Forest exotics: there are many, but some are more dangerous than others

Forests need our help monitoring for exotic invasive species.

What are the most current big exotic (non-native) invasive threats to forest health?  Arguably, the short list would be Asian long-horned beetle, hemlock woolly adelgid, balsam woolly adelgid, Heterobasidion root disease and oak wilt. Then, there are a few plants, such as buckthorn, Japanese barberry and garlic mustard. Other species could easily be added. 

Some might ask about the emerald ash borer and beech bark disease. However, those are lost wars. Our future children and grandchildren will be hard-pressed to find ash and beech leaves for their school science projects, much like chestnut, butternut, bitternut hickory and others. 

Asian long-horned beetles (ALB) are not yet in the Lake States but the threat from ALB is immense. The tree host list is long but maples are their favorite. The nearest infestations are in the Toronto area and southern Ohio. The large exit holes of the adult beetles are indicators of an infestation. So is the large size, long antennae and black color with white blotches of the adult. 

Hemlock woolly adelgid (HWA) is a tiny sap-sucking insect that forms colonies on the small twigs of hemlocks. It’s hard to spot at the top of tall hemlocks. Much of the hemlocks in eastern states have been killed, leaving landscapes looking like those of the western forests killed by the mountain pine beetle. There are a few outbreaks in Michigan along southern Lake Michigan. The Lake States have some of the last reserves of old hemlock stands. 

Balsam woolly adelgid (BWA) is a cousin to the HWA, but feeds on balsam fir.  Currently, balsam mortality (and white spruce) from the native spruce budworm might be incorrectly attributed to BWA. This adelgid is not yet in the Lake States and we need to be on the lookout, just like with the ALB. 

Heterobasidion root disease is a soil pathogen that affects red pine, especially well-managed plantations. As large volumes of Lake States red pine are reaching maturity, so this disease may become increasingly important. It invades freshly-cut stumps, from thinning practices, and then spreads to other red pine via root connections. The fungus can linger in the soil for decades.

Oak wilt is a particularly pernicious disease. A group of picnic beetles unwittingly carry spores to uninfected oaks through tree wounds. Thus, the warning about wounding oaks from mid-April through mid-July. Once in a stand, especially those trees in the red oak group, the disease quietly spreads through the root systems until all the oaks are dead.  At that point, the fungus in that stand dies as it requires live tissue to survive.  One could argue that much of our oak resources exist due to human influence and is, therefore, not natural. However, humans seem to have an uncanny connection to oak forests. 

Exotic invasive plants are fewer in forest systems than in other vegetation types.  However, they can seriously alter the ecology of the understory, which can have major impacts on tree regeneration and wildlife habitat. These plants often work in concert with each other and with certain animals, such as earthworms (another exotic), mice, white-tailed deer and Pennsylvania sedge. 

Buckthorn (two exotic species), Japanese barberry and garlic mustard, are among the worst of the worst. Other species could be among this blacklist, such as black locust, Norway maple, several honeysuckles, privet, Japanese stilt grass and oriental bittersweet. Then, there is a group of nasty species that occupy the sunnier edges of forests, such as Autumn olive, knotweeds, mile-a-minute weed, Phragmites and black and pale swallow-worts. The northwoods is currently less affected than the forests near urban and agricultural areas, but that may simply be a matter of time. 

Monitoring and early attacks can eradicate infestations. There have been success stories. Accidental introductions can be reduced through sanitation practices, such as cleaning recreation equipment (e.g. ATVs and RVs), shoe-scraping and avoiding firewood movement. 

Thousands of “eyes on the forest” is the best weapon against exotic species. Observation and education might be the most important tool in the arsenal needed to keep our forests as healthy as possible. Those who care enough, will take the time to learn. And, there is new information every year. 

In Michigan, one of the best references for current forest health issues is the annual DNR Forest Health Highlights and the Michigan Natural Features Inventory. The Wisconsin DNR also has a  website for forest health information. 

Usually, management is the best way to maintain the health and vigor of a forest, promote biodiversity, alter wildlife habitat, and meet many other ownership goals. A consulting forester is the best way to learn how to take care of a forest.  

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