Insects are being deployed in the war against invasive species in Connecticut

  • 10 min to read
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Claire Rutledge, associate agricultural scientist with the Connecticut Agricultural Experiment Station, shaves the bark from a young ash tree looking for emerald ash borer larvae and parasitoids.

After wandering through the forest at Cromwell Meadows Wildlife Management Area, Claire Rutledge selects a dying ash tree and goes to work.

She pulls out her drawknife — a foot-long sturdy blade with handles on either end — and slams it into the tree at chest height, then draws it downward until the bark can be easily peeled from the tree in long vertical strips.

As she does so, she searches for evidence of emerald ash borers, an invasive beetle from Asia that is expected to kill all of the ash trees in the Northeast in the coming decade. After peeling away several strips of bark, she reveals a series of winding tunnels like switchbacks on a hiking trail that were created by the beetle’s larva as it consumed the tissue between the tree’s bark and wood. She also points out several holes in the bark created by adult beetles as they emerged from the tree to find a mate. 

But Rutledge, an entomologist with the Connecticut Agricultural Experiment Station in New Haven, and her team of seven colleagues aren’t just seeking evidence of the beetle. They’re also looking for tiny parasitic wasps, offspring of a species Rutledge had released several years earlier to kill the beetles. It’s a strategy called biological control, whereby the natural predators of the beetle in its native range in the Far East are released locally in an effort to keep the beetle in check.

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An emerald ash borer larvae under the bark of an ash tree.

The emerald ash borer was first discovered in the U.S. in 2002 near Detroit, and it slowly expanded into ash forests in nearby states. It was found in New York in 2008 and Connecticut and Massachusetts in 2012, though it probably arrived a few years earlier. Although its rampage through the region isn’t expected to end before every mature ash tree is dead, scientists like Rutledge hope that efforts to control the insect by releasing the parasitic wasps will allow future generations of the trees to fend off the invader.

The wasps use their long stinger-like ovipositor to lay their eggs through the bark and into the beetle larvae. When the wasp larvae hatch, they kill the beetle larva by eating it from the inside out.

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“For a little while, the beetle larva keeps eating the tree and looks fine, but eventually it stops looking so fine and looks like a bag of Cheetos with a bunch of wasp larvae in it,” says Rutledge, who has released at least one of three species of parasitic wasps at 14 sites around the state, beginning in 2013.

She measures the success of her efforts by whether the wasps are sustaining themselves in the environment and by collecting and dissecting emerald ash borer larvae to determine how many have been parasitized by the wasps. “We’re recovering the wasps all over the place, so they seem to be doing pretty well,” she says. “And 20 to 40 percent of the beetle larvae we find are killed. So we consider it a success.” 

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A beetle species native to Asia, the emerald ash borer was first spotted in the U.S. in 2002 and in Connecticut a decade later. Their larvae live within and feed on ash trees; with no native predators, these destructive insects have proliferated in the Northeast and now threaten the entire ash tree population. 

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Holes and bark damage on the stump of a removed ash tree, which was damaged by the emerald ash borer


Non-native insects and plants have been invading the U.S. for more than a century, costing billions of dollars and causing significant ecological harm. Removing these invaders by conventional means — the application of chemical pesticides and herbicides or manual removal of plants — is a labor-intensive exercise that seldom works for long. And although biological control does not completely eliminate the problem either, practitioners say it is a self-sustaining strategy that is cost-effective and causes less harm to the environment than chemical methods.

“With biocontrol, we’re dealing with a pest that comes from someplace else, and it left all of its natural predators behind,” Rutledge says. “We’re trying to reintroduce them to those predators to help keep them at manageable levels.”

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Jian Duan (left), entomologist with the USDA Agricultural Research Service, and Claire Rutledge, associate agricultural scientist with the Connecticut Agricultural Experiment Station, look at emerald ash borer larvae under the bark of an ash tree in a study area at the Cromwell Meadows Wildlife Management Area.

