What is Killing Our Trees?

Beech Bark Disease on the Move

This is the first in a six-article series called "What Is Killing Our Trees." The series will explore some of the lesser-known diseases that affect Michigan trees and potentially impact wildlife.

While much attention has been focused on the spread of the emerald ash borer and the related bans on moving ash trees and firewood, many people are unaware that another disease involving an exotic, tree-killing insect is slowly making its way across Michigan. Beech bark disease (BBD)Ñcaused by the interaction between the beech scale insect (Cryptococcus fagisuga) and native fungiÑis killing American beech trees by the thousands in the Eastern Upper Peninsula and along the Lake Michigan shoreline in the Lower Peninsula. It is widespread from Maine to northern Ohio and has shown up as far south as Tennessee.

On Labor Day weekend in 2008, I saw hundreds of dead or dying beech trees in the Ludington area of Mason County where the disease is firmly entrenched. Some were highly-valued, majestic yard trees; others were dominant trees in forested dunes in and near Ludington State Park. Beech nuts are important food for deer, squirrels and turkeys, and because beech trees are prone to heart rot they provide nesting or denning cavities for woodpeckers, raccoons, owls, porcupines and a host of other species. So, BBD is taking a toll on wildlife.

It all began when the beech scale insect entered Nova Scotia, Canada in 1870 in a shipment of ornamental trees from Asia. The insects gradually spread south and west, probably entering Michigan around 1990. The tiny insects, which are just a millimeter long, reproduce by parthenogenesis. That is, there are only females and they do not mate. The immature scale insects are aptly referred to as "crawlers," and are mobile in August through November. The crawlers feed on the tree's sap, and secrete a white, waxy substance that can be readily seen on the bark of heavily-infested trees.

Technically, trees don't have BBD until fungi enter openings in the bark made by the crawlers. For reasons not fully understood, when fungi appear the scale insects work deeper into the tree's cambium layer, where nutrients and water move up the tree from its roots. When fungi follow the scale insects into the cambium, the flow of nutrients and water is slowed and the tree suffers leaf loss.

Infected beech trees die very slowly, sometimes lasting ten years or more after they are first attacked by scale insects. There are resistant trees which apparently secrete substances that harden the inner bark and keep fungi from entering the cambium. In the Northeast, up to 25% of beech trees may be somewhat resistant to BBD. However, more than three-quarters of beech trees that do get BBD die and the rest stop growing and/or producing nuts. So, the impact on beech-dominated forests can be dramatic.

As in the case of the emerald ash borer, the spread of BBD can be slowed by not moving firewood. Officials are considering banning movement of beech logs from infected areas. Beyond that, and not cutting down resistant trees, it seems little can be done to protect the American beech tree from this exotic disease. Citizens can report potential BBD outbreaks to the DNRE Forestry Division at 517-373-1275 and get more information from www.na.fs.fed.us/spfo/fidls/beechbark/fidl-beech.htm.

Dr. Patrick J. Rusz
Director of Wildlife Programs