Invasive Species


In 2001, I read a sky-is-falling story about how earthworms were profoundly impacting the soil in northern forests. I initially dismissed the story, thinking that the beneficial earthworm could hardly be an exotic invader like zebra mussels or starlings. After all, gardeners are thrilled to see worms in their compost pile, which they know help to quickly convert their kitchen wastes into good soil. So how could earthworms possibly have any negative impact?

Soon after reading this earthworm story, I joined a hike near Eagle River with a botanist for the Nicolet National Forest. She took our group to a state natural area where the forest understory was almost entirely Pennsylvania sedge, a species which looks to most people just like grass. I’d seen forest floors like this before and had always wondered why this monotype of sedges occurred when many forest floras were populated with a wide diversity of wildflowers.

The botanist had brought along a shovel, and she dug up some soil, asking what was different about it from what we ordinarily see. Well, the soil was well mixed, relatively dark, and crumbly – in other words, pretty good stuff, and an anomaly in our usually sandy loams. There were worms in the soil, too, and they’d done their job, converting the leaves into soil, mixing and aerating the whole shebang. We then looked around, and noticed that the forest floor had virtually no spongy leaf litter like one always expects in a hardwood forest. The leaves had been quickly converted to soil by the earthworms.

You’d think that would be good.

And you’d be dead wrong.

The story is yet another in the ever-growing tome of invasive species, because after the last glacial retreat, no native earthworms lived in the Great Lakes region. We have, however, introduced at least 15 species of worms in the last century. This means that since the glacial retreat some 10,000 years ago, our forests developed in the complete absence of earthworms. Thus, annual leaf decomposition evolved to be controlled by fungi and bacteria, which work so slowly that the accumulation of leaf litter far exceeds the rate of decomposition, resulting in the formation of a thick, spongy duff layer on the forest floor. In a rich sugar maple forest, the duff layer can be up to four inches thick, insulating the ground and keeping it cool and wet. Herbaceous plants like trillium and trout lily have adapted over thousands of years to germinate and root exclusively in this thick mat.43

In the last century, however, along came the detritus-loving earthworm, and as their population grew, they rapidly consumed the duff layer in as little as a year or two, literally eating the duff out from under the seedling plants. The duff turned into a much denser layer of darker soil, and total plant cover dropped from nearly 100 percent to less than 25 percent. Forests besieged by earthworms are often now totally dominated by Pennsylvania sedge where other native plants once flourished, species like large-flowered and nodding trillium, Solomon’s seal, blue cohosh, sweet cicely, Canada mayflower, wild ginger, red baneberry, lady fern, rattlesnake fern, bloodroot, bellwort, and many others.

In sites that were invaded by earthworms over a decade ago, the native understory herbs and tree seedlings have still failed to recover. A sharp drop has also occurred in animal species that live in the moist duff layer, like salamanders and an array of tiny insects and arthropods that ordinarily are numerous. How their decline is affecting the life up the food chain – the shrews, and snakes, birds, bears, and others – is still unclear.

A study undertaken from 2007 to 2010 on 101 randomly selected northern hardwood sites in the Chequamegon-Nicolet National Forest found that 90% of the sites were impacted by earthworms.44 The earthworms reduced or eliminated the soil litter and thoroughly mixed the soil resulting in major reductions in mychorrhizal fungi, soil microbes, soil-dwelling invertebrates, and a wide array of herbaceous and woody species that ordinarily are found in forest understories. Sugar maple and basswood seedlings declined, and Pennsylvania sedge took over, reigning in this new environment because of its adaptive abilities to withstand drier conditions, disperse its seeds early in the season, and to aggressively reproduce through spreading rhizomes. Add in the effects of browsing by over populations of white-tailed deer, and the vegetative impacts were seen as profound.

Where did the earthworms come from? Mostly from anglers who toss their excess worms into the woods at the end of a fishing trip.

The only good news is that earthworms move very slowly, less than a half mile over 100 years. If we can stop introducing them, we may be able to slow the change of natural woodlands.

The bad news is that in the northern lakes district, lakes are often less than a mile apart, so the worms eventually spread from the lakeshores and meet in the woodland middle between the lakes. And, at this point, no one knows how to get rid of earthworms.

If you fish and ordinarily dump your leftover worms in the woods, cease and desist. And if you have a compost pile or garden full of worms next to the woods, please bury a metal barrier around the garden to prevent the earthworms’ dispersal. The understory of our northern forests, and all the associated wildlife, is literally at stake.

Hemlock Wooly Adelgid

The hemlock woolly adelgid (pronounced a-DEL-jid), a native of Asia, is only 1/32 inch long – perhaps the size of a period in one of these sentences – but its arrival in Virginia in the mid 1950s loomed as an Armageddon for Eastern hemlocks.62 The hemlock woolly adelgid feeds on the sap at the base of hemlock needles, sucking the tree’s juices and injecting a toxic saliva which disrupts nutrient flow and causes the needles to eventually fall off. Without needles, the tree starves to death, usually within three to five years of the initial attack. White, woolly masses, which resemble tiny cotton balls at the bases of hemlock needles, indicate an infested tree.

