A previously unidentified species of invasive parasitic wasps is now stalking their prey on both coasts of the United States coasts, according to some long-term entomological data. Unlike the maligned murder hornets of 2020, the millimeter-sized Bootanomyia dorsalis isn’t likely to threaten humans. But the effects on North America’s roughly 800 species of oak gall wasps—and their ecosystems—remains to be seen. The discoveries are detailed in a study published in the Journal of Hymenoptera Research.
Meet Bootanomyia dorsalis
Native to Europe, B. dorsalis is one of the world’s many parasitoid wasps, insects that lay their eggs in living plants and animals. It frequently targets both their fellow oak gall wasps, which measure only one to eight millimeters in length. Galls are small, tumor-like growths formed on plants that act as incubators for oak gall wasp larvae. Parasitic wasps often lay their own eggs in these galls, which later feed on their victims.
North America currently hosts about 90 oak tree species that collectively house an estimated 800 gall wasp species. And the number of parasitic wasp species is even larger.
“Parasitic wasps are likely the most diverse group of animals on the planet and are extremely important in ecological systems, acting as biological control agents to keep insects in check, including those that are crop or forest pests,” Kirsten Prior, a biological scientist at Binghamton University and study co-author, said in a statement.
Prior and colleagues found B. dorsalis thousands of miles from their native habitat during work on a larger research project focused on North American oak gall wasp and parasitoid diversity. Specifically, the team is looking at how gall-forming insects evade ever-evolving parasitic wasp varieties (or clades), and how those same wasps pursue them. Their strategy involves collecting oak gall wasps from around the continent, allowing parasitic wasps to hatch from those galls, and then conducting genetic sequencing to identify the various species.
“We are interested in how oak gall characteristics act as defenses against parasites and affect the evolutionary trajectories of both oak gall wasps and the parasites they host,” Prior explained, calling the broader study the most extensive of its kind.
An out of place parasite
After years of research, the team has so far amassed an estimated 25 oak gall wasp species while also identifying over 100 parasitic wasp species. In doing so, they recently noticed that a handful of those parasitic insects weren’t where they were supposed to be.
“Finding [B. dorsalis] on the two coasts of North America inspired our group to confirm this parasitic species’ identity and whether it was, in fact, an introduced parasite from Europe,” said Prior.
But while the specimens are all technically B. dorsalis, the genetic differences between the Pacific and East coast varieties (or clades) are so striking that they may warrant separation. Using the Cytochrome Oxidase Subunit (aka the universal barcoding gene, mtCOI), Prior’s team determined that the New York clade was related to B. dorsalis found in Portugal, Italy, and Iran. In contrast, the Pacific wasps were related to parasitoids in Spain, Hungary, as well as Iran, too. The East Coast wasps were also slightly more genetically diverse than their West Coast counterparts, which hints at potentially less of a population bottleneck or multiple introductions to the continent.
There are a couple main theories about how they got to North America in the first place. It’s possible that B. dorsalis arrived when Europeans colonists planted non-native oak tree species as far back as the 17th century. But given that an adult B. dorsalis can live as long as 27 days, they may also be more recent transplants aboard airplanes.
Get ready for Gall Week
While this research confirms that B. dorsalis has already made it to North America, it’s less clear how their introduction may affect insect and plant populations. Prior conceded that other invasive parasitic species have been documented as harming native insect populations.
“We did find that they can parasitize multiple oak gall wasp species… [so] they could be affecting populations of native oak gall wasp species or other native parasites of oak gall wasps,” she said.
Regardless of their ramifications, studying and identifying these insects can only continue with both proper funding—and the help of citizen scientists. The platform iNaturalist regularly hosts Gall Week to encourage specimen collection, while Prior and colleagues will continue to update their progress on the website Gallformers.org.
“Only when we have a large, concerted effort to search for biodiversity can we uncover surprises—like new or introduced species,” said Prior.
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