Annapolis, MarylandHanging a cluster of twigs on a tree might sound like a pointless exercise, but Heather Disque, a forest pest entomologist for the state of Maryland, isn’t hanging just any sticks in Martinak State Park.
Hundreds of immature parasitic wasps are hidden in Disque’s bundle, and when these Spathius galinae mature in several days, they’ll be driven by a single need: to lay their eggs on the larvae of the emerald ash borer, an invasive beetle that has over the past few decades ravaged many species of ash trees across the United States.
Scientists have been using parasitic wasps for 10 years, with some success, but this particular species, originally from the Russian Far East, is the best hope for keeping the emerald ash borer in check for decades to come. S. galinae can do what other wasps can’t: inject its eggs into ash borer larvae living on older, larger trees (because of a longer body part called an ovipositor)—and tolerate cold.
Once the female deposits eggs in the beetle’s gooey, white larvae, the developing wasps will consume the ash borers from the inside out. With billions of invasive emerald ash borers in 35 states and the District of Columbia, the wasps should have no shortage of food.
“It’s taken a long time to figure out how to get the right parasitoids to the right locations at the right time,” Disque says. “If we’re putting millions of dollars into this biocontrol program, we need it to work in the real world.”
A deadly invasion
In the summer of 2002, Michigan was focused on the West Nile virus, which was infecting some bird populations and had sickened more than 600 people. Soon, however, the state was trying to manage a very different kind of outbreak.
Professional arborists in Detroit had begun receiving calls about dead and dying ash trees the summer before, but it took them nearly a year to identify the culprit. Native to China, Agrilus planipennis likely arrived in the mid-90s as a stowaway in wooden pallets built from trees in China’s northeast. North American ash trees had no resistance to the Asian beetle, nor did the pest face any predators, so populations boomed. By the time entomologists discovered the invasive insect, it had already spread 250 miles south, to Cincinnati.
Officials tried to quarantine affected areas to stop the emerald ash borer from spreading, but it can fly several miles. Pesticides had little effect. The hordes of beetles simply overpowered any barriers humans threw in its path. A 2014 study found that emerald ash borers had killed more than 99 percent of medium and large ash trees in the Detroit area. In just over a decade, the insect had spread throughout the Midwest.
Almost 20 years on from the borer’s initial discovery, few counties east of the Mississippi remain free of the ash borer, and it’s moving west too. The invasive emerald ash borer cost the U.S. upwards of $10.7 billion between 2009 and 2019, including both the costs of control efforts and in the loss of the trees themselves. The U.S. Department of Agriculture calls it the most destructive and costly of several invasive wood-boring insects that have recently caused problems in the U.S.
The borer lays its eggs in the nooks and crannies of the ash tree’s knobby bark. Once hatched, the larvae begin their destructive binge. Although they can (and will) eat any kind of wood, they prefer to nosh on nutrient-rich phloem, the woody tissue that transports the sugars made in the leaves during photosynthesis to the rest of the tree. It’s particularly rich in energy and nutrients, which is what makes it attractive to the emerald ash borer and crucial to the tree.
With enough damage to its support system, the ash tree can’t move nutrients from its leaves to the tips of its roots, so it withers. Leaves turn a jaundiced yellow, then fall off completely. Death follows in four to five years.
Scientists learned all this in the frantic first years after the ash borer’s discovery, says Jian Duan, an entomologist at the U.S. Department of Agriculture. Study of the beetle in its home range—the eastern part of China that elbows its way into the Yellow Sea—revealed that the destruction there was nearly non-existent. Further research revealed the species of ash in Asia had evolved resistance to the emerald ash borer and were better able to defend themselves against the pest, but it didn’t immediately provide a solution for the dying ash trees in North America.
But the researchers discovered that it wasn’t just the ash trees’ immunity to the borer that was keeping it in check in China and the Russian Far East. Parasitic wasps there laid their eggs on the eggs or larvae of the emerald ash borer.
The wasps “keep the emerald ash borer under control, so it’s not a pest in China,” Duan says. Biologists began to wonder, could they also do it in the U.S.?
Finding the right parasite
Introducing one species to control an invasive one isn’t without risk; examples abound. In late 1800s Hawaii, biologists imported mongoose from India to deal with the archipelago’s rat infestation. But not only did the mongoose fail to reduce rat numbers, the weasel itself became an invasive species. To avoid introducing a solution even worse than the problem, biologists at the USDA’s Animal and Plant Health Inspection Service spent several years intensively studying parasitic wasps from China. Any wasp that would be used for biocontrol would need to avoid parasitizing resident North American beetles.
Several of the species came close to being effective at reducing ash borer numbers, but weren’t quite right: They were sensitive to cold and were too small to lay eggs in some tree bark.
“When the trees are young, the bark is thin, and the wasps can reach the (beetle) larvae,” says Inés Ibáñez, an ecologist at the University of Michigan. “But when the trees are big, they can’t.”