Without any native predators to keep it in check, this tiny aphid-like insect spreads at an average rate of 15 to 20 miles per year, blown by winds, carried by birds and other wildlife, and transported by infested nursery stock. As of 2016, it had killed or caused extensive declines in millions of hemlocks in eighteen states from Georgia to Maine, and has edged its way west into Ohio. The adelgids produce several generations per year and have an extremely high reproductive potential (up to 300 eggs per female), so in computer parlance, they go viral very quickly.

This insect now infests about one half of the native range of hemlock in the eastern United States, and researches have been working furiously to find a way to counter it. In 2002, the Great Smokey Mountains National Park began releasing predatory beetles that feed exclusively on adelgids, and by 2011, the park had released over half a million beetles. It will take years before populations of beetles increase enough to naturally control adelgid infestations, but preliminary monitoring results have some researchers encouraged, while others remain doubtful.

The good news for hemlock in the northernmost states is that cold winters may still stop the HWA in its tracks. Hemlock wooly adelgids use supercooling, equivalent to an antifreeze, to survive the winter. But they are tolerant of the cold only to -5°F, and then they begin to die.

Of course, the bad news is that with climate change, our winters are warming, and all bets are off as to whether Wisconsin will remain cold enough to deter the wooly adelgids.

As of 2018, the tiny hemlock wooly adelgid is poised not too far from Wisconsin’s horizon. If the adelgids arrive and thrive, how will we describe to someone 100 years hence what the dark hush of a hemlock forest looked like, smelled like, and felt like?

Chestnut Blight

Chestnut Blight – A Non-Wisconsin Story with a Wisconsin Twist

In the 1800s, one in every four hardwood trees in North America's eastern forests was an American chestnut. Together, chestnuts and oaks dominated nearly 20 million acres of forest from Maine to Florida and west to the Ohio Valley. Every spring so many chestnut trees erupted in white blossom that, from a distance, the hills appeared white with snow.[i]

Reaching heights of 130 feet and over 6 feet in diameter, American chestnuts were home to bears, deer, squirrels, and all manner of small mammals and birds, all feasting on fallen chestnuts. Native Americans, too, relied on the large crops of nuts, and European settlers utilized the lightweight, rot-resistant, straight-grained wood to build houses, barns, telegraph poles, railroad ties, furniture, and musical instruments, while the bark provided tannin for tanning leather. Ecologically, the American chestnut was king, determining in large part the physical structure and microclimate of the forest.

The death knell rang for the chestnut-dominated Eastern forests when a New York City nurseryman named S. B. Parsons imported Japanese chestnut trees in 1876. Other nurseries followed suit, and somewhere within one or many of these shipments the pathogenic fungus Cryphonectria parasitica hid. First discovered in New York State in 1904, the fungus spread throughout eastern hardwood forests at a rate of 24 miles per year. Within 50 years, C. parasitica had killed nearly four billion chestnut trees, radically changing the species composition of eastern North American forests in a relative heartbeat.

None of this happened in Wisconsin, or Michigan, or Minnesota for that matter. The natural range of American chestnut barely reached the border of Ohio and Indiana. So why is this story included in a Wisconsin book? Because the largest remaining stand of mature American chestnut trees left in America grows in West Salem, La Crosse County, Wisconsin, and thus Wisconsin, though it never was home to chestnuts, may have a pivotal role in reestablishing native American chestnuts.

How is this possible? In the late 19th century, a Wisconsin settler named Martin Hicks planted nine chestnut trees that quickly multiplied. Approximately 2,500 chestnut trees now grow on 60 acres, all of which were planted hundreds of miles outside the natural range of American chestnut, and thus escaped the initial onslaught of chestnut blight.[ii]

That was the good news. The bad news is that in 1987, scientists found blight in the stand, perhaps brought in on the boots of visiting New England scientists. Early efforts included the immediate removal and destruction of any tree suspected to have blight. But the fungus continued to move through the stand, and eventually a team of scientists proposed a new tactic – inoculating the trees with a strain of the fungus that had its own disease caused by a virus. In all, about 310 trees have been treated, while other trees are recipients of the virus through the natural spread of the fungus.

The largest remaining stand of mature chestnut has now become the largest natural laboratory in the fight against blight. At first, the inoculated virus appeared to be failing. But in 2008, people began noticing changes when one of the largest trees that was initially treated and thought to be dead began putting out new branches 30 or 40 feet off the ground. Other chestnuts have also begun rebuilding their crowns. It’s unclear, but the virus may have worked, though the jury remains out.

