How do you find an insect the size of your fingertip in a densely-packed forest?
For Jian Duan, the answer is simple: follow the dead ash trees.
On a rainy day in eastern Connecticut, Duan, a federal research entomologist with the U.S. Department of Agriculture, walked to a dying ash covered with holes. Peeling back the bark with a draw knife, he revealed a mess of serpentine tunnels. Curled up inside was one of his targets: a larva of emerald ash borer.
“Let’s collect it,” Duan said, gesturing as his assistant handed him a pair of tweezers tied to a brightly-colored ribbon.
(In case you’re wondering, the ribbon makes the tweezers easy to spot when they’re dropped on the leaf-covered ground.)
But today Duan isn’t just collecting emerald ash borers. He’s also looking for its predator, one released here on purpose in 2019 and 2020: a wasp known as spathius galinae.
“It’s from the Russian Far East,” Duan said, smiling. “Unfortunately, there are no common names for these parasitic wasps.”
The stingless wasp is tiny — about the size of a mosquito. But scientists have big hopes for it.
In Russian forests, this wasp naturally targets and attacks emerald ash borer.
“Emerald ash borer, in its native range, northeast Asia, [does] not kill trees like this,” Duan said.
And if this experiment works, the borers won’t kill as many trees here either.
A biological solution to a biological problem
Solutions like this, known as biological control, are one way scientists can deal with biological problems like the invasive emerald ash borer. Right now, there are experiments across New England to see if the wasps can help save the region’s ash trees.
Claire Rutledge, with the Connecticut Agricultural Experiment Station, says when emerald ash borer feeds on trees, it cuts off key nutrient pathways. For ash trees, that’s death by a thousand cuts.
“One larva is not a big deal. Twenty larvae are not a big deal. Two thousand larvae kill the tree,” Rutledge said.
Since it was first detected in Michigan in 2002, emerald ash borer (EAB) has killed tens of millions of ash trees. Federal officials estimate it has cost municipalities, nursery operators and the forest-product industry tens of millions of dollars. The massive die-off of ash trees has also disrupted the making of culturally important products like baskets and baseball bats.
Duan said the idea of biocontrol is to find the natural predators of an introduced pest and bring them to the new environment to slow the pest’s spread.
“Because in the native range, these natural enemies co-evolved with the pest,” Duan said.
Shortly after EAB arrived in the U.S., DNA testing traced its origin to northeast Asia. Duan traveled to Russia — trekking through cold forests to collect wasps that only prey on EAB. Samples were brought to America, quarantined and carefully tested for years to ensure the wasps wouldn’t kill any other non-target species.
Now that some of these wasps have been living in the forests of Connecticut and Massachusetts for about 10 years, scientists are trying to find out if the intervention is working.
Scientists are ‘cautiously optimistic’
As Duan and his assistant peel more bark and pull more larvae from the dying tree in Connecticut, they still can’t find the Russian wasp or two other species that were introduced here. So they decide to cut down the tree to study it further.
They cut the wood into meter-long segments and continue looking.
Rutledge said it’s too late for the wasp to stop the massive wave of EAB that’s killing older ash trees. But there is hope for the younger trees that are just starting to grow.
“When regeneration starts to happen, after the EAB levels drop, the parasitoids will be able to keep those populations down so that the new ash can grow and escape,” Rutledge said.
On this old ash tree, they found a few EAB larvae — and no wasps. But they did find parasitoids just a few miles away at another site they’re studying. And as recently as last year, Rutledge said they found reproducing populations of wasps initially introduced in 2013.
So, she said, it’s looking like success: the wasps are sticking around and spreading.
“I’m really cautiously optimistic,” Rutledge said. “The problem with bio control is it’s going to be 10 or 15 years later when we see how much of a resurgence the ash manages.”
Even then, she said, it’s going to be a long time before we see big, healthy ash trees in New England forests again.