It’s a practice that has its origins as a means of controlling crop pests in China more than a thousand years ago. In the U.S., it was first used by the Department of Agriculture in the 1880s, and by the turn of the century it already had its first success story — the eradication of an invasive insect called the cottony cushion scale that was wreaking havoc on California’s emerging citrus industry. When an Australian ladybug was released to kill the scale, it succeeded beyond all expectations. Since then, hundreds of insects have been identified to control exotic forest pests, aquatic weeds and many other invasive species.

In New England, one of the most successful biological control efforts focused on a sawfly called the birch leafminer, an insect native to Europe that was first discovered in Connecticut in 1923. The pest makes the leaves of birch trees turn brown and fall off. In the 1970s, several insects known to parasitize the leafminer in Europe were released at numerous sites from Pennsylvania to Newfoundland, and by 2007 the invader was no longer detected in the region.

Not every attempt has been as successful, however. Periodic news reports still raise the issues that resulted from hundred-year-old biocontrol efforts that have become unfortunate examples of what not to do. Most point to cane toads in Australia and mongooses in Hawaii, which were released to fight invasive pests but which became even bigger problems themselves. Similarly, several non-native parasitic insects were released to control gypsy moth caterpillars in the U.S. a century ago, but they were later found to also kill the caterpillars of numerous beneficial moths and butterflies.

“The science has advanced significantly since those days, and a lot of it is with an eye toward avoiding horror stories like those,” Rutledge says. “We know a lot more about what works, and we do extensive testing to make sure that what we introduce is going to be specific to the host we’re targeting. Now we have a much better handle on things.”

Or, as Rutledge’s colleague at the University of Massachusetts, Roy Van Driesche, says, “You wouldn’t judge your risk of having open heart surgery by outcomes from the 1950s, would you? We’ve learned a lot since then.” 

Emerald ash borer

The invader: Emerald ash borer, a beetle native to Asia that’s killed untold numbers of ash trees in the U.S.

The defenders: Parasitic wasps, (from top) Spathius galinae, Tetrastichus planipennisi and Oobius agrili, whose larvae feast on the beetle larvae.


Today, testing of potential biological control agents is undertaken in high-security quarantine labs where years of “host-specificity tests” are conducted to ensure that the insect being released won’t kill non-target native species. It can take up to 10 years of testing and about $1 million in research funding before scientists are convinced that an insect is safe to release. Then they must petition the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service for a permit to release the insect.

At the quarantine lab at the University of Rhode Island, Lisa Tewksbury and a team of students are testing insects for the control of a variety of invasive plants and raising biocontrol agents for release around the region to fight pests. In collaboration with Gail Reynolds at the University of Connecticut Cooperative Extension, she is working to release a parasitic wasp known to control the lily leaf beetle, a blood-red beetle native to Asia and Europe that has killed populations of native and ornamental lilies throughout the Northeast. The beetle is no longer a serious problem in Rhode Island and eastern Massachusetts, thanks to the wasps, but control efforts in Connecticut are still underway.

Lily leaf beetle

The invader: Lily leaf beetle (left), aka scarlet lily beetle, indigenous to parts of Europe and Asia; feeds on the leaves, stem, buds and flowers of lilies.

The defenders: Parasitic wasps, Diaparsis jucunda (top right) and Tetrastichus setifer, whose larvae feed on the beetle larvae.

According to Reynolds, the adult beetles graze on lily leaves and flowers, but the beetle larvae are more destructive, eating almost the entire plant and leaving nothing but a dead stalk. “For people who love lilies, it’s heartbreaking,” she says. “It’s a problem throughout Connecticut, especially for gardeners who like to grow Asiatic lilies, and it’s a really big problem for commercial growers.” Although the beetles can be picked off by hand, that’s not a practical solution for most gardeners.

