The emerald ash borer, an invasive beetle, has devastated millions of ash trees across North America, costing billions in damage. Despite massive government efforts, the real saviors have been native predators—woodpeckers, nuthatches, and a tiny native wasp—that prey on the beetle larvae, gradually helping ash trees to survive and recover in infested areas.
It is 6:00 in the morning somewhere in southern Michigan. The autumn air smells like cold metal, and the leaves are halfway down. A man named Henry, representing every American landowner affected by a devastating forest pest, walks the edge of an 80-acre stand of timber his grandfather bought in the 1950s. He has walked this stand every fall for over 50 years.
Stopping under a dead ash tree, still standing 40 feet tall with bark peeling off in pale, dry sheets, Henry listens. What he hears is a sharp, hard, mechanical hammering—six or eight birds creating a continuous sound rolling across the woodlot like rain on a tin roof. He says aloud, "They came back."
The "they" Henry refers to are native birds returning to prey on the emerald ash borer (EAB), a beetle that has caused one of the most destructive forest pest outbreaks in modern American history.
The disaster began in the summer of 2002 in a Detroit suburb, where forestry workers noticed thousands of ash trees dying simultaneously. Upon investigation, they found a small, metallic green beetle unknown to them: Agrilus planipennis, the emerald ash borer, native to eastern China. It likely arrived years earlier in wooden packing crates on a freighter.
The beetle spread rapidly across the United States and Canada, reaching over 36 states and five Canadian provinces by 2018. The EAB kills ash trees by laying eggs in bark cracks; larvae hatch and tunnel through the phloem, disrupting the tree's nutrient flow and starving it within 2 to 4 years.
With approximately 8 billion ash trees in North America, the beetle has killed over 100 million by 2026, causing tens of billions of dollars in economic damage.
The federal government responded with quarantines on firewood movement, bans on transporting ash logs, fines for contaminated wood, and pesticide injections costing hundreds of dollars per tree every two years. At its peak, the USDA spent tens of millions annually on EAB management.
Despite these efforts, the beetle continued to spread.
A breakthrough came from bird scientists analyzing unrelated data from Project FeederWatch, a citizen science program tracking backyard birds. Dr. Walter Koenig at Cornell and colleagues found that in counties heavily infested by EAB, populations of four bird species increased instead of declining.
Three of these were woodpeckers, and the fourth was the white-breasted nuthatch, a small bird known for running headfirst down tree trunks.
Further studies confirmed that these birds were preying on EAB larvae, increasing their populations as the beetle spread.
The hairy woodpecker and the downy woodpecker are the primary predators drilling into ash bark to extract EAB larvae. The hairy woodpecker, about 9 inches long, strikes bark with extraordinary speed and force, while the smaller downy woodpecker specializes in thinner branches.
These woodpeckers locate larvae by listening to faint vibrations as the larvae chew through the phloem. They then chisel through bark to extract larvae using their long, barbed tongues.
Studies show woodpeckers can remove 30 to 40% of EAB larvae and pupae in an area, sometimes even more.
The red-bellied woodpecker, a medium-sized bird with a striking red crown, increased most during winter when fruit was scarce. Unlike the drillers, it picks at exit holes, cracks, and shallow galleries, targeting larvae near the bark surface.
The white-breasted nuthatch complements this by working upside down on tree trunks, accessing bark angles woodpeckers cannot. It cleans up larvae exposed by woodpeckers but not yet extracted.
Together, these birds cover the entire vertical face of the tree, ensuring no larvae escape predation.
Atanycolus cappaerti, a tiny native wasp described in 2009, is a parasitoid that drills through solid bark to lay eggs inside EAB larvae. The wasp's larva then consumes the beetle larva from within.
Field studies in Michigan, Ohio, and Ontario show parasitism rates ranging from less than 10% to over 70% in some sites.
This wasp was present in North American forests long before the beetle's arrival and has adapted to prey on this new host.
In counties where EAB has been present for 15 years or more, researchers observe young ash saplings surviving under sustained beetle pressure. These "lingering ash" trees indicate that native predators have reduced beetle densities to levels young trees can withstand.
This recovery was not achieved by quarantines, injections, or introduced biocontrol wasps alone but by the combined efforts of native birds and wasps.
However, the beetle continues to spread, and ash forests will suffer significant losses for decades. The recovery is slow but hopeful.
Interestingly, ash wood retains its strength after the tree dies from EAB infestation because the damage is limited to the soft layer beneath the bark. Salvage operators in Michigan, Ohio, and Indiana have been harvesting EAB-killed ash for over a decade, using it for baseball bats, tool handles, flooring, and cabinets.
The story of the emerald ash borer and its native predators teaches us that sometimes nature has its own solutions. While billions were spent trying to kill the beetle, the forest was quietly fighting back through native birds and wasps.
Henry, walking his timber stand, hears the hammering of woodpeckers and sees young ash saplings surviving. The forest does not need rescuing; it needs time and the presence of native predators to recover.
This story challenges us to reconsider our approach to forest management and conservation, recognizing the power of native ecosystems to heal themselves.
If you hear hammering in the woods next time, remember the unseen battle and the hopeful recovery unfolding quietly among the trees.
I'm Edmund Hale, and this is one of the most quietly hopeful recoveries in North American conservation today.
