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Nature Notes: Cold snaps and special beetle help combat hemlock woolly adelgids

Polar vortexes, those bone-chilling cold snaps that occasionally dip into Connecticut and other parts of the country, and a tiny, specialized beetle may be the two best weapons yet besides chemical controls to combat the hemlock woolly adelgid (HWA), an aphid-like insect that’s decimating the state’s hemlock forests.

That’s the assessment of Dr. Carole Cheah, research entomologist at the Connecticut Agricultural Experiment Station’s Valley Laboratory in Windsor. She has been studying the destructive pest for 15 years and collected some interesting data.

“The increased frequency and penetration of polar vortex outbreaks in winter, extending deep into Connecticut in 2014, 2015 and 2016 (including the first week of 2018) have profoundly affected the survival of HWA,” Cheah recently told a group of scientists in New Haven.

“The greatest HWA winter mortality occurred on Feb 14, 2016, when temperatures dropped to minus-25 degrees in some parts of the state for a few hours, killing 97 percent of the insects,” Cheah said.

The challenge is that hemlock woolly adelgid infestations seem to bounce back after mild winters, and that’s when HWA management and control is highly necessary, Cheah said.

The non-native pest first appeared in Richmond, Va., in the early 1950s.

It’s believed the bug hitchhiked to the U.S. on imported nursery stock from the island of Honshu, Japan. Since then, HWA has spread from Georgia to Maine and out to Minnesota, a total of 20 states. It was first discovered in Connecticut in 1985.

Cheah said the tiny insect can kill a hemlock in four to five years.

Its methods are insidious. “HWA inserts its long, piercing-sucking mouth parts into the base of hemlock needles and feeds on nutrients stored in the xylem ray cells.

In addition to removing the tree’s nutrients, the adelgid is thought to cause a hypersensitive response that restricts water transport in the tree,” according to USDA’s Forestry Service.

One option Cheah and others have been exploring is to implement a federally approved ladybeetle called Sasajiscymnus tsugae in small scale releases to target surviving HWA after heavy winter kills.

Why this beetle?

S. tsugae is native to Japan, like HWA, and specifically eats adelgids. They can also survive hot, humid Connecticut summers, and cold snaps to minus–7 degrees, Cheah said.

Armed with this information, Cheah said scientists released 178,000 ladybeetles into controlled areas in Connecticut from 1995 to 2017, recording positive results.

For example, in the late summer and fall of 2016, when HWA lay an overwintering generation of eggs, released S. tsugae adults consumed more than 1,200 HWA in tested areas, and in the spring and early summer, when HWA lays another generation of eggs, scientists projected 5,000 ladybeetles would devour some 6 million HWA.

This unique beetle is the only species that can be reared commercially and is now available to homeowners through a company in Pennsylvania called Tree Savers.

When asked if the ladybeetle could possibly run amok and eat other insect species, Cheah said in an email, “All biological controls must undergo an environmental risk assessment at the federal and state level, which approves the subsequent field release.”

“This is a very safe biological control agent,” Cheah said, adding 22 years of tests have shown that it only eats other adelgid pests like the balsam woolly adelgid and pine bark adelgid. “I think it has great potential.”

Bill Hobbs lives in Stonington. To comment, email


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