By Claudia Geib
The yellow-legged hornet is a predator: after it sets up a nest in a new neighborhood, its workers head out in search of smaller wasps, flies and bees to feed the hive’s growing brood. One of its favorite snacks is honey bees. Lingering outside a hive, these hornets, Vespa velutina, capture flying honey bees mid-air, stopping on their way home only to chew the bee up into pellets to feed their young. In the hornets’ native range across Southeast Asia, local bee species have evolved defenses against the yellow-legged hornet’s attacks. But in Europe and the United States, native bees and agricultural honey bees are defenseless.
“As invasives, these hornets are released from any kind of control agents, whereas in southern Asia there are things that prey on them,” says Lynn S. Kimsey, an entomologist at the University of California, Davis. “The threat is that they would expand their population to take up all resources available without any effective control, except maybe us humans.”
In the face of this new threat, scientists are turning to an old technology: radio tracking, which has been used to follow wildlife since the 1950s. Thanks to recent advancements, radio tracking tags can now be made small enough to attach to the hornets’ tiny bodies, and in doing so lead scientists back to their nests. These miniature tags are also being tested on other species, such as the northern giant hornet (Vespa mandarinia), better known as the “murder hornet.”
With global shipping allowing insects to hitch a ride on ocean-crossing cargo and pop up halfway across the world, such technology could be a boon to finding and destroying unwelcome arrivals — if scientists can keep up with the invasion.
A close-up image of a Japanese giant hornet. Image by Yasunori Koide via Wikimedia Commons (CC BY-SA 4.0).
The road to radio-tracking a hornet
The first radio trackers were born from the Cold War. With the invention of the transistor, radios no longer relied on bulky, high-voltage glass vacuum tubes to carry electricity, allowing devices to shrink enough that they could be attached to animals without weighing them down. In the late 1950s, Cornell University professor Dwain Warner was among the first to develop radio-tracking devices to study animal behavior, which he later applied on birds, starting with geese and owls.
According to science historian and author Etienne Benson, Warner was compelled both by his scientific interest and by his frustration that the U.S. was falling behind: the Soviet Union’s Sputnik program had not only put dogs into space, but also monitored their physiology through the brief flights.
From there, radio tracking quickly rose to prominence in wildlife research. Today, it’s used to study everything from whales to mice. Even as newer technologies have come online, such as satellites and GPS, radio tracking remains prominent. But for many decades, insects remained too small for any of these trackers.
Behavioral ecologist Peter Kennedy was studying beetles in the U.K. when he heard that researchers had adapted a version of this technology to track insects, originally created to find avalanche victims. It was the early 2000s, and the device worked a bit like a metal detector, pinging off a small diode, a type of electrical conductor, stuck to the bug’s carapace. (The same diodes are still sewn into the clothing of more than 150 outdoor brands , and used by rescue organizations worldwide.) To do the actual tracking, scientists had to carry a large, handheld radar with a heavy, waist-worn battery pack that only worked within a limited range. But Kennedy was intrigued, especially when he later moved to another organization that was using similar technology, adapting marine radar mounted to a trailer, to track honey bee movements.
Even so, Kennedy himself didn’t get involved with using the technology until September 2016, when the U.K.’s Department for Environment, Food & Rural Affairs (DEFRA) got in touch with worrying news. DEFRA told him that an apiary in Gloucestershire had reported yellow-legged hornets snatching honey bees from their hives. The hornets had been a pest in France since at least 2004, and the new sighting suggested they had finally found their way across the English Channel.
“There was an urgency to locate that nest,” says Kennedy, a research fellow studying pollinator behavior at the University of Exeter. Timing was everything: during winter, V. velutina hives produce new queens, which then leave the original nest the following spring — “which means they could potentially found multiple nests in the subsequent year,” Kennedy says.
After being anesthetized on ice, a yellow-legged hornet is attached to an acrylic restraining plate by a wire gently looped around its petiole, holding it in place so that a radio tag can be tied on. Image by Juliet Osborne.