Reynolds calls the parasitic wasps that control the beetles “teeny tiny parasitoids you can barely see, like a tiny speck of dirt,” but they can overpower the much-larger beetle. “The beetle is an eye-catching red, but the larvae are not endearing because they have a fecal shield — they carry all their poop on their back,” she says. “They’re really disgusting.”

Like the parasitic wasps that control the emerald ash borer, the wasps used against the lily leaf beetle insert their ovipositor into the larvae of the beetle to lay their eggs, and when they hatch, the wasp larvae kill the beetle larva from the inside. “Then, when the wasps emerge, you have more wasps to keep the lily leaf beetle at bay,” Reynolds says.

When the wasps are ready to be released, typically in May or June, Tewksbury sends them to Reynolds in a cooler via overnight mail, and Reynolds releases as many as 100 at a time at various sites around the state. Selecting those sites, however, has been more challenging than she imagined because the wasps aren’t an overnight success.

“At first, I sent an email to garden clubs and master gardeners, and they were really interested,” she says. “But many people are impatient; they’re looking for a silver bullet. It takes four or five years for the wasp population to build up, and many people couldn’t just sit on their hands and wait for it to happen. A lot of them sprayed pesticides or pulled their lilies out instead of waiting for the wasps to do their job.”

Swallow-wort

The invader: Swallow-wort, a close relative to the milkweed plant that is toxic to the caterpillars of the struggling monarch butterflies.

The defender: Hypena opulenta moth, native to Eastern Europe and the Middle East; feeds exclusively on swallow-wort leaves.

But after finding enough people willing to give the wasps the necessary time, the wasps are spreading throughout the state and lily leaf beetle numbers are declining. The project will likely be discontinued in a year or two as biocontrol agents are approved for other pests and funding shifts to more damaging invasive species.

Next up is the release of a moth whose caterpillar feeds on swallow-wort, a European plant introduced as an ornamental by the horticulture industry that has spread into the wild. Swallow-wort is a close relative of milkweed, which monarch butterfly caterpillars feed on, but swallow-wort is toxic to monarch caterpillars.

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Tewksbury has made test releases of the moth at the home of an entomologist in Redding who is tracking its success, and she hopes to release them this year at Bluff Point State Park in Groton, where a large area is covered in swallow-wort. The moth is already having modest success controlling the swallow-wort population in parts of southern Ontario, and releases have begun at several other Northeast states.

“We release adult moths, egg laying happens soon after, and their larvae do the feeding damage on the plant,” Tewksbury says. “We want them to pupate and have a second generation of adults lay eggs and those larvae do some feeding damage. Then those pupate and remain in the soil and emerge next year to start the process over again. At least that’s the hope.”


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Carole Cheah, a scientist at the Windsor office of the Connecticut Agricultural Experiment Station, raises insects to be released to kill invasive pests.

At the Windsor office of the Connecticut Agricultural Experiment Station, Carole Cheah raises poppyseed-size black ladybugs in her laboratory in an underground bunker. In dozens of clear plastic containers on shelves lining the walls are sprigs from native hemlock trees infested with a tree-killing pest from Japan called the hemlock woolly adelgid. And feeding on the pests are the nearly invisible ladybugs.

Cheah has been studying the adelgid for 26 years, beginning not long after it was first discovered in the New Haven area in the 1980s, though it first appeared in the U.S. in the 1950s at a private arboretum in Virginia. Her mentor, Mark McClure, traveled to Japan to identify the adelgid’s natural enemies and found a mite that he thought was promising as a biocontrol agent. In the course of studying the mite, he stumbled upon the ladybug, and Cheah was hired to investigate whether the ladybug was the adelgid predator they were looking for. It was. She has been raising and releasing them ever since.

The adelgid, an aphid-like insect that spins a white wooly cocoon around itself on the underside of hemlock needles, feeds on cells in the tree’s stems, which inhibits the tree’s ability to produce new foliage. Hemlocks throughout Connecticut — except in the high elevations of the northwest part of the state — were infested with the adelgid in the 1990s, but the ladybug appears to be succeeding at keeping it under control.

