Foresters are calling it the most destructive tree pest to ever invade North America, and its march of devastation continues. Emerald ash borer has now spread to 35 states, killing hundreds of millions of ash trees in its wake. A unique bird monitoring method might help slow the spread.
Emerald ash borer (EAB) is a small, green, iridescent beetle native to Asia and first discovered in the United States in 2002 in Michigan. To date, the insect’s presence has not been confirmed in North Dakota, but it is as close as Winnipeg, Manitoba, Sioux Falls, S.D., and Sauk Centre, Minn.
Although the insect only attacks ash (Fraxinus species), and no other tree types, ash is the most common tree in North Dakota, accounting for over 78 million trees. Neighboring Minnesota has more ash trees than any other state, estimated at 1 billion.
Adult EAB female beetles lay up to 200 eggs in bark crevices of ash trees, which hatch into larvae that feed under the bark, creating S-shaped galleries that disrupt movement of water and nutrients, killing the tree within years.
Larvae take one or two years to mature, spending winters in these galleries, eventually emerging as adults, chewing telltale D-shaped holes as they exit branches. The adults, which can fly, seek out new trees, and the cycle continues.
Insecticides are neither practical nor effective for control. Cities are gradually removing ash and replacing with diversified tree types. Our best defense is early detection of EAB outbreaks and removal of infected trees to slow the devastating spread until newly planted trees fill the gaps that will be left when ash trees are lost.
Peter Gag, forest health manager with the North Dakota Forest Service, is promoting a unique and effective method of early detection of EAB infestations, and we can all participate. Gag indicates that because some birds, termed bark-foraging birds, actively seek EAB larvae as a food source, these birds can lead us to EAB outbreaks.
In our region, woodpeckers are the primary bark-foraging birds, which create a tracking device more noticeable than the small, 1-centimeter beetles and their small D-shaped exit holes in dead or declining trees.
A definite correlation has been found between the presence of EAB and the five woodpecker species that forage for these insects. If woodpeckers can lead us to the elusive small pockets of EAB activity, we can eradicate recent invasions, preventing widespread infestations by early removal of affected trees.
Gag says the most effective method to survey for spread of EAB yet is the visual inspection of ash trees for woodpecker damage, which is easier to observe in winter and early spring when leaves are off trees.
What are we looking for? Inspect ash trees for woodpecker holes in bark, “blonding,” which is removal of strips of bark that exposes the inner light tan wood, and finding wood scattered on the ground beneath the tree.
Because EAB only invades ash, it’s important to identify trees as ash. The leaves are compound, consisting of five to seven leaflets, which doesn’t help in winter. Ash tree bark is quite distinctively divided into diamond-shaped furrows, and the outermost surface of the ridges are flat-topped. Buds are arranged opposite one another along twigs.
Gag says once you’ve detected a bark-foraging bird, and confirmed that it’s on an ash, notice where on the tree the bird is actively foraging. EAB tends to first establish itself in the small, upper branches of the crown, 4 to 5 inches in diameter or smaller.
How long the bark-foraging woodpeckers work on a tree is also important. Woodpeckers will go randomly from tree to tree searching, and the longer they remain on a tree, the more likely it’s found a good food source, possibly EAB. Use binoculars to view small upper branches for signs of EAB, including split bark, extensive woodpecker holes, or blonding. These indicate that the food source is likely valuable and plentiful enough to warrant the bird remaining in place.
For all of us to participate, Gag has set up an easy-to-use survey through our cellphones to collect data and guide a tree professional to the site of a tree that might hold EAB. The QR code can be scanned by the camera on your phone or tablet to bring you to a survey in which you provide information that goes directly to the North Dakota Forest Service, where it will be determined if a site requires further investigation.
For details on how to access the QR code and survey, as well as invasive species threats to trees in North Dakota, visit https://www.ndinvasives.org/learn-about-eab-and-bark-foraging-birds.
Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at firstname.lastname@example.org.