Yet the hornets’ nests can be difficult to find, hidden in dense scrub and high treetops. So, Kennedy and his colleagues decided to try radio tagging: in essence, catching and turning one of the hunting hornets into a homing beacon that would lead them back to the nest.
With funding for a small pilot study, Kennedy and his colleagues pioneered the first method used to track hornets using radio telemetry. After catching a hornet, the researchers anesthetized it in a tube submerged in ice for 10 minutes. They then balanced the sleeping, 22-millimeter-long (0.8-inch) insect on a custom-made restraining plate that secured a wire around its tiny waist — the joint between the thorax and abdomen called the petiole. From there, the researchers tied a cotton thread around the petiole and through the metal loops of the radio tag. The key to success, Kennedy found, was attaching the tag to the hornet’s underside, creating the same weight distribution of it carrying prey back to the nest.
Once the hornet flew off, researchers could follow it by ear: when an antenna is pointed in the same direction as the hornet, the listener will hear a beep. Like a game of Marco Polo, if the beeps get louder, that means you’re getting closer. Over the course of their research, Kennedy and his team used this technique to find five nests, with the furthest more than 1.3 kilometers (0.8 miles) from the point where they’d caught and tagged their “Trojan horse.” They destroyed each nest, in the hopes of preventing the hornets from spreading further.
Kennedy’s team published their results in 2018. Today, other researchers are using this model in new places and for new species as the threat of invasive insects grows.
After a rest period to let it wake up from cold anesthesia, Peter Kennedy releases a recently-tagged yellow-legged hornet, which flies off to lead the researchers to its nest. Image by Peter Kennedy.
Meet the ‘murder hornet’
If people in North America are familiar with an invasive hornet, the name they probably know is the “murder hornet.” This nickname became a buzzword after a May 2020 article in The New York Times introduced it into the public lexicon and inspired a minor hornet panic in the U.S.
Scientists know the “murder hornet” as Vespa mandarinia, or the northern giant hornet, native to East Asia. (It was previously referred to as the Asian giant hornet, but entomologists in 2022 adopted the new name to fit rules that dictate species names not refer to ethnic or racial groups.) In addition to preying on pollinators, V. mandarinia is known for its intense sting; multiple stings, such as from a group of hornets defending a nest, can be fatal to humans. It’s also the world’s largest hornet.
The incident that introduced the northern giant hornet to North American audiences was the same incident that, quite literally, put this pest on Sven-Erik Spichiger’s radar. In December 2019, a northern giant hornet turned up in Spichiger’s proverbial backyard: northern Washington state. It was this hornet, and another nest found the following spring, that inspired the NYT article; for Spichiger, it was a sign to get to work. As the managing entomologist at Washington State’s Department of Agriculture, he had actually produced a template for managing a
theoretical V. mandarinia invasion in a previous job. But he quickly found that some of the common techniques for finding nests wouldn’t work in Washington, such as tying a lightweight piece of tinsel to the hornet and following its shimmer back to the hive.
“I’m sure that works very well in some other places, but I’m about 6 feet tall and [even] I can’t see over the Himalayan blackberry hedges in the places where we were working,” Spichiger says.
Then Spichiger came across Kennedy’s work from the U.K. After reading his research and communicating directly with Kennedy by email, Spichiger and his team found a U.S.-made radio tag that was small enough to be mounted on northern giant hornet workers, which are usually about 25-38 mm (1-1.5 in) in length.
A restrained yellow-legged hornet with a radio-tracking tag attached. This tiny tag weighs about 0.15 grams, around half to a quarter the average weight of these hornets—and about the same weight as its honeybee prey. Image by Peter Kennedy.
Using these tags, Spichiger and his colleagues found and destroyed four northern giant hornet nests in Washington in 2020 and 2021. In the process, his team also identified a new nest behavior for V. mandarinia: most of the literature about this hornet indicates it nests in the ground, but all of the Washington nests were in hollows in tree trunks.
“The first nest was one of those moments when we’re looking at a pile of sawdust, saying, that really looks like the spoils of a hornet nest, but there’s nothing at the base of a tree — only to notice they were flying out above our heads,” Spichiger recalls.