Forest scientists have toiled for decades to restore the American chestnut to its native habitat. One semi-successful strategy has been mating American chestnut with the blight-resistant but much smaller relative, the Chinese chestnut, selectively breeding the hybrids to achieve a tree that is as genetically and physically similar to an American chestnut as possible, yet still resilient.

Another promising route may be through genetic engineering, a pathway that is fraught with its own unknowns. By taking genes from wheat, Asian chestnuts, grapes, peppers and other plants and inserting them into American chestnut trees, researchers have created hundreds of transgenic trees that are almost 100 percent genetically identical to wild American chestnut, yet immune to C. parasitica.

In the meantime, American chestnut still lives in the East as a shrubby minor understory component, existing as sprouts from old stumps and root systems. The living stumps send up young, skinny shoots but these almost always succumb to blight by their teens or 20s, never getting old enough to flower and reproduce.

If you’d like to see a unique mature American chestnut in Wisconsin, visit Bayfield in far northern Wisconsin, where Susan Larsen and Neil Howk, innkeepers at the Grey Oak Guest House, will graciously tell you the story of the old chestnut in front of their home. It’s failing in its old age – they had to remove the top of the tree in 2013 – though not due to the blight. But given that it’s 700 miles out of its normal range and next to Lake Superior, it’s still an iconic historical treasure that is worth taking the time to visit and to contemplate.

[i] “Every spring so many chestnut trees erupted in white blossom” see The American Chestnut Foundation website,

[ii] “Approximately 2,500 chestnut trees now grow” Gina Childs, “Chestnut’s Last Stand,” Wisconsin Natural Resources Magazine, (August 2002).


Emerald Ash Borer

Emerald ash borer (EAB,), an Asian insect first identified in Detroit, Michigan, in 2002, has become the most destructive forest insect to ever invade the U.S. In southeast Michigan, scientists have documented 99 percent mortality in forest stands dominated by green ash, white ash, or black ash. More than 60 million ash trees, ranging from one inch to five feet in diameter, have been killed by EAB in this area alone. As of 2017, populations of emerald ash borer have been found in 31 states, along with Ontario and Quebec, and the insect continues its march westward. Across Wisconsin, communities continue to closely track the spread of the tree-killing emerald ash borer. Forty-six counties are now under quarantine.

Adult EAB beetles are remarkably good at finding and colonizing ash trees. The beetles use their vision and finely tuned sense of smell to find their host trees and one another. The adult beetles are relatively innocuous, only nibbling the margins of leaves throughout their short life span. After a few weeks of leaf feeding though, the females lay their eggs beneath loose bark or in bark crevices, and then the trouble starts.

The tiny EAB larvae hatch in mid-summer and chew their way to the phloem, the tissue used by the trees to transport nutrients from the canopy down to the roots. The larvae feed in s-shaped tunnels, called galleries, and eventually move further into the tree, etching the outer ring of sapwood, which ash trees use to transport water up from the roots to the canopy. As the number of larvae builds, the ability of the tree to transport nutrients and water is disrupted by the galleries. The canopy then begins to thin, large branches die, and eventually, the entire tree succumbs, often within only a few years.

The long-term ramifications of ash mortality isn’t known, but can be expected to cascade through ecosystems, affecting nutrient cycling, hydrology, understory forest composition, and the habitat available for birds, mammals, insects and other animals.

Millions of dollars have been spent to identify, evaluate, rear and release parasitoid wasps that attack EAB in China. These are now being released in states with EAB infestations, but whether these Asian imports will be able to actually control EAB populations will take years to determine.

In the meantime, scientists are trying to find native, natural enemies of EAB in the U.S. Historically, native parasitoid wasps have evolved to find and attack the larvae of native beetles, such as bronze birch borer, that colonize stressed or dying trees. At least one native parasitoid, Atanycolus cappaerti, a wasp that did not even have a scientific name until 2010, is becoming increasingly common in heavily infested ash trees. Whether it will be able to slow the population growth of EAB remains a closely watched evolution.

Perhaps the best deterrent to EAB is severe cold. The larvae can supercool, but they die if they freeze. A 2010 study in Minnesota showed that 5% of the insects die at 0°F, 34% at -10°F, 79% at -20°F, and 98% at -30°F.64

That’s great news, but given that the larvae spend the winter under the bark of trees where they may be insulated by the bark itself and possibly by the snow, there is the question of what temperatures the insects actually experience. Temperatures under the bark can be 2 to 7°F warmer than the air temperature. However, prolonged cold well below zero minimizes the insulating effect of bark. So, that’s just one more reason why we need to embrace, and work to maintain, continued cold winters.

Some caveats: Whether EAB can become more resistant to cold isn’t known. And great variability in minimum temperatures occurs across landscapes depending on factors like south or north facing slopes and elevation.