War of the Frogs

Anyone who has heard the mating call of the coqui frog remembers it. The nocturnal native of Puerto Rico, believed to have arrived in bromeliads shipped to Hawaiʻi from a nursery there in the 1980s, has a two-note chirping call that can exceed the legislatively-established…...
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Are we winning the struggle to take back the night from coqui—or are they here to stay?

Anyone who has heard the mating call of the coqui frog remembers it. The nocturnal native of Puerto Rico, believed to have arrived in bromeliads shipped to Hawaiʻi from a nursery there in the 1980s, has a two-note chirping call that can exceed the legislatively-established state health standard of 70 decibels. According to biologists, the first note, “ko” is intended to warn off other males, while the second note “ki” is supposed to attract females. The call gives the frog its onomatopoeian name, coqui. 

While some find them cute (they have cartoonishly big eyes and rounded, padded toes that make them kind of adorable) and don’t mind the chorus of chirps that peal through the night, others find it impossible to sleep due to the constant noise. The Maui Invasive Species Committee’s (MISC) strategic plan for eradicating the frog states that its continued presence on the island will affect tourism, real estate values, human health, quality of life, cultural resources, and native ecosystems. Ominously, it concludes that “without a significant and sustained increase in efforts, the vision of a coqui-free Maui will become impossible.” So, are we winning or losing the war against the frogs?

This invasive species, Eleutherodactylus coqui, has become well-established in Maliko Gulch and the surrounding area of Haʻikū. They love the moist, cool environment of the gulch and find plenty of perches to sing from. They tend to be territorial and return to the same perch, night after night. 

The frogs have also been trapped on almost every other part of the island, except for Upcountry and the dry, sparsely populated stretch of coastline from Makena to Kaupo—and that may just be because there’s nobody living there to hear them. Nineteen outlying population centers have been eradicated by the group, meaning that no frogs have been heard there for a full year, but three remain in Kihei, Wailuku, and Kula, all of which involve plant nurseries.

Coqui are small—about the size of a nickel. They’re well-camouflaged by their brownish-tan coloration, have no natural predators here, and reproduce like crazy. A single female can lay four to six egg clutches as often as eight weeks apart, totaling 1,400 eggs each year, or over 8,000 in her lifetime. And for each calling male, biologists claim there are likely six or seven other frogs nearby that are either female or juveniles. Only the mature males chirp. And the coqui doesn’t waste time in a tadpole stage. Eggs develop directly into froglets in 17-26 days, which will mature into adult frogs in about eight months.

Thanks to their high reproductive rates and lack of competitors in Hawaiʻi, where food is abundant, their density here is unparalleled in the world, reaching nearly 37,000 frogs per acre in the highest densities. That’s two to three times those reported in its native Puerto Rico. 

Following genetic analysis, it is believed that the frogs which arrived on Maui were a separate population from those that infested the Big Island, rather than island hoppers. Maui’s population have a distinct stripe across their faces and are believed to have come from a former nursery site at the edge of the 425-acre Maliko Gulch. But most of the single frogs that MISC has captured elsewhere on the island appear to have re-invaded from the Big Island, where control efforts are no longer considered effective. So, even if we exterminate every frog in Haʻikū, we could face future invasions from our neighbor island. 

The first coqui was identified on Maui in 1997. Early arrivals which had hitchhiked from Puerto Rico on bromeliads that were then planted at resorts in Kaanapali shocked tourists with their shrill calls, prompting guests to cancel reservations and change hotels. Hotel management responded by sending their gardeners out with spray bottles full of a caffeine solution which, while effective at killing the frogs by speeding up their heartbeat until it failed, was also absorbed by workers, who found themselves feeling like they’d pounded a pot of coffee. That prompted the switch to citric acid. 

Today, a full-time crew spends nights hunting and spraying the frogs. And not with little spray bottles. Now they use fire hoses. Pallets of bagged citric acid, typically from China, are stacked at the MISC baseyard where it is mixed into a 12-16 percent solution with water and distributed to reservoirs in various neighborhoods where volunteer partners assist with the effort. 

