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Invasive Alert: Hemlock Woolly Adelgid is Now Prevalent – and Spreading – along Maine’s Coast 

Release date: April 20, 2023

Hemlock woolly adelgid will kill a healthy hemlock in a matter of years, so it’s critical that people act. Thankfully, Maine’s land trust community is taking a coordinated approach. 

 (APRIL 11, 2023, Topsham, Maine) The hemlock woolly adelgid has made its unfortunate entrance into the state of Maine. According to Maine Forest Service entomologist Colleen Teerling, the pest was first found in the southernmost forests of Maine in 2003. Over the last two decades the adelgid has spread, making its way through the eastern hemlocks of Maine’s coast. While the adelgid was initially more prevalent in southern Maine, its range is growing. Damariscotta and the Pemaquid Peninsula, Camden, Thomaston, Rockland, and now Mount Desert Island are all seeing it. In Waldoboro, some of the state’s oldest eastern hemlocks (trees that pre-date the Revolutionary War) have been infected. And the insect’s range is growing as it moves inland and eastward. 

 Thankfully, there’s a lot that Mainers can do to manage the spread of this invasive. Maine’s land trusts are at the center of a coastwide effort to curtail its spread. 

 “Eastern hemlock is an incredibly important species of tree in Maine,” says Joan Ray, Director of Land Conservation for the Coastal Rivers Conservation Trust. Ray is also facilitating a Hemlock Woolly Adelgid Working Group within the 12 Rivers Conservation Initiative, which includes seven land trusts between Bath and Belfast, all of which are seeing growing evidence of the invasive pest.  

 “If we lose our eastern hemlocks, Maine’s landscape will be altered dramatically and in some ways be unrecognizable,” noted Ray. 

 Hemlocks are important for many bird species, including hermit thrush, black-throated green warbler, Blackburnian warbler, and blue-headed vireo. Because they overhang rivers, streams, lakes, and ponds, they are also critical for providing shade and cooling for cold-water fish and amphibians. Their large boughs provide protection from heavy snow and create important deer wintering yards too. 

 “Hemlock trees are extremely important in riparian areas – on the edges of streams and lakes – because they help to regulate the temperature of cool water,” noted Teerling. Keeping streams cold is critical for trout, salmon, and a wide range of native species. 

 “We are seeing quite a lot of hemlock woolly adelgid in southern and coastal Maine, but it is slowly moving eastward and inland,” warns Teerling. “Cold winters slow the spread, so with climate change we just don’t have the reliable protection of winter anymore,” she added. 

 On Mount Desert Island hemlock woolly adelgid has been found in Acadia National Park, Seal Harbor, the Long Pond-Somes Pond watershed, and on a few local preserves. The Land & Garden Preserve and the Park are working to save a beloved stand of hemlocks along Jordan Stream and Maine Coast Heritage Trust plans to keep a close eye on public preserves, using kiosks to offer information to visitors.  

 Maine Coast Heritage Trust is also monitoring preserves in Brunswick, Topsham, Harpswell, and several islands along the Maine coast, including Clark Island in St. George and Sheep Island, Monroe Island, and Ash Island in Owls Head.  

Quick Action Needed to Slow the Spread  

Because the hemlock woolly adelgid can kill a healthy hemlock tree in a matter of several years, it’s critical that people take action to help. Teerling emphasizes, the actions of a few can make a big difference. It’s important to know that the mobile stage of the hemlock woolly adelgid is from March to July. This is when the juvenile can easily latch onto clothing, pets, cars, and equipment. “All it takes is to move one adelgid and you can start a new infestation,” says Teerling. Because it’s an invasive species that does not belong here, we don’t have natural control measures, so slowing the spread is important, she noted. 

 Recommendations to Slow the Spread 

 Prune infested hemlock trees (look for white fuzz on the underside of needles).  

