By Gwendolyn Craig
Think of a typical predator-prey scenario and you might consider a lion attacking an antelope or a fox attacking a rabbit. Does anyone think about bug-on-bug combat?
Yes. Nick Dietschler does, and so do his colleagues at the New York State Hemlock Initiative out of Cornell University.
On April 22, gathered in the parking lot of the Edmund Niles Huyck Preserve in Rensselaerville, Dietschler pulled out a large jar and set it on his truck. Inside were flies, lots of tiny flies, just a bit bigger than your common fruit fly. These adults were ready for dispatch—a mission to mate, lay their eggs and let their hatched babies devour the eggs of their prey, the speck-sized hemlock woolly adelgid (HWA).
HWA is an invasive insect that slowly kills hemlock trees. Mark Whitmore, head of the initiative, said the trees die within four to 10 years. All hemlock woolly adelgids are females that reproduce asexually. The adelgids colonize a hemlock branch. The adults insert their mouthparts into the hemlock twigs, Whitmore said, which indirectly kills the trees’ buds.
It’s perhaps an insidious way for the tree to go. Typically its needles look green and healthy for a while as the infestation grows. During the winter, an infested tree is easier to spot because HWA develops a white, waxy wool that can be seen underneath the tree’s branches.
The invasive bug made its second known appearance in the Adirondack Park last year on the eastern shores of Lake George. The state Department of Environmental Conservation used insecticides on hundreds of infected trees, sometimes sprayed on the basal bark and other times (when closer to water sources) injected. It’s the most effective method for treating HWA, Dietschler said, but the long-term fight may include predator bugs.
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An HWA predator beetle, called Laricobius nigrinus, was released on Lake George at the end of October. They are originally from the Pacific Northwest where there is a native population of hemlock woolly adelgid. Whitmore will travel out west and collect branches infested with HWA, which also includes branches infested with their predators.
He collects both beetles and two species of silver fly—the subjects in Dietschler’s jar at the Huyck Preserve. They are reared in a lab at Cornell, and when hatched, transported to HWA infestations.
The same fly species will be released on Lake George later this spring. But the Huyck Preserve welcomed these new residents first with great hopes that they will help save its 350 acres of hemlocks.
On this late-April day, Dietschler was surprised by the amount of snow falling (and sticking). He walked part of the preserve with Anne Rhoads, executive director of the preserve; Garrett Chisholm, stewardship coordinator for the preserve; and Nicole Campbell, terrestrial invasive species coordinator with the Capital Region PRISM.
Rhoads and Chisholm guided the group to their most infested stands of hemlocks. Looking up at the canopy, clumps of white sticking like gum at the base of hemlock needles seemed to stand out, even with the snowfall. Rhoads said HWA was first spotted on the 2,000-acre preserve in 2015, but it reached a high infestation level by 2019. The preserve has treated some of its hemlocks with the same insecticides used on Lake George, and it expects to use another round of it later this spring.
“But in terms of landscape-scale management of HWA across a very large area, chemical treatment is not feasible over a very long period of time,” Rhoades said. “I feel like the only hope for saving the species as a whole over a very long period of time is through biological control.”
Enter the flies.
Dietschler had hoped to release the silver flies while at the preserve, but he wanted to give them their best chance at survival. That meant releasing them on a warmer, sunny day. But he, Rhoads and Chisholm identified the stands of trees where Chisholm would return with the big jars of hope, unscrew the top and let about 800 flies fly.
Dietschler and Whitmore are not worried that the introduction of the flies will become some new nuisance to deal with, because they are specialized to feed on HWA alone. They hope over time the flies will have enough adelgid to feed on that they can establish their own populations. But how will they know if it’s working?
The New York State Hemlock Initiative is using a burgeoning technology called environmental DNA to track not only HWA but also its predators.
Paul Simonin is studying environmental DNA, also called eDNA for short, at Cornell University. During an Adirondack Park Invasive Plant Program meeting on Thursday, Simonin described the process for tracking eDNA. Take a plant, for example, he said. If there is only one, your chances of seeing it out in the environment might be slim.
“With eDNA, these organisms are giving off bits of tissue all the time, so it’s almost certain that there are more bits of tissue out there than individual organism,” Simonin said. “So, our probability if we can do the sampling right of detecting bits of tissue, bits of DNA, is higher.”
Simonin mostly studies aquatic invasive species. By grabbing a water sample from a lake, he can bring it back to a lab and figure out whether or not the DNA of hydrilla, for example, is present.
In Whitmore’s case with HWA, he will grab branches and possibly rainwater from hemlock trees and search the samples for DNA evidence of his predator bugs. Traditionally he has used sticky traps to capture his silver flies to make sure they’re still out there doing their jobs, but it’s difficult to identify between the two silver fly species when on the trap, Whitmore said.
He is also using eDNA to track the amount of adeglid DNA showing up in samples collected on Lake George. The technology could help with earlier detection of infestations.
For now, though, a tiny insect battle is beginning before a microscopic process for finding the winner can happen.
“Next year at this time, we might have more information,” Whitmore said.