big green frog
The Idaho Department of Fish and Game controls bullfrogs to keep them from expanding into the warming range of northern leopard frogs and western toads. Photo © Michael Lucid, Idaho Department of Fish and Game.
The deep “jug-o’-rum” of the male bullfrog resonating through spring twilight is a call from Earth’s distant past. Bullfrog progenitors arrived ages before the dinosaurs, when amphibians staggered out of Devonian seas to dominate the land for 70 million years. For those who love nature, bullfrog vocalization awakens what Rachel Carson called our “sense of wonder” — that is, if they live in the eastern half of the U.S., the bullfrog’s native range.

In the western half, where bullfrogs were unleashed in the 19th and early 20th centuries to augment frog-leg markets, it has precisely the opposite effect.

For instance, when Yosemite National Park’s bullfrog-control team totes gigs, air rifles and nets into the meadows it’s cheered by visitors for silencing the ugly reminder that native ecosystems throughout the West are being trashed by alien invaders. Also imperiled by bullfrogs are native species in China, France, the United Kingdom, Italy, South Korea, Taiwan, Japan, Haiti, Jamaica, Mexico, the Netherlands, Colombia, Cuba, Dominican Republic, Puerto Rico, Venezuela and Uruguay. Bullfrogs compete with and prey on indigenous wildlife. They devour anything they can stuff into their cavernous maws with their “hands,” including native frogs, salamanders, bats, fish, crayfish, snakes, turtles, rodents, insectivores, lagomorphs and birds as big as goslings.
dead bird
Brewers blackbird from a bullfrog stomach. Photo © David Hall, University of Arizona

What’s more, they carry and spread the devastating amphibian disease chytrid fungus, to which they’re pretty much immune.

Bullfrogs, which can weigh two pounds and produce clutches of 20,000 eggs, dwarf native amphibians in size and fecundity. Where they’ve been introduced, they have few predators. And as global warming destroys habitat of the natives, it expands habitat suitable for bullfrogs.

Eradicating bullfrogs from the West appears impossible, but where they’re controlled locally native amphibians surge back. Benefiting are the California red-legged frog , northern red-legged frog, northern leopard frog, Chiricahua leopard frog, Pacific tree frog, canyon tree frog, Woodhouse’s toad, red-spotted toad, Great Plains toad, western toad, narrow-mouthed toad and dozens of salamander species.
man with frog
Assistant Wildlife Biologist Chris Prewitt holding bullfrog he just removed. Photo © David Hall, University of Arizona

Chiricahua Leopard Frogs

The largest bullfrog control project is in southeast Arizona, mostly for federally threatened Chiricahua leopard frogs. In Arizona bullfrog tadpoles gorge on abundant algae, then transform to juveniles that crowd banks. Adults eat many in a self-perpetuating food chain, even as populations grow. With summer rains adults and surviving juveniles disperse, sometimes traveling five miles. “Once you have bullfrogs you don’t have native frogs; they’re able to wipe them out in two weeks,” says David Hall, a University of Arizona research scientist.
frog in pond
A Chiricahua leopard frog in Sycamore Canyon, Arizona . Photo © Jim Rorabaug, USFWS /Wikimedia Commons
But with aggressive bullfrog control natives move in from refugia. In the Las Cienegas National Conservation Area Hall and partners, including the Arizona Fish and Game Department, Bureau of Land Management, Cienega Watershed Partnership and the U.S. Fish and Wildlife Service, have built a robust Chiricahua leopard frog “metapopulation” — i.e., a group of populations separated by space. If one population flickers out, the vacant habitat gets recolonized from a neighboring population. Chytrid fungus is less problematic in metapopulations. It’s also less problematic in running water. In 2013 the partners finished clearing bullfrogs from seven-mile Cienega Creek. “It took us three years,” says Hall. “It’s a real accomplishment; the creek is a gem.”

Then, with Chiricahua leopard frogs cultured in ponds built with solar pumps, the partners established seven breeding populations that covered 16 square miles. Today that metapopulation has expanded to 32 square miles and 19 breeding populations.

Hall showed me a PowerPoint presentation chronicling how the partners have facilitated enormous expansion of Chiricahua leopard frogs across an area 66 by 155 miles.
American Conservation Experience (ACE) crew members help pull seines through stock tanks to capture bullfrogs. Photo © David Hall, University of Arizona

Yellow-legged Frogs

In the Plumas National Forest in northern California bullfrogs threaten the foothill yellow-legged frog. “We’ve been focusing our effort on a series of 8 ponds,” declares Forest Service wildlife biologist Colin Dillingham. “At the end of this season we didn’t find bullfrogs in two of them, and catch numbers were down in the others.” The Plumas also sustains the federally endangered Sierra Nevada yellow-legged frog, which clings to existence in watersheds easily accessible to bullfrogs. These frogs face another alien predator — trout stocked in naturally fishless water before the age of ecological awareness.
frog in water
A yellow-legged frog. Photo © Colin Dillingham, Plumas National Forest
Eastern brook trout were eliminating Sierra Nevada yellow-legged frogs from the Forest’s Gold Lake. So the California Department of Fish and Wildlife removed the trout with gillnets, apparently successfully because the nets have been coming up empty for the last two years. Despite the fact that abundant brook trout will remain throughout the Forest and beyond, a multitude of anglers, public officials and editors were apoplectic, accusing the state and feds of conspiring to end Sierra trout fishing. “If the yellow-legged frog disappears, would anyone notice?” demanded one newspaper chain. But enlightened anglers spoke up, too. Ralph Cutter, who runs a fly-fishing school and guide service, released this public statement: “I would much rather leave a legacy of as natural an ecosystem as possible, rather than an artificial and synthetic landscape designed for the amusement of certain enthusiasts including myself.”
bucket with dead tadpoles
5 gallon bucket of euthenized bullfrog tadpoles collected during seining. © David Hall, University of Arizona

