Linda Stathoplos, a retired NOAA researcher and oceanographer, searches the base of the jetty for remnants of bugs that washed ashore on Wells Beach in June 2021. Stathoplos used a microscope to determine that the remains of small black flies likely stained the feet of beachgoers. Ben McCanna/Staff Photographer

Piles of millions upon millions of dead insects that stained beachgoers’ feet in southern Maine last summer were the end result of an exploding population of an invasive pest slowly killing the region’s native hemlock trees, according to new research from state and federal scientists.

Alarmed residents reported a strange black substance that stained the bottom of their feet while walking on beaches in Wells and nearby coastal towns last year. Scientists quickly identified the odd stuff as the collective corpses of unidentified winged insects, which yielded a black ink-like substance when crushed.

Hemlock wooly adelgid in fly stage Nathan Havill/USDA Forest Service

But it took further research and DNA analysis to identify the flies and reach a disturbing conclusion – the phenomenon was caused by a massive die-off of hemlock woolly adelgid, a species native to East Asia that threatens hemlock trees from Nova Scotia to Georgia. The findings illustrate the scale of the problem in coastal Maine, aided by mild winters likely to become more common in future years.

“It makes you think – if that many are washed up on the beach, how many more are on the trees damaging them?” said Nathan Havill, an entomologist with the U.S. Forest Service and lead author of a paper investigating the incident recently published in the Journal of the Acadian Entomological Society.

The foreign adelgid’s stunted lifecycle outside of its native environment and a series of mild winters created a scenario in which clouds of trillions of doomed insects were likely swept out to sea, died, and were brought back on the tides, researchers found. Last year was the first documented incidence of a mass die-off at this scale, but it could have happened before and may happen again.

The Maine Department of Environmental Protection has received one report of black masses and stained feet this year in Kennebunk, but was unable to verify it.

The chain of events that led to piles of dead insects on beaches and shorelines from Maine to Massachusetts is tied to the life cycle of the invasive adelgids, introduced to the East Coast in the 1950s.

A hemlock branch infested with hemlock wooly adelgid. Photo courtesy of the USDA Forest Service

Adelgids have two generations each year. In its flightless form, the insect develops white, woolly nests on the underside of native hemlock branches. Where it is invasive, the insect feeds off the trees and injects a toxin, eventually weakening and then killing its host over a period of years.

In the second stage, some of those insects transition to a tiny winged version that emerges in early June, moves on to spruce trees and eventually reproduces. But its survival as a fly depends on a type of spruce tree that does not grow on the East Coast – so the winged form is doomed from the moment it emerges.

The shortened lifespan also means that adelgid on the East Coast are all females that reproduce asexually – producing genetic clones, generation after generation.

When adelgid populations grow too big, more of the insects transition to their winged form, Havill said. And the adelgid is particularly abundant right now in areas of southern Maine and New Hampshire. “It is just producing lots and lots of offspring. A proportion of those became this winged form that apparently got blown out to sea.”

While first reported in Wells, the same adelgid piles were found in York, Kennebunk, Ogunquit, Great Common Island, New Hampshire, and Gloucester, Massachusetts. A boater in Beverly, Massachusetts, in early June last year reported thousands of tiny insects falling from the sky, littering the boat and water.

Earlier die-offs could have happened without anyone recognizing it, said Maine State Geologist Steve Dickson.

Last year’s winds and tides may have hit the right balance to bring the flies to people’s attention. Scattered reports of black feet and other observations suggest the phenomenon could have occurred this year, but was obscured by high tides or unfavorable winds, Dickson added.

“No one has seen them in large numbers. Either they are spread out in more time or the wind and the weather spread them out,” he said.

Maine forest managers in the last two years noted that mild winters allowed a growing population of invasive adelgids in coastal Maine, said State Entomologist Allison Kanoti.

“It is not something that anyone has in their memories, it is not something we could find readily recorded anywhere,” she said. “The extent of the population being so high you could see them deposited on beaches and staining people’s feet – that is an enormous population of insects.”

Coastal hemlocks that often grow in thin soil are at risk from adelgid infestation worsened by recent droughts. Peninsular towns midcoast Maine have widespread hemlock decline and scattered mortality and the problem extends to Casco Bay and York County, Kanoti said. Pests will likely move farther inland as winters warm and coastal hemlock are depleted.

Kanoti credits watchful citizens for alerting scientists in the first place.

“It’s a good example of how folks actually recognizing a new issue can be really helpful in further advancing our understanding of what is going on,” she said. “In this case, it was concerns of beach quality that turned out to be a forest health concern.”