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Pests, Predators, and Parasitoids: Wasps Target Flies That Eat Hemlock Woolly Adelgid

hemlock woolly adelgid, Leucotaraxis argenticollis fly, and Pachyneuron wasp

In the effort to manage the hemlock woolly adelgid in eastern North America, two fly species deployed as biological control agents are themselves targeted by parasitoid wasps. A new study examines how these parasitoids affect the flies and other predators of the hemlock pests. Shown here is a hemlock branch infested with hemlock woolly adelgid (Adelges tsugae), left; an adult female Leucotaraxis argenticollis fly, center, whose larvae feed on the adelgid; and a wasp in the genus Pachyneuron that parasitzes L. argenticollis larvae or pupae. (Fly photo courtesy of Cornell University Insect Collection and New York State Hemlock Initiative [NYSHI]. Hemlock and wasp photos courtesy of NYSHI.)

By Melissa Mayer

Melissa Mayer

Melissa Mayer

It may look like cottony puffs decorating hemlock trees like Christmas flocking, but the hemlock woolly adelgid (Adelges tsugae) is an invasive pest that decimates the hemlocks upon which it feeds. On the east coast of North America, losing hemlock affects the entire ecosystem. Animals like moose and black bears rely on hemlock, and the trees prevent erosion and regulate stream water temperature for cold-water species like trout.

Over the course of dealing with the hemlock woolly adelgid (HWA) problem, a team of researchers from Cornell University and the U.S. Department of Agriculture looked at the interactions between the invasive pest, the beetles and flies that serve as biological control agents for HWA, and the wasps that parasitize the flies. In a paper published in August in Environmental Entomology, the team use emergence data for Melanips and Pachyneuron wasps to better understand how the parasitoids interact with HWA and reveal limited competition between the beetle and fly biocontrol agents.

Battling HWA on the East Coast

Sabrina Celis

Sabrina Celis

In the Hemlock Woolly Adelgid Biocontrol Research Lab at Cornell, scientists rear and release insects that gobble up HWA: a specialist predator beetle, Laricobius nigrinus, and two species of silver flies, Leucotaraxis argenticollis and Leucotaraxis piniperda.

To do that, the researchers travel to the Pacific Northwest, where the three predators naturally occur and specialize in targeting HWA. There, they collect hemlock foliage, ship it to New York, and quarantine it in specially designed clear acrylic cages. Then the team waits for those beetles and silver flies to crawl out of the imported plant matter so they can be reared to adulthood and released to manage HWA infestations.

Sabrina Celis, lead author on the study, joined the lab as an undergraduate research assistant when she was a freshman entomology major at Cornell. In addition to the biocontrol agents, she watched lots of other insects climb out of the foliage—and that piqued her interest.

“That kind of became my side project as a freshman,” Celis says. “So, during the summer, I would ID everything that came out of the foliage to family, and what became pretty clear was that what we were collecting the most of, aside from our biological control agents, were parasitoids. At that point, it was known that the Leucotaraxis flies that we were rearing were susceptible to parasitism—but not a lot was known about the parasitoids.”

In fact, those parasitic wasps targeting the Leucotaraxis flies are so cryptic that, even when the team sent specimens out to the taxonomy experts at the Smithsonian, they were only able to ID the wasps to genus: Melanips and Pachyneuron.

Uncovering the Parasitoids

After three summers, Celis had amassed a trove of emergence data, so she focused her senior thesis on using that information to better understand the parasitoid wasps and how they interact with and affect the biocontrol agents.

“It seems that Melanips prefers the flies Leucotaraxis piniperda, while the wasp Pachyneuron prefers the fly Leucotaraxis argenticollis,” says Celis. “And, just from a biology standpoint, I thought that was really interesting. Since we got to this point from emergence data, we don’t know if this is a super specific preference or if the wasps are actually able to parasitize either species, but that would be an interesting thing to see in the future.”

Both wasps are solitary endoparasitoids, so a single adult wasp lays her eggs inside the host. Then, the wasp larvae can develop and eventually pupate inside the host fly’s puparium—but the specifics of that process are still unclear. While the team has watched the Melanips wasp parasitize fly larvae and then emerge from the fly’s pupal case, they don’t yet know if the Pachyneuron wasp parasitizes fly larvae or pupae.

The emergence data also shows that the wasps’ life cycles lag the flies’ life cycles. The flies begin emerging around February and remain active until July. Parasitism increased over that time, with the highest rates of wasp emergence in June and July.

Perhaps the best news is that the team could use the emergence data to estimate how many silver fly larvae would be at a site and determine that they aren’t likely to compete with beetle larvae at the same site. While those beetles can reduce the population of overwintering HWA, the HWA population rebounds in the spring. The data suggest that it may work well to use the beetle and silver fly biocontrol agents together.

More Research to Come

Celis says this paper is a stepping stone and that more work on live parasitoids—including behavioral tests and choice assays—could help confirm the wasps’ life histories and host preferences and firm up how these parasitoids affect the silver flies as biocontrol agents. She says that, if the silver flies become established in eastern North America, it will also be important to see if these wasps are already in the region or if other wasps parasitize the flies there.

“There are so many layers of interactions,” Celis says. “It’s really important for biological control researchers to look not just at the pest they’re trying to kill and the biological control agent they’re studying, but really look at all the different interactions happening in the native range and see how that informs what could happen when they try to introduce a biological control agent.”

For Celis, the opportunity to invest in this entomology research as an undergrad shaped her future goals. She’s now headed to the University of Minnesota this fall to begin a Ph.D. program focused on parasitoid wasps.

Melissa Mayer is a freelance science writer based in Portland, Oregon. Email:


  1. This might be a dumb question, but how does the Hemlock Woolly Adelgid cause damage to the trees from eating their sap? Or what kind of damage does it create and how severe?

  2. Hi Elizabeth. For a good intro to HWA, see this past post from 2019: In short, as that post explains, “The hemlock woolly adelgid (HWA) does its damage by sucking nutrients from its host trees.” The associated article in the Journal of Integrated Pest Management delves further into its impact: (Scroll down to “Sampling and Scouting Procedures” section and see the paragraph labeled “Damage.”)

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