In May I discussed the status and biology of Emerald Ash Borer (EAB) and its effects on hedgerows. This month I am concentrating on forest landowners and how we should approach our management of woodlots in Caledonia County over the next 10 and 20 years.
How Long Do We Have?
It’s hard to predict how rapidly EAB will spread. In Wisconsin and Minnesota, a climate similar to ours, EAB was first discovered in 2008-2009. In the 10 years since then it has generally infested an area about 75 miles out from the first infestation. But EAB has popped up in spots all over those states, almost certainly carried there in firewood or other wood products. While EAB may only fly on its own power a mile or two per year, all it takes is a few adults being blown along in a storm or stiff breeze, and they will land several miles away. Given our proximity to the known infestations in Groton and Plainfield, we should expect to find EAB in our own woods within about 5 years, and possibly this year.
Remember that EAB is generally not noticed for the first three or four years that it is present. Then the population builds, slowly at first, and then very rapidly. As you can see represented by the dashed line in the graph, it will only be four or five years from the time we first notice damage until the EAB population reaches its maximum, at which point there will be a killing wave and all our ash trees will be dead or nearly so. After the trees die, the EAB population will crash due to a lack of food, but it will persist at a low level with surges in population whenever there are enough residual ash trees grown big enough to provide food for the EAB. Then those trees will be knocked out, and so forth into the foreseeable future.
Woodpeckers as Allies
The objective of our management is to delay the killing crescendo for five years or so. Research in Michigan has demonstrated a significant slowing of the exponential growth in EAB populations through use of girdled ash trees and encouraging woodpecker predation. With no management, EAB populations grow by a factor of 3.6 times per year as the crescendo builds to a peak population in the eighth or ninth year after the first insect lands. With girdling management and the subsequent feeding by woodpeckers, the populations grew only 1.3 times per year, delaying the peak population until about year 18. The solid line in the graph shows this effect.
Woodpeckers are the top agent of biological control of EAB. Studies have shown that woodpeckers eat 90% of the last larval stage (just before emerging as adults). Woodpecker populations will build with the surge in their food source. Standing dead trees are good for lots of reasons, and it makes sense that we should be increasing the number of snags in our woods to increase the food available and thus increase the population of woodpeckers before the arrival of EAB.
EAB is attracted to stressed ash trees. The technique for stressing trees and maximizing woodpecker predation is now well established after years of work in the Midwest. This is a way to detect the arrival of EAB in your woods, and also to delay the increase in the EAB population once it is here. About a month before adult emergence, lightly girdle some mature ash by cutting the phloem but not the sapwood. In response, the trees apparently emit some sort of stress chemical which attracts EAB. If the insect is already well established, these trees then act as sinks, attracting large numbers of EAB, which lay their eggs later in the summer. Leave the tree standing through the next winter to give woodpeckers the chance to feed on the larvae. Then in March or April cut the tree and buck it up. You can burn it yourself, but DO NOT TRANSPORT IT! Continue to do this each year to delay the build-up of EAB. In late May I girdled two ash trees and several cull trees of other species to act as food for woodpeckers. I encourage all forest landowners to study up on this technique and get started.
Forest Management in Anticipation
Talk with your forester about when you should cut ash. If a logger knocks on your door offering to cut your ash “before it’s too late”, do not agree until consulting a forester. The value of logs grows exponentially as they increase in diameter from 12 to 20 inches, so hesitate to cut too soon. In Peacham we may have only six or eight years if the insect is already here, or perhaps as many as 15 to 20 years if EAB moves slowly and/or we practice girdling management as it arrives. A healthy 12 inch ash will grow an inch in about four years. If EAB does not start killing on my land for 16 years, the board feet in that tree will more than double.
Your forester will also track the price of ash logs. Last year ash was selling for $800/1000 board feet, a very good price that was driven by Chinese demand. This spring it was down to about $600 and falling, thanks to the trade war. You might think a glut on the market would drive prices way down, but that has not happened in other places.
A well informed decision about when and how to cut your ash will involve a discussion about the size and maturity of the trees (are they 12-16” diameter or 18-24”), the current price of logs, and your tolerance for risk, both in terms of market price and the arrival of EAB. If you wait too long and the tree is already infested when you cut it, the log will be worth less because the mill will trim off the outer inch. Personally, I have been cutting ash that are 18” or bigger. For now, I’m leaving smaller diameters, hoping to get another couple of inches growth before having to cut them.
Northern White Ash trees are the tallest hardwoods in Caledonia County and their big crowns can do a lot of damage as they fall through the trees around them. Ash should be cut between October and April. During this period the bark of surrounding trees is more resistant to damage. It is also the period when your ash logs can be trucked without risk of spreading emerging EAB adults.
For the latest updates on EAB in Vermont and links to national information, go to vtinvasives.org where you will find the latest map of the infestation along with information on EAB and all other invasive plants and insects in Vermont. This is also where you should report possible detections of EAB in your area. If you want to be on the statewide list-serve to receive direct email updates, you can sign up there.
For the latest national information go to emeraldashborer.info.
A very informative webinar given last fall can be found at http://www.urbanforestrytoday.org/videos.html. Look for the “Emerald Ash Borer Update” given by Dr. Nate Siegert, USDA FS at UMass Amherst 9/18/2018. It contains both practical stuff and some dry research data.
Tim McKay is a retired natural resource conservationist and current woodworker and tree farmer in Peacham.