a bit outdated already. It will be interesting to see how fast woodpeckers learn to use these beetles as food in the newly infested states. They learned pretty fast in the midwest, given this is a fairly recent pest in the US.

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