“Every six weeks the neighborhoods will spray,” Fukushima explained. “They have training ahead of time and then we bring them the equipment and provide the citric acid for them. Every week there is a community that is spraying coqui in their backyards. That lets our crew get out more in those gulches and wildland areas to try to attack multiple places where the coqui are. It really helps us when those residential areas are being covered by neighbors.”

“It’s absolutely critical to our long-term success to have the community involved,” said acting MISC director Teya Penniman, who authored the group’s ten-year strategic plan for eradicating the frog from the island. “It was a number of years ago that we said, ‘This isn’t working. We can’t get to everywhere,’” she explained. So, they began asking residents for help.

John Phelps has been one of the organization’s most committed volunteers. A retired Lockheed Martin engineer, he received the 2022 Community Hero Award during Hawaiʻi Invasive Species Awareness Month in February for his leadership in organizing his neighbors to combat the frog’s spread in Haʻikū Hill, a community of luxury homes with million-dollar views right on the edge of Maliko Gulch, site of the island’s worst infestation. A veteran of NASA’s Viking Mars lander mission in the 1970s, he is an obsessive problem solver. 

This coqui male caught in Haikū shows the tan-colored bar between the eyes common to the frogs on Mauiʻs North Shore. Photo by Dan Collins

With materials provided by MISC, he and neighbor Norm Clark ran a PVC line along the edge of the gulch with valves that can accommodate hoses so they can spray the steep, hard-to-reach slopes from above. Clark’s wife Cathy manages a database where each household on the block can schedule their time to use the shared 100-gallon spray rig, which is mounted on Phelps’ pickup truck. At first they used it to feed the PVC pipe, as well, but 100 gallons didn’t last long, so he got his mauka neighbor Ian Walsh to put a reservoir tank on his land which now feeds the pipeline.

When Phelps and his wife bought their home, nights were quiet in that part of Haʻikū. Then they began to hear the frogs singing in the distance. Soon, they were in their backyard. “When we started spraying, all of the properties had hundreds of frogs making noise on every one of them,” recalled Phelps. 

“Every property every month had frogs coming back,” he said. “The reduction was slow because of the proliferation and the fact that they were just streaming from the perimeter.”

At one point, they had pushed the frogs out of everyone’s yard and stopped spraying, but within a month or two, they had returned. So, he and the rest of the community are vigilant again about keeping their monthly spray schedule. If coqui were to become widespread, it’s estimated that the collective loss of property values in the state could exceed $200 million, according to MISC.

Because coqui eat slugs as well as insects, they often carry rat lungworm. So, when they hand-capture the frogs, MISC workers always wear gloves. Sadly, there is no evidence that the frogs help us out by eating mosquitoes, as they haven’t been found in coqui stomachs. In fact, research indicates that the number of flies and other flying insects actually increases in the frog’s presence, perhaps due to the abundance of frog carcasses and feces. And while coquis do eat lots of ants, the MISC staff have noticed anecdotal evidence that the little fire ant will actually drive the frogs out of an area. So, in terms of biological control, fire ants might be effective. However, then we’ve got a much bigger little fire ant problem. “I would much rather have coqui than fire ants,” Penniman said. 

At this time, there aren’t any known biological control agents that can help with the frog problem, however one with unexplored potential is chytric fungus, which has been linked to amphibian mortality worldwide. Research is needed, but again, that requires funding.

In addition to capturing and spraying the frogs, other control methods include habitat modification—mainly removing undergrowth to eliminate refuges and egg-laying sites, and barrier fences at the edges of infested areas where adjacent neighborhoods are under active control.

Barrier fences made of mesh with a 90 degree angle that requires the frogs to walk upside down can be an effective barrier, if placed on the edge of an infested area like the one MISC installed where Kaluanui Rd. crosses Maliko Gulch. Additional barriers are being considered, but funding remains uncertain. 

