Watch out for these Invasive species that threaten our forests

Forests are experiencing a wide range of threats from pests. Forest-lovers can better assess forest health by knowing more how to recognize these pests and reporting them to the Michigan Invasive Species Information Network.

All trees die. However, the manner in which they die can matter greatly.  Processes of natural succession, management-induced regeneration, and recovery from natural events are part of what we expect. Yet, mortality from exotic pests and native-species-gone-awry signals an ecological system under stress.

Climate change will have an increasing level of influence over the next several decades, although our resilient forests are rather slow to respond in most ways. This gradual trend will exacerbate the impacts of some forest pests. Firewood movement and other human activities are the main vectors for exotic insects and diseases. 

Here is a selection of what to learn about and watch for. If you suspect one of the exotics, report the sighting through either “MISIN” or, in some cased, the DNR. Keep in mind, that this list is not exhaustive, by any means.

Asian Longhorned Beetle

Maples are this beetle’s favorite host, although the host list is long. The damage is largely done by the larvae, by chewing large holes through the wood, then weakening the tree that leads to breakage. Large pencil-sized exit holes are a good clue. While not yet in Michigan, public vigilance for this pest is critical. 

Emerald Ash Borer         

Most people are now familiar with the emerald ash borer, although it still has lots of work to do in northern Michigan and the Upper Peninsula. First found in the Detroit area in 2002, the borer has spread to dozens of states. The larvae chews through the living tissue under the bark of ashes, eventually killing the tree, from the top, down.

Hemlock Woolly Adelgid

This adelgid is a small, aphid-like insect that inserts its mouth parts at the base of hemlock needles. The eventual heavy infestation will drain the hemlocks of juices and cause death. It is difficult to spot because of its small size and position high in the crown of hemlocks. 

Spotted Lanternfly

A colorful exotic detected in eastern Pennsylvania in 2014, this fly feeds on maples, oaks, walnut, sycamore, and fruit trees. The adults are mobile from mid-summer through fall. It’s not yet in Michigan, so it’s another species to keep an eye out for. 

Oak Wilt

Likely an exotic disease, the pathogen has slowly been moving through oak stands since the 1940s. In many states, including Michigan, it has become epidemic. Oak leaves, especially those in the red oak subgroup, turn brown and fall-off the tree within a couple of weeks. Oaks have other maladies that can resemble oak wilt, but a complete loss of all leaves (no survivors or sprouts) is a pretty good indicator.

Heterobasidion Root Disease

This is an old pathogen in a new package, so to speak. It affects conifers, especially red pine, and commonly introduces itself to a stand through a cut stump. The pathogen moves through root grafts, a common practice, and can persist in the soil for long periods of time. 

Forest Tent Caterpillar

The FTC is a native moth that has population epidemics on a 10-15 year cycle. The larvae chew-up the leaves of aspen, maples, oaks, and apples. Not typically fatal, but successive defoliation can lead to tree death. The FTC is sometimes mistakenly called “army worms” (for good reason) and should not be confused with the tent-making eastern tent caterpillar and fall/spring webworms.  

Spruce Budworm

This is another native, cyclical moth, that prefers mature balsam fir and white spruce. Outbreaks have been spotty across much of northern Michigan. Current-year foliage turns brown, making a tree appear dead. However, this budworm prefers not to eat the older needles and, many times, when the dead needles slough-off the tree in the fall, the green appearance “returns”. 


There are two very common exotic species and a native. The exotic species can quickly overrun certain soil types, especially moister, richer soils. Buckthorns have almost no wildlife value, even though the fruits are commonly eaten by birds, and spread widely. Buckthorn can work in concert with other species, too. 

Earthworms and Slugs

The Lake States have no native worms or slugs. Together, with other species (including deer and with climate change), these lowly creatures drastically change soil and soil-surface conditions that can prevent native forest species from regenerating. Earthworms often have good reputation for gardens and fish bait, but in the forest they are gradually causing significant damage. 

To learn more about these species, and others, considerable amounts of information are on the Internet. A few links would be the Midwest Invasive Species Network MISIN, the DNR (2018 Highlights available), and the U.P. Tree ID website. Many County Conservation Districts and Land Conservancies have active search and destroy programs. Volunteers are always welcome!


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