  • It’s also important to prune branches that are not infested, especially those along driveways, roadways, and trails. 
  • Pruning in driveways should be done high and wide, especially where garbage trucks, delivery trucks are likely to be travelling from one property to another providing a vector for widespread dispersal of hemlock woolly adelgid.  
  • Leave pruned branches (both infested and not infested) on the ground or pull them deeper into the woods. They can be transported safely in the fall. 
  • Avoid parking underneath hemlocks (the crawlers latch onto cars). 
  • Avoid brushing up against hemlock branches from March to July. This includes dogs too. 
  • If you do brush against hemlocks that may be infested, be sure to run your clothes in the dryer on high for 20 minutes to kill the crawlers. 
  • Take bird feeders down in areas where hemlocks are near during the active crawler phase (March-July). 

How the Adelgid Spreads and Multiplies
 The hemlock woolly adelgid is small and difficult to see with the naked eye. According to Teerling, the white fuzz that you see on the underside of hemlock needles is a protective coating, not the insect itself. Adelgids feed all winter and spring, sticking their mouthparts into trees and feeding on its starches, effectively killing a tree. The adelgids hibernate and are inactive in late summer. All adelgids on hemlock are female, producing two generations of eggs per year with as many as 250-500 eggs per generation, says Teerling. 

Land Trusts Deploying Biocontrol Beetles as Predators 

The members of the Woolly Adelgid Working Group and other organizations are experimenting with release of a biocontrol beetle known as Sasajiscymnus tsugae. This beetle, native to Japan (where hemlock woolly adelgid originated), is a specialized predator of hemlock woolly adelgid. In states like Connecticut and New Jersey the release of the beetle, which is grown in labs in Pennsylvania, has shown significant improvements in hemlocks. Releases of beetles are planned for this May and in early June in several locations along the Maine coast.  

  • Maggie Cozens, Southern Maine Outreach Manager for Maine Coast Heritage Trust, says her organization has purchased 10 colonies of the biocontrol beetle and is planning on releasing five colonies at Woodward Point Preserve in Brunswick and five colonies at Cousins River Field & Marsh in Yarmouth. There are about 100 beetles per colony. MCHT plans to host workshops so individuals can release biocontrol beetles on their own. 
  • Coastal Rivers Conservation Trust has been working on the adelgid issue for four years and will be participating in a third beetle release this May or early June at several locations.
  • The 12 Rivers Conservation Initiative formed the Hemlock Woolly Adelgid Working Group last year to tackle the wider area from the Kennebec River up to Penobscot Bay. They are also planning beetle releases this spring.
  • The Land & Garden Preserve has released Laricobius osakensis predator beetles into a beloved stand of hemlocks that shades an iconic cobblestone bridge along the carriage roads. The preserve plans to use Sasajiscymmus tsugae this year or next, depending on the population of adelgids. The Somes-Meynell Wildlife Sanctuary plans to do the same in the Long Pond-Somes Pond watershed. 
  • Cape Elizabeth Land Trust experimented with a release in May 2013 in Robinson Woods. It seems that hemlock woolly adelgid is not prevalent there.
  • Acadia National Park is collaborating with other MDI land managers on their collective response and is developing an Integrated Pest Management program for hemlock woolly adelgid that may include biocontrol in the future.  


When asked about employing beetles as a control method, Teerling is cautiously optimistic. “It’s hard for me to answer, especially as a scientist, because in a forest situation we don’t have controls and experiments that can prove their efficacy,” she says. But she can confirm that in almost every location in Maine where beetles have been released since 2007, they are still out there, surviving, and presumably doing their job of consuming the hemlock woolly adelgid. 

There is not yet enough evidence to prove whether these beetles alone can control hemlock woolly adelgid in Maine’s forests. Although the state does not specifically recommend that people buy them, Teerling says they will “absolutely support those who want to do it and will do all they can to help ensure that the beetles successfully establish.”

Maine has time to slow the spread of this invasive pest, she says, and she appreciates the concerted work of Maine’s land trust community. “I’m so grateful to land trusts for their willingness to take on a complex situation,” noted Teerling. “They are doing an amazing job to raise awareness about the hemlock woolly adelgid.”