MSU Extension

Summer and the warm growing season offer plenty of food for forest plants and forest wildlife to reproduce. Some wildlife species are a bit less desirable to many people, such as forest tent caterpillars, spruce budworms and Lecanium scales. 

To a forester, the outbreaks of native insects and pathogens can be an interesting part of the job. From an ecological viewpoint, these impacts can be regenerative and fascinating. To a backyard barbecuer, picnicker or a home landscape enthusiast, some of these events can be annoying.

Forest tent caterpillar (Malacosoma disstria or FTC) population eruptions occur every 10-15 years. The FTC is among a group of “tent caterpillar” species but the FTC does not form tents, like the spring-time eastern tent caterpillar. The FTC has a row of yellow spots in between a pair of baby-blue lines. Aspen leaves are among the favorite food, sometimes rendering summer canopies leafless. Mass migrations by large populations of the larvae can be dense enough to cause cars to slip off roads. 

Spruce budworm (Choristoneura fumiferana) populations continue to erupt in various locations in the northwoods. Also cyclical, the budworms appear for several years every couple of decades, or so. They prefer eating the new needles on balsam fir and, secondarily, white spruce. Multiple defoliations can lead to extensive tree death (and regeneration). Firs grow brown in July and August, but after the killed new growth drops in the fall, the older green needles make the trees appear recovered. 

Population eruptions of both the forest tent caterpillar and spruce budworm can be likened to multiple bursts across the statewide landscape, with variable locations from year to year, until their populations crash. Little can be done about treating these outbreaks. 

Lecanium scale (Lecanium spp.) is a tiny insect the feeds on the sap of host trees from a stationary place on the twig. The scale builds a protective coating around itself. Sometimes the coating is hard, other times it is soft or cottony. Different species of Lecanium feed on different host tree species. Twigs of heavily-infested trees have lots of bumps on them. 

Usually, scales have only minor impacts on tree health, but their greatest impact when populations are high have more to do with human inconvenience. The “honeydew” secreted by the scales create sticky surfaces. This sticky substance supports the growth of a black mildew called sooty mold. These secretions can be troublesome to clean from decks, outdoor furniture and other surfaces. 

The emerald ash borer (Agrilus planipennis) has raged through most of the communities and woodlands of southern Michigan and portions of the rest of the state. This insect has been the most expensive forest pest to date, with most of the costs incurred by cities and homeowners for tree removal. All species of true ash (Fraxinus spp.) are killed but researchers are looking more closely at the uncommon blue ash (F. quadrangulata), which appears to exhibit some resistance. Also, some insect parasitoids have been identified that might be useful in managing EAB populations. 

Oak wilt (Ceratocystis fagacaerum) is a fungus, related to Dutch elm disease, which kills oaks. Species in the red oak taxonomic group are particularly vulnerable. This disease has become increasingly important within the extensive oak woodlands of the northern Lower Peninsula. Oak wilt typically kills a tree within a few weeks, where wilting begins at the top of the tree and works its way down. Commonly spread by a group of sap-feeding beetles, it is important avoid wounding oaks from mid-April to mid-July. 

Hemlock woolly adelgid (Adelges tsugae) has established itself in Michigan in some of the counties along the Lake Michigan shore of the Lower Peninsula. The Michigan Department of Agriculture and Rural Development has recently emplaced quarantine regulations on the movement of hemlock. Adelgids are another tiny sap-feeding insect, similar to scales. Hopefully, aggressive treatment will, once again, eradicate the insect from the state. 

Not yet known to be in Michigan is the Asian long-horned beetle (Anoplophora glabripennis). Nearest infestations are in Toronto and southern Ohio. This large wood-boring beetle has a wide range of host tree species, but prefers maples. Federal, state and provincial agencies are aggressively working to contain and eradicate ALB populations in North America. There have been a number of successes. 

Each year, the Michigan DNR prepares a “Forest Health Report” that highlights the major events from the previous year. Those reports can be found on the DNR website on their forest health webpage. 

One of the best ways to maintain forest health is to manage woodlands for maximum tree vigor. Healthy trees are better able to resist pests than stressed trees. A professional forester can provide advice on how to best care for particular woodlands.