New Invasive Species a Threat to Native Ash Trees

If one was to fly a helicopter or plane over the state of Maine, you would see trees, trees, and even more trees. Maine has the most trees in the United States; ninety percent of the state is covered by forests. Maine’s forests contain a wide variety of tree species with over 55 species sampled in 2017. Out of those 55 species of trees, a few of them are in danger of extinction, One of which is the ash (Fraxinus) tree.

New invasive species a threat to native ash trees

Ash trees are vital to natural forest environments and shelterbelts as they are a fast growing species, and are often one of the first hardwoods to colonize disturbed forests and along river banks. A shelterbelt (windbreak) is a planting most often  made up of one or more rows of trees or shrubs planted in such a manner as to provide shelter from the wind and to protect the soil from erosion. Ash trees are incredibly tolerant of a wide variety of soil conditions including wetlands and riparian corridors.  What is not commonly known is that ash trees are a key component of North American forests. They provide food and shelter for birds, squirrels, and insects, and they support important pollinator species such as butterflies and moths. They also support a rich and diverse ground flora. Shed ash leaves retain more nutrients and are recycled more rapidly than those of most other trees, making the soil rich for the plant life found close by. 

Approximately four percent of trees in Maine are ash trees; this number may seem small, but it translates to a whopping 400 million ash trees over an inch in diameter. Knowing that 4% of the trees in Maine are ash trees, and they number 400 million trees, the sheer magnitude of the actual amount of trees we have in Maine is enormous.  

There are three species of ash trees native to Maine. They are the green ash (Fraxinus Pennsylvanica), the white ash (Fraxinus Americana), and the black ash or brown ash (Fraxinus Nigra). The Green ash is the most widespread of the three ash species native to the state of Maine. Both the green and the white are the two most common of the three ash species. Ash trees have several useful features; the wood of the green ash is often used as a source of firewood while the bark of the north eastern subspecies is used for dye and a source of potash.

The black ash, however, is especially important to the Wabanaki tribes of Maine. The tribes - Mi’kmaq, Maliseet, Passamaquoddy, and Penobscot - consider the black (or Brown) ash to be the source of life in Wabanaki creation stories, and central to Wabanaki culture. The black ash also plays a critical role in basket-weaving practices because the wood possesses the flexibility and strength needed to make such intricate works of art.

The ash species are beautiful and are easily distinguishable by several key features. They sport a smooth bark at a young age, but when they mature, they develop deep furrows resembling a diamond pattern or a corky appearance. Their branches are also easy to spot; opposite branching makes it easy to pick out an ash tree, while its compound leaves have anywhere between 5 to 11 leaflets on each leaf. Ash tree seeds are single, oar-shaped samaras that hang in clusters. Samaras typically stay on the tree until late fall. 

Now imagine 4%, or 400 million, ash trees missing from the landscape. It wouldn’t happen overnight, of course, but over a long period of time it's possible. And it’s happening right now. Four hundred million ash trees in Maine are left, and that number will continue to get smaller. 

Why, who, or what is the culprit for this devastation?

Enter the emerald ash borer. 

The emerald ash borer (Agrilus planipennis) is a relative newcomer to the United States that was first discovered in southeastern Michigan near Detroit in the summer of 2002. The EAB, as it is often referred to by its acronym, is a jewel beetle native to north-eastern Asia that feeds on ash species. Scientists believe that the emerald ash borer was introduced during the early 1990’s when they were discovered to have infested wood packing materials from shipments overseas. 

The EAB is one of the most serious of the invasive species attacking our forests in Maine and across the country. These green beetles are especially dangerous in America because they threaten many species of ash trees. What makes them singularly unique is that native species of boring beetles would bore into dead ash trees, while the emerald ash borer would bore into still-living ash species. They also lack a natural predator since they are newly arrived to the northern American continent, but that may change in the future as they are becoming targets for woodpeckers and nuthatches. 

This northeastern Asian green beetle was first found in Northern Aroostook county in May of 2018, western York county in September 2018, and Cumberland county in September of the following year. Since then, the EAB has been spreading. It has become such a danger to our native ash species that efforts to stop the spread of the new invasive species are now well underway. As of February 11th, 2022, all of Cumberland and York counties, and parts of Aroostook and Oxford counties, have been placed under EAB quarantine. Outside of Maine, several other states also have been placed under EAB quarantine, including Massachusetts, New York, New Jersey, New Hampshire, Vermont, and many others. 

Besides quarantining areas where EAB is detected, other methods of prevention are being used to prevent the spread of the green beetle. The purple prism trap, provided by the USDA APHIS, is one of the most effective methods of EAB detection among traps. Trap trees are also being used with a method called “girdling”, which strips the tree of a large portion of its bark, releasing compounds into the air that attracts the EAB. Yet another method of EAB detection is through biosurveillance. The Cerceris fumipennis, a native non-stinging wasp, can help entomologists with early detection. These ground-dwelling wasps provision their nests with buprestid beetles, including the emerald ash borer. The Maine Forest Service is currently looking for colonies of this wasp in order to help detect and contain the EAB. 

The beetle is small, growing up to a mere one and a half inch; they are narrow, bullet shaped with a metallic green sheen on their shells. They are usually spotted between the months of June to August, and are often seen flying near canopies of ash trees as they are very rarely on the ground. They are similar in appearance to the native species of Tiger Beetles, and among others. Larvae, however, are roughly a half an inch to one and a half inches long, are creamy white, narrow with bell shaped segments along its body. They are usually seen year round, and are often located in the cambium layer of the ash trees.

Because they are not so easily seen as they are so small, other ways of detecting the EAB is through recognizing signs and symptoms of EAB infestation. Woodpeckers love the emerald ash borers. If you find a woodpecker on an ash tree, chances are you’ll find an infestation of EAB. Known as the “blonding” effect, woodpeckers hunt for EAB larvae and pupae, which is often found under the bark of ash trees. The larvae feed in a serpentine pattern under the bark of the ash species, making it look like S-shaped tunnels. When larvae grow to adults, they emerge from the trees, leaving behind a D-shaped exit hole. EAB infestations often cause the sprouts of ash trees to grow in an abnormal way, called epicormic shoots. Larvae feeding under the bark would cause the bark to split; excessive feeding would cause the crown to die. 

The Arboretum has many ash trees on the property, including our Green Ash Collection, which includes specimens from across the United States. This collection represents subspecies of green ash from across North America. It was initially planted as a research collection to observe differences in each of the subspecies. This collection of green ash species could offer a unique research opportunity in the future to observe differences in resistance to the EAB. As part of its ongoing efforts to provide unique research opportunities for our ecosystem, the Green Ash Collection is a vital piece of our fight against climate change, invasive species and diseases. 

While the native ash species of trees only number four percent of our hardwood forests in Maine, 400 million trees is still a staggering amount. Leaving the emerald ash borer to wreak devastation to our ash species is not an option; the EAB is yet one of many of the invasive species threatening our ecosystem, and we must do all we can to contain or stop the damage to our forests before it is too late. Viles Arboretum is doing its part to help ongoing efforts to preserve our way of life with the many events it has planned in the future.