Another research expedition to the Russian Far East in the mid-2000s found Spathius galinae. It was just right, with body parts big enough to attack insects in larger trees and tolerance to cold. The first wasps were released in spring 2016 in infested hardwood stands in Connecticut, New York, and Massachusetts.
The preliminary results showed promise. When Duan and other U.S. Fish and Wildlife Service scientists sampled trees near the release sites of S. galinae, they found that anywhere from 13 to 49 percent of ash borer larvae were infected with wasp parasitizes. A previous wasp from China only parasitized 0.1–5.6 percent of larvae.
“In some places, they’ve gone from one or 2 percent parasitism to anywhere from 30 to 70 percent of the ash borer larvae. And that’s having a huge impact on the population,” says Claire Rutledge, an entomologist with the Connecticut Agricultural Experiment Station.
Effects of ash tree deaths
In her eight years working for the Maryland Department of Agriculture, Disque has watched the emerald ash borer march steadily eastward. Ash trees are a favorite of urban foresters, especially on roadsides due to their hardiness and easy maintenance, and the ash borer feasted along the roads connecting Baltimore and Washington, D.C. The ash borer arrived at Martinak State Park, a small parcel of land just east of Annapolis, Maryland’s capital, along the Choptank River.
From her van’s trunk, Disque pulls out a shoebox-size container that she calls her borer kit and pulls out a C-shaped section of ash tree bark. From the outside, the folded bark looks like it was taken from a healthy ash at least a foot in diameter. When Disque flips it over, however, lines zigzag across the bronze surface like tracks from a child’s game of Chutes and Ladders. The tracks are small—a Q-tip might barely fit through a straight section—but Disque says they riddled the entire tree, ultimately killing it.
“If you’re seeing lots of beetles, then the next year you’re probably going to see a majority of trees start to crash,” Disque says.
It’s what happened to many ash trees along Route 50, which connects Washington to Annapolis. They’re easy to miss if you aren’t looking for them. The oaks and maples still sport full summer crowns of leaves, but clumps of bare twigs appear regularly throughout. Unlike massive stands of hemlock and chestnut, which are being wiped out by the woolly adelgid and an invasive fungus, respectively, ash rarely comprises the majority of tree species in natural forests.
In that sense, says Wendy Klooster, a horticulturalist at Ohio State University, biologists dodged a bullet with the ash borer. The country may be losing most of its ash trees, but the emerald ash borer is not single-handedly depopulating entire forests like the woolly adelgid and chestnut blight.
But that doesn’t mean the ecological impacts of the ash borers aren’t felt. Outside of the financial impact, the loss of ash trees in dense forests has opened the canopy, which paves the way for vines and invasive plants that thrive on sunlight. And since ash trees often live along streams and marshes, their abrupt loss has left those areas wetter and more erosion-prone, according to Klooster.
“It could have cascading impacts on the other plant and animal life in those areas,” she says. “We're just starting to explore and understand the repercussions.”
Native Americans have disproportionately felt the brunt of the ash tree die-off as these species are crucial to many indigenous practices, such as basket weaving, says Anthony D’Amato, a forest ecologist at the University of Vermont. The tree also sits at the heart of some tribes’ origin stories. To the Wabanaki people, D’Amato says, Creation began with a black ash.
“There’s a deep connection to the tree, so when we talk about the impacts of the ash borer, we can’t overlook its cultural importance,” he says.
What success looks like
Large numbers of ash trees live along the creeks that empty into the Choptank River, so Disque hopes that S. galinae will help save Martinak’s remaining ashes. She targets one tree in particular, a lone ash near the boat launch that just began to show signs of infestation last year. Several lower branches are dead, but the crown remains healthy looking—for now.
“We don’t want to release (wasps) on anything that’s going to die in the next year,” she says. “The beetles are just going to move away faster than the parasitoids can attack them.”
She hangs the pencil-size twigs that harbor the wasps, hoping that the adults will target the tree’s ash borers. The wasps have been so successful elsewhere that she’s cautiously optimistic for Martinak’s remaining ash trees, she says. This past year, Disque’s team released S. galinae at a handful of sites around Maryland, and in 2022, she intends to measure how far the wasps have spread beyond their release site—a key measure of their ability to survive in the wild.
“By bringing the co-evolved natural enemy to the ecosystem, we hope to restore a badly needed balance with the emerald ash borer,” says Jian Duan, an entomologist at the U.S. Department of Agriculture.
“I’m convinced this species will play an important role in regulating or controlling the emerald ash borer.”
The ash borer is here to stay, experts agree, and the three species of wasp from China still in use, plus the relative newcomer from Russia, should help us live with the invasive insect. But since the borer can co-exist with ash trees in its native habitat, Disque and Duan are hopeful that they can keep enough trees alive for long enough that such an adaptation can happen here.