Beginning in 1995 at a town forest in Windsor, Cheah released about 10,000 of the ladybugs at 16 sites around the state — a total of 178,000 ladybugs — in an effort to quickly eradicate the adelgid.

Hemlock woolly adelgid

The invader: Hemlock woolly adelgid (left), an aphid-like insect native to East Asia that feeds on hemlock and spruce trees.

The defender: Asian lady beetle (Sasajiscymnus tsugae), tiny, ladybug-like beetles that devour adelgids.

“We came up with the idea of doing a high number of releases at each site, and it has really paid off,” Cheah says. “Our idea was to spread them around the landscape in every county of the state, and if it survived and multiplied, it would have the effect we wanted.”

During a late-winter visit to Salmon River State Forest in Colchester, where 10,000 of the ladybugs were released in 2001, Cheah inspects each of the 15 trees she monitors every year and assesses their health using a variety of metrics. She stands back to look at the whole tree to estimate how much has live foliage, then stands beneath the tree and looks straight up to rate how much skylight can be seen through the foliage as a measure of foliage density. 

After completing several other measurements, she approaches the next tree on her list. She grabs a low branch to look for signs of the adelgid and, with obvious satisfaction, says, “no little wool balls here.” At a third tree, she notes plenty of dead twigs, but she decides that the tree’s poor condition is more likely the result of a recent drought rather than the adelgid.

“When I see a dying tree, I want to know why it died. It’s not always the adelgid,” Cheah says. “When we started this, a lot of people said that the trees were going to die. But I’ve got news for you; they don’t die. Give them a chance; they’re resilient.” 

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The next invasive pest that entomologists expect to fight using biocontrol methods is the spotted lanternfly, a planthopper native to China, India, Vietnam and eastern Asia that was first detected in Pennsylvania in 2014. It feeds by sucking the sap from the leaves, stems and trunks of more than 70 different plants, from wild and cultivated grapes to hardwood trees, vegetables and roses. It has already had a devastating impact on the wine industry in Pennsylvania. Officials in that state have been imploring residents to kill them on sight and scrape their egg masses off trees before they hatch in May. Although spotted lanternflies have not arrived in Connecticut in big numbers yet — they were first spotted in Southbury last October — staff at the Connecticut Agricultural Experiment Station expect it to eventually threaten the state’s agriculture industry. So scientists are already working to identify the pest’s native predators in China and testing them in quarantine to see if they will be successful biocontrol agents. So far, a parasitic wasp and a fungal pathogen are showing promising results.

As she returns to the parking lot at the state forest, she smiles and says, “These trees are all clear. It’s better than I could have hoped for.” The ladybug is doing equally well elsewhere around the state, and it’s doing a better job of beating back the adelgid than the three or four other biocontrol agents released in other states.

“It’s been a success, but it’s not all due to biological control,” she acknowledges. “It’s important to know the whole ecology of the tree and all the factors that influence it. We had four years of severe winters that single-handedly brought down the adelgid population by a lot.”

Determining the success of most biological control efforts usually takes many years of monitoring, which can be challenging for those seeking immediate results.

“Are we going to have ash trees as a component of our forest in the future?” asks Claire Rutledge, staring up at a dead tree. “That’s going to be a 10- to 15-year answer. We have high hopes, and things are looking good so far, but we have to be patient, and that can be really hard.”

(Correction: The original version of this article included a photo of a different insect that was misidentified as an emerald ash borer.)

This article appeared in the May 2020 issue of Connecticut Magazine. You can subscribe here, or find the current issue on sale hereSign up for our newsletter to get the latest and greatest content from Connecticut Magazine delivered right to your inbox. Got a question or comment? Email editor@connecticutmag.com. And follow us on Facebook and Instagram@connecticutmagazine and Twitter @connecticutmag.