Emerald ash borer strikes in Thetford

It was the persistent tapping of a woodpecker that drew attention to the two ash trees beside Poor Farm Road. Peering upward, it was evident that the crowns had lost over half their leaves, revealing many dead branches. The characteristic "woodpecker flecking," the result of woodpeckers chipping off flakes of bark to reach the invading pests, was also plainly visible on the upper trunk and main branches.

A visit from the town tree warden confirmed the dreaded news — the trees were infested with emerald ash borer.

Sadly, this was inevitable. Thetford is flanked by towns where the pest has been confirmed. It was only a matter of time before the beetle made its way to the trees on Poor Farm Road. The emerald ash borer, an elongated metallic green insect about 3/8–1/2 inch long and 1/16 inch wide — that's roughly the size of a cooked grain of rice — spreads at a rate of 2 miles a year.

The US government has given up on trying to quarantine ash borer outbreaks and deregulated the pest in 2021. Some of us remember the triangular purple boxes, baited with pheromones and sticky glue, that were hung in ash trees to survey for the borer. They haven't been around for the last decade or so, as trying to contain the spread has proved futile. So, now it is up to towns, communities, and individual landowners to address the problem. Most towns have volunteer tree wardens or tree committees, but no full-time experts. Towns also have limited budgets to cover the management of vegetation throughout the entire town, not just dealing with ash trees.  

Most insects in nature are our allies, pollinating crops and plants, providing irreplaceable nourishment (proteins and fats in particular) for birds, bats, amphibians, etc, and keeping each other's numbers in check by predation. Plants have developed multiple defenses against the insects they habitually coexist with. These associations have evolved over eons, and when examined in detail may reveal a surprising level of complexity and specificity. However, when a non-native insect that didn't co-evolve with north American ecosystems appears on the scene, there's nothing to keep it in balance. The emerald ash borer was first discovered near Detroit, Michigan, and in Ontario Canada, where it most likely emerged from wood packing crates imported from Asia. 

The beetles feed on various parts of ash trees to complete their life cycle and can infest all species of ash. Black ash appears particularly susceptible. The adults feed on ash leaves, but it is the larvae that do the deadly work. They hatch from eggs laid in bark crevices and bore under the bark where they consume the trunk's growing layer, or cambium, and also the tender sapwood and the sap-conducting phloem. This disrupts the flow of water and nutrients, and a large infestation girdles a tree and kills it.  Ash trees can take up to three years to show symptoms of infestation and up to five years to die. The trees first infested in Vermont are starting to die en masse.

Initial research suggested the ash borer larvae were killed at temperatures below minus 35.3 degrees Centigrade. However it was recently discovered that they exhibit what is known as phenotypic plasticity, meaning they can change their characteristics without the slow process of evolution at the genetic level. The new findings suggest that phenotypic plasticity allows them to survive even minus 50 degrees C.

At present only a small proportion of Vermont ash trees are infected. The best we can do is to slow the spread of emerald ash borer while hoping that resistant trees may be discovered. If most ash trees are lost in a few years, the effects on forests would be devastating. If the same number of ash trees are lost over the course of decades, it would allow forests and towns more time to adjust and allow for replacement tree species to establish. The other hope is that biologists will find some antidote. In fact, the state is working with the USDA’s Animal and Plant Health Inspection Service program to release parasite wasp species that attack the ash borer. The small, stingless wasps lay their eggs in ash borer larvae, which “dramatically slows” how fast the beetle can reproduce and spread. The wasps have been released at six sites in Vermont with more locations to come. 

The VT Department of Forests Parks and Recreation issued guidelines which it recommends for handling all ash timber, because small infestations are hard to see.

When harvesting ash trees, examine them for signs of borer damage and determine if the trees are in an area of infestation. It is illegal in Vermont to transport wood infected with the borer without proper treatment.

If the area is infested, delay cutting till the end of flight season of the adult beetles, which is between June 1st and September 30th.

Ash wood that must be moved must be heat treated first, so that the temperature in the center of the wood reaches 71.1 degrees Centigrade, or 160 degrees F, for 75 minutes. This will necessitate cutting the lumber into 3-inch thick boards. A treatment facility is required that has to be approved by the Vermont Agency of Agriculture Food and Markets (VAAFM). Once approved, a compliance agreement will be issued, subject to annual recalibration of the temperature monitoring equipment.

Alternatively, sawlogs must be stripped of all bark plus an additional half-inch of wood, making sure that no bark or cambium remain.

A method more amenable to small woodlot owners is to leave ash wood in place to season for 24 months before transport. Chipping ash trees and leaving them on site is another option.

Some towns and cities, like South Burlington, are proactively removing ash shade trees and replacing them with other tree species. However, the proactive cutting of the many roadside wild ash trees in rural towns would be prohibitively expensive.

The state of Vermont is tracking the progress of the ash borer. If you think you have an infested tree, please report it.