As soybean gall midge (Resseliella maxima) grows as a pest of concern, researchers are searching for ways to keep it in check. Recently, the discovery of the wasp Synopeas ruficoxum parasitizing soybean gall midge in Nebraska offers hope that it could be deployed in biological control efforts. Shown here are lateral views of the 2005 holotype of S. ruficoxum found in New Brunswick, Canada (A), a 2017 unidentified Synopeas found in Ontario, Canada (B), and 2021 Synopeas “Y” (C), illustrating the elongate metasoma. (Image originally published in von Gries et al. 2025, Journal of Hymenoptera Research)

By Sarah von Gries

Sarah von Gries

Tiny, squishy, and deceptively cute, bright orange maggots tucked inside soybean stems are causing big trouble. Resseliella maxima, better known as the soybean gall midge (SGM), has become a concern for soybean growers, chewing through stems and cutting into yields. Despite several years of efforts, there is still no reliable way to keep SGM populations in check. However, nature may already be offering reinforcements: parasitoid wasps that consume SGM maggots from the inside out.

A Lone “Oddball” Wasp

In 2021, Justin McMechan, Ph.D., and his lab team at the University of Nebraska–Lincoln began rearing insects from SGM-infested soybean stems. Among the hundreds of midges and a handful of familiar soybean-associated insects, the team recovered an unusual wasp. It stood out, not just from the known insects, but also from another wasp species discovered around the same time in Minnesota.

This lone wasp was sent to Gloria Melotto, a graduate student at the University of Minnesota studying natural enemies of SGM. At the time, Melotto was working to identify a different wasp species associated with SGM in Minnesota, collaborating with two taxonomists, Elijah Talamas, Ph.D., of the Florida State Collection of Arthropods and Jessica Awad, Ph.D., at Naturalis Biodiversity Center in the Netherlands.

Melotto forwarded the Nebraska specimen, along with her Minnesota parasitoids, to Talamas and Awad for identification. More specimens were available from Minnesota, so the team’s attention focused on that species, now known as Synopeas maximum (family Platygastridae, subfamily Platygastrinae). The Nebraska wasp remained unidentified, waiting patiently in a collection drawer for its time to shine.

By 2023, I had joined McMechan’s team as a graduate student, just in time for a new field season surveying Nebraska soybean fields for natural enemies. That same “oddball” wasp reappeared, only this time it was not alone. We collected 26 more wasps, scattered across multiple sites. Suddenly, the curious singleton revealed itself to be part of a much larger story. It was clear this wasp warranted closer attention, and we published the resulting discovery in August in the Journal of Hymenoptera Research.

Tiny, bright orange maggots of Resseliella maxima, better known as the soybean gall midge are causing big trouble for soybean growers, chewing through stems and cutting into yields. Despite several years of efforts, there is still no reliable way to keep soybean gall midge populations in check. However, nature may be offering reinforcements: parasitoid wasps that consume soybean gall midge larvae from the inside out. (Adult soybean gall midge shown in inset.) (Image courtesy of Sarah von Gries)

The Mystery of Synopeas “Y”

Talamas and Awad identified the wasp as a second species in the genus Synopeas, and we began referring to it as Synopeas “Y.” Its elongate metasoma, or tail end of its abdomen, set it apart from most of the 400 described species in the genus, narrowing the possible matches to a subset of 34, which we later named the “craterum group.”

While Talamas and Awad combed the literature for descriptions that matched Synopeas “Y,” we generated genentic barcode sequences from the 26 fresh specimens and queried them against the Barcode of Life Database. Two matches surfaced: unidentified Synopeas specimens from Montreal and Guelph, Canada. Fortunately, voucher specimens were available for closer examination, so Talamas requested them to compare against Synopeas “Y.”

“The Canadian specimens had more brightly colored appendages [than those from the U.S.], but otherwise they appeared to be conspecific,” says Talamas. The morphological match and a genetic barcode similarity of 99.5% provided strong evidence that the Canadian and U.S. populations were the same species.

Then came a breakthrough. Awad found a potential match with a single female wasp collected in 2005 near Belleville, New Brunswick, Canada, Synopeas ruficoxum, which she had seen during a previous visit to the Natural History Museum of Denmark, Copenhagen.

“I was about 90% confident that Synopeas “Y” was Synopeas ruficoxum,” Awad says. “The morphology was a very close match and the known distribution matched as well. I wanted to double-check the holotype, but I examined it closely when I was in Copenhagen and took detailed notes, so I had a pretty good idea of what it looked like.”

Geographic distribution of soybean gall midge (Resseliella maxima) and the parasitoid wasp Synopeas ruficoxum in the U.S. and Canada. (Figure originally published in von Gries et al. 2025, Journal of Hymenoptera Research)

Identity Confirmed

After borrowing and comparing the museum specimen and two voucher specimens, Talamas and Awad confirmed Synopeas “Y” was indeed Synopeas ruficoxum, expanding the known geographic range of the species.

Using the genetic barcodes, we developed species-specific primers to screen field-collected SGM larvae for parasitism by S. ruficoxum. This method provided DNA evidence for the host-parasitoid relationship, confirming that S. ruficoxum parasitizes SGM.

Although all wasps in Platygastrinae are assumed to parasitize gall midges (family Cecidomyiidae), validated host-parasitoid relationships at the species level remain rare. “Something that’s kind of interesting,” Awad says, “is that other members of the Synopeas craterum group also parasitize species of Resseliella. This is only the third host association known for this group.” The function of the extra-long metasoma remains unknown, but more ecological data could help explain this unusual feature.

Looking Ahead: Biological Control Potential

With S. ruficoxum now identified as a parasitoid of SGM, it opens the door to more questions. We still do not know much about its basic biology, like how long adults live, or even how they reproduce, since only females have been found. We also lack data on parasitism rates, which is key to evaluating its potential as a biological control agent in integrated pest management. And all of these questions need to be considered alongside the other known SGM parasitoid, S. maximum.

Molecular tools, including DNA barcoding, offer opportunities to advance our understanding of platygastrines and their gall midge hosts, especially in pest management. These techniques can reveal host-parasitoid associations, detect new species, and monitor population dynamics in agriculture. Broader implementation has the potential to enhance biological control strategies but is presently limited by major knowledge gaps about the diversity and life history of gall midges and their parasitoids.

Here, the agricultural significance of the insects helps to provide both the impetus and funding to advance our understanding of their biology, and the tools used to do it. These tools can now be used to study gall midge-parasitoid interactions more broadly, including those in both agricultural and natural ecosystems.

Sarah von Gries is a Ph.D. candidate in Entomology at the University of Minnesota, co-advised by Robert Koch, Ph.D., and Amelia Lindsey, Ph.D. Email: [email protected].