Methods of Control

The Santa Rosa Plateau Ecological Reserve in southern California, managed by The Nature Conservancy, has ponds small enough to be seined and drained. “We separated the bullfrog tadpoles, euthanized them, and held the native stuff,” reports the Conservancy’s Zack Principe. “Then, after we got the ponds pretty clean, we pumped them out.” With bullfrogs eradicated, the Conservancy will reintroduce California red-legged frogs in the spring of 2020 from a population in Baja California. The Wheatley Ranch in the mountains of San Diego, one of the Conservancy’s conservation-easement properties, has a pond too big and deep to seine or drain, so the U.S. Geological Survey has eradicated the bullfrogs with gigs and non-toxic copper rounds fired by .22 rifles. Next spring it will reintroduce California red-legged frogs from that same Baja population.
An adult California red-legged frog/ Photo © Zach Behrens, National Park Service / Wikimedia Commons
The challenge is different on British Columbia’s Vancouver Island, where bullfrogs imperil the northern red-legged frog, western toad, Pacific tree frog and six species of salamander. In 1989 Stan Orchard, a biologist with the Royal British Columbia Museum, identified bullfrogs near Victoria. Despite the impending ecological disaster he couldn’t get funding to research control methods. Ten years later the World Wildlife Fund invited him to Australia to set up a national amphibian conservation program. When he returned in 2003 bullfrogs had proliferated and dispersed to the point they threatened to invade the reservoirs supplying greater Victoria with potable water. So Orchard started a company called BullfrogControl.com Inc. and invented and patented the Electro-Frogger Pole. It works like electro-fishing gear but the powerpack isn’t worn on the operator’s back. Instead, it’s placed in an inflatable boat, thereby increasing mobility, safety and efficiency. “It generates an electrical field about 15 inches across, so you just have to get the anode in the vicinity of the bullfrog,” says Orchard. “With gigging and shooting you’ve got a very small target. But with the Electro-Frogger if a bullfrog moves, you simply engage your field in front of it. If it’s swimming, you can stop it in the water. If you accidentally stun a native frog, it will recover in about a minute.” Funded by municipalities, Orchard’s crews work at night with spotlights that reveal frogs by eye reflection. So far they’ve kept bullfrogs out of the reservoirs by killing at least 80,000 adults. It’s not worth going after the tadpoles because in such habitat only about 10 percent make it to adulthood. The Idaho Department of Fish and Game uses Orchard’s Electro-Frogger to keep bullfrogs from expanding into the warming range of northern leopard frogs and western toads. In Arizona’s Chiricahua leopard frog habitat (mostly stock ponds) Hall and his team wade, catching tadpoles in 30-foot-long bag seines and shooting adults with copper .22 rounds.
man with pole in water
Electro-frogging in Idaho. Photo © Mac Kobza, wildlife biologist for Boulder County, Colorado

Politics of Bullfrog Control

It’s hard to imagine that bullfrog control could be controversial within the scientific community, but Orchard offers this: “The resistance comes from biologists. In BC they pretty much set up their own little kingdoms, and there are subtle degrees of skullduggery to protect budgets.” Orchard is especially frustrated by a myth, originally issuing from the University of Victoria and circulated still, that killing adult bullfrogs may be counterproductive because they control their numbers through cannibalism. “That’s just nonsense,” he says. “People who make that claim are either extremely ignorant or have an agenda.” On Hall’s PowerPoint presentation red dots, adjacent to the southernmost and biggest buffer zone, mark land owned by a rancher who hates government and claims to love bullfrogs. He denies Hall’s team access.

“There’s a huge population of bullfrogs on his land,” says Hall. “Most landowners will work with us, but one of our biggest problems is landowners who won’t.” Hall hopes to hire a sociologist to address this problem.

A male American bullfrog. Photo © Fir002 /Wikimedia Commons
Then there’s the politics of money. In the U.S. vanishing native amphibians no longer seem a budgetary concern, so the Plumas National Forest finds itself without funds for bullfrog control. Dillingham and his team do it on their own time.

And this from Hall: “Our funding is gone. Apparently the successful restoration of aquatic habitats is not a high priority of the current administration or they are unaware of what the partners have accomplished. We’ve had huge success. We’ve established buffer zones around Chiricahua leopard frog populations, and they’ll get re-invaded unless they’re maintained.”

Every attempt is made to utilize the dead bullfrogs, but it’s not always possible. Hall and his crew frequently work in desert environments with no refrigeration, and they’re “getting sick of eating frog legs.” He gives them away when he can.

“Pretty much all our big bullfrogs get eaten,” says Dillingham. And he provides smaller carcasses to schools for dissection.

ACE crew and University of Arizona Assistant Biologists Chris Prewitt and Julia Muldoon-Marquez. Photo © David Hall, University of Arizona

Hall has kept a low profile, which may explain why he has yet to encounter organized resistance from animal-rights groups. “Now,” he says, “I’m ready to take them on. Bullfrogs are amazing animals. I have great respect for them in their native habitat, but in the West they’re just wreaking havoc.”

Orchard hasn’t encountered organized animal-rights resistance either, perhaps because the public abhors the prospect of its drinking water filtered through millions of bullfrog kidneys. But when he’s on the lecture circuit individuals scold him for alleged cruelty.

Invariably he silences them with this response: “You know, bullfrogs swallow baby ducks live and then dissolve them in hydrochloric acid and proteolytic enzymes.”