Forest workers in Maine are counting on hundreds of beetles to combat a growing infestation of potentially harmful pests in Vaughan Woods in South Berwick. The Maine Forest Service released 1,500 black lady beetles into the state park on May 5 to help reduce its population of hemlock woolly adelgid. 

HWA, as they are commonly called, are invasive, aphid-like insects native to Japan. They resemble miniature cotton balls and form white, woolly masses on the underside of hemlock twigs and at the base of needles. The insects cause infested trees to have off-color needles, premature needle drop and twig dieback.

“They can cause pretty serious decline in hemlocks and they can kill hemlocks, as well,” said Allison Kanoti, an entomologist with the Maine Forest Service. “The hemlock is a unique tree species in that it’s very shade tolerant and so it keeps its branches down toward the ground, which provides important structure for wildlife. And it’s often found along streams and rivers, so it can be important to preventing erosion.” 

HWA were introduced to the Eastern United States in the 1950s and were first observed in Vaughan Woods last December. Researchers conducted a targeted survey for the insects in March and found the population had spread to additional spots. They have been found in less than 2 of the trees but are concentrated along streams.

“When you have decline of hemlock, you end up with things like decline of water quality,” Kanoti said.

The tiny, oval-shaped black lady beetle (Sasajiscymnus tsugae), also native to Japan, was brought to the United States under quarantine and has been released into several forests to fight HWA. Authorities hope the beetles will prey on the HWA and keep their population at a level the trees can tolerate. 

Although the beetles have been closely monitored, Kanoti said, there is no guarantee their introduction into Vaughan Woods won’t have unintended consequences.

“You can never eliminate this possibility,” she said. “Ecosystems are very complex, so there can always be things that you don’t anticipate.”

Kanoti said the Forest Service will continue to monitor the park, although a precise management plan is still being developed.