MSU Extension

By the time the frosts begin and leaves are down for the year, most forest insects and pathogens have completed their annual life cycle. Observers might be able to detect evidence of those pests, some of which is more important than others.

Trees are host to hundreds of species of insects and pathogens. The majority of these pests seldom present serious health risks to trees. So, it can be difficult to figure out which symptoms are important and which to simply enjoy as part of the forest and tree microhabitat environment. Exotic species can sometimes cause particular concern.

During the growing season, when significant portions of a tree crown dies, then that is usually a symptom of consequence. Of course, during the dormant season, only the evergreens present this opportunity for observation.

A good example would be to re-inspect those fir and spruce trees that displayed lots of browned needles during the summer. Once the dead needles have fallen and the annual fall needle drop is over, the trees may look less hopeless. Of course, if there are few or no green needles, then the tree is dead. A homeowner will need to decide whether or not to keep a sparsely-needled residential tree. Blue spruce and Austrian pine can be particularly troublesome.

Tree health problems are viewed differently between urban/residential environments versus wildland forests. Homeowners are more concerned about individual trees and their appearance. And, urban and residential environments are usually more hostile for trees than their natural forest environments. Oftentimes, a homeowner will require the services of a certified arborist, rather than a more traditional forester.

Tree-watchers can provide an especially good service by watching for new infestations of exotic species, particularly the Asian long-horned beetle, hemlock woolly adelgid, and 1000 cankers disease of walnut. These species are the focus of the MSU “Eyes on the Forest” program and the development of a “Sentinel Tree” network.

The Asian long-horned beetle (ALB) is not yet known to occur in Michigan, but infestations are nearby in southern Ohio and Toronto. The ALB feeds on many tree species but maples are their favorites. Maples are the most common genus of tree in Michigan and occupy much of the urban forest.

The ALB adult beetles are very large and distinctive but are not likely to be seen in the fall. However, their large exit holes from trees can be seen, especially after the leaf cover has disappeared. They’re about the size of tapping holes used in the maple syrup process. With a trained eye, an observer might also be able to recognize the shallow oviposition sites, from mid-summer, where the eggs were laid. The ALB typically attacks the top of the tree first, so using binoculars to inspect the tree from the top down is helpful.

The hemlock woolly adelgid (HWA) is known to be in parts of the Lower Peninsula along the Lake Michigan shoreline. The tiny insects are difficult to see but colonies of their puffy white protective coatings can be seen along the twigs. Binoculars are needed for taller trees.

It’s possible to confuse HWA with the similar elongated hemlock scale. However, the scale attaches to the undersides of hemlock needles and the HWA attaches to the twigs.

The 1000 cankers disease (TCD) attacks black walnuts and butternuts. It’s a combination of a tiny bark beetle that carries a canker-forming fungus under the bark. The spores germinate and form a patch of dead tissue. If wilting walnuts were seen during the growing season, inspect the trunk or branches for tiny beetle exit holes. Shave the bark around the exit hole and look for blackened tissue.

In addition to these exotic pest threats, there are many other damaging agents associated with trees. Most common, perhaps, are wood-rotting fungi that enter a tree through a wound and begin breaking-down the wood. The softened wood can become good habitat for secondary invaders, such as ants, bats, flying squirrels, and many others. Woodpeckers are sometimes a good sign that this process is underway, as they sense the insect larvae and attempt to excavate them for food.

The living part of a stem or branch is restricted to several thin layers (cambium) around the outside, under the bark. The interior wood has value mostly for structural support. Nevertheless, when wood-rotting fungi soften enough wood, weakened trees become a hazard.

With the wet fall this year, many of these fungi have good environmental conditions for producing robust numbers of fruiting bodies, such as conks and brackets. Some of these fruiting bodies are edible, have artisan value, and other uses.

Lichens deserve a special mention, as they are harmless to trees but are often mistaken for or suspected as harmful organisms. They commonly grow on tree bark, similar to some moss species, but simply use the bark as a substrate. Lichens are interesting life forms, worthy of the curiosity by those who like natural things.