In addition to another nest destroyed in the neighboring Canadian province of British Columbia, this work in Washington state appears to have effectively rid North America of the vanguard of a “murder hornet” invasion — for now, at least. Neither the U.S. nor Canada has had a northern giant hornet sighting since the last nest was destroyed in 2021, though Spichiger and his team are ready if there is. They recently ordered a radio telemetry drone from the Australian company Wildlife Drones, which will allow them to track tagged hornets from the air, making it even easier to navigate through the state’s often tangled woodlands.
The experience Washington entomologists gained from dealing with V. mandarinia is helping out in another part of the U.S. as well. In the summer of 2023, beekeepers in Georgia discovered yellow-legged hornets preying on their hives for the first time. Georgia’s entomologists, too, are turning to radio tracking in response to the new threat: Spichiger’s team shared what they had learned about the process in a conference call, and even temporarily loaned them some equipment.
Radio tracking isn’t a silver bullet; for example, it’s expensive to operate, so it often isn’t feasible in places where an invasive insect has spread widely.
Known as White 32, this yellow-legged hornet was the first that researcher Peter Kennedy successfully tagged and tracked to its nest, which he found within 45 minutes and only 528 meters (around a third of a mile) from where it was caught and tagged. Image by Peter Kennedy.
“The number of nests in France, for example, have exploded to such a level that to track down all those nests would cost in the region of 250 million euros,” or about $270 million, Kennedy says of the yellow-legged hornet scourge there.
In such cases, he recommends using a combination of the old-fashioned streamer method and radio tracking, reserving the tech for the hardest-to-find nests. “The nest numbers have expanded to such a point [in France] that eradication is no longer possible, so the best you can hope for is to mitigate the effects,” he adds.
In places where these invasives haven’t yet become established, like Georgia and Washington, entomologists are hoping radio tracking can help prevent the situation from progressing to the point it’s reached on mainland Europe.
And for those of us in hornet-free zones, entomologist Kimsey has a simple piece of advice: “Don’t panic.” Kimsey’s specialty is “stinging things,” and while she welcomes the growing ability to track invasive insects by radio, her prediction is that these species have little threat of spreading too widely. She points out that in their native distribution, both V. velutina and V. mandarinia require wet summers, which somewhat limits their potential habitat.
“These things come and go, and you have to figure with the enormous amount of shipping across the Pacific and Atlantic, the fact we have so few of these [invasives] making their way successfully is actually pretty impressive,” she adds.
Banner image: A yellow-legged hornet “hawking,” hovering in front of a honeybee hive to pick slower-moving bees out of midair. Image by Karine Monceau.
Citations:
Kennedy, P. J., Ford, S. M., Poidatz, J., Thiéry, D., & Osborne, J. L. (2018). Searching for nests of the invasive Asian hornet (Vespa velutina) using radio-telemetry. Communications Biology, 1(1). doi: 10.1038/s42003-018-0092-9
Kissling, W. D., Pattemore, D. E., & Hagen, M. (2013). Challenges and prospects in the telemetry of insects. Biological Reviews, 89(3), 511-530. doi:10.1111/brv.12065
Lima, C. G., Vaz, A. S., Honrado, J. P., Aranha, J., Crespo, N., & Vicente, J. R. (2022). The invasion by the yellow-legged hornet: A systematic review. Journal for Nature Conservation, 67, 126173. doi: 10.1016/j.jnc.2022.126173
Mascanzoni, D., & Wallin, H. (1986). The harmonic radar: A new method of tracing insects in the field. Ecological Entomology, 11(4), 387-390. doi:10.1111/j.1365-2311.1986.tb00317.x
Rhodes, M. W., Bennie, J. J., Spalding, A., ffrench-Constant, R. H., & Maclean, I. M. D. (2022). Recent advances in the remote sensing of insects. Biological Reviews, 97, 343-360. doi: 10.1111/brv.12802
This article was originally published on Mongabay