The sad reality is that, as dedicated as they are, MISC can’t win this war alone. Federal assistance dried up a few years ago, and while the group enjoys robust support from the state and county, funding remains insufficient to tackle the problem effectively. Due to lack of resources, it has been eight years since they sent crews into Maliko Gulch. Instead, they have concentrated their efforts on the edges of the gulch and surrounding areas to prevent the frogs’ spread. 

That’s clearly a strategy for containment, not eradication. It’s been so long since MISCdeployed a crew to Maliko that they’re not even sure if the system of pipes they laid to deliver the citric acid is still intact. Their strategic plan sets a goal of returning crews to Maliko by 2025, almost a decade after efforts there were suspended. So, are the people on the front lines of this war giving up? 

The short answer is, no, these folks don’t throw in the towel easily. However the reality is that the longer it takes to eradicate the frogs, the more difficult and expensive it will become to do so. In 2007, the total cost of eradication was estimated at $3 million. By 2015, that cost had risen to $20 million. Today, MISC estimates an initial cost of $8 million per year, decreasing to $4.5 million over ten years, for a total of $63.5 million. Says the strategic plan, “Available resources have never been adequate and, as a result, costs have increased exponentially as coqui have continued to spread.” However, it also asserts that “without previous management efforts, coqui frogs would already be beyond control for Maui.”

MISC crew members prepare to enter a gulch where coqui frogs have gained a foothold. Photo by Dan Collins

How is progress measured? Historically, crew members have estimated numbers by listening for frog calls and counting how many individuals they hear. Multiply that by six or seven and you have a rough number of frogs for that area. However, new digital acoustic monitoring systems can do that for us, and they tend to be much more accurate when there are large populations. It’s hard to differentiate between hundreds and thousands of frogs by ear, but an algorithm can do it. 

“When you’re in a place where coqui are really abundant, it gets pretty hard to tell if you’re making a difference,” explained Penniman. “I mean, we know we are. But these devices, once they get [calibrated], then we’ll have a more objective reference.”

The difficulty is that the frog calls aren’t consistent, even when the population is. The males call much more when it’s wet out, and fall silent when it’s dry. So, unless you’re monitoring the weather conditions along with the number of frog calls, your data could be misleading. You think a population has been wiped out, and then the rains come and suddenly they’re back? Well, not really. “A lot of it is weather dependent,” explained Megan Archibald, acting coqui program coordinator. “So, when it’s dry for months and months, like the beginning of this year, we don’t hear anything, but that doesn’t mean the coqui frogs aren’t there.”

Let’s be honest, though. Digital technology may be able to help us count the frogs, but only hands-on hard work will eliminate them. And securing more funding, while essential, isn’t the only challenge. Hiring people in the current environment is increasingly difficult. We can find some solace in the last line of the strategic plan, “A coqui-free Maui is still possible.” But for how long?

To learn more about the Community Coqui Control Program or to report an invasive species that you’ve encountered, visit MauiInvasive.org

WELCOME TO THE JUNGLE: Down the Gulch with the MISC Coqui Frog Patrol

It’s well past dusk as the crew descends into the gulch. It’s their second attempt at hunting the elusive coqui frog on this drizzly autumn night. The first expedition, into another gulch alongside Haʻikū’s Giggle Hill, was cut short by roaming wild pigs. 

“We heard some boars, and then we peeked around the corner and Emily saw some piglets and we could hear the mom snorting at us, so we got out,” explains crew member Margaret Angel. Crew safety is the top priority.

This time, the target area is a thickly overgrown ravine just east of the old Haʻikū Cannery. Crew members connect a fire hose to the 400-gallon truck-mounted tank of citric acid that they’ve positioned near the edge of the gulch. The other end is connected by a camlock fitting to a length of white PVC pipe that extends deep into the darkness below. Armed with headlamps, thick boots, and protective gear, the six-person, all-woman team begins to descend a slippery, barely visible trail. Heavy lengths of fire hose are slung over their shoulders. Periodically, a pair of tiny eyes peer back from the foliage, reflecting the teams’ headlamps. But these women are used to the cane spiders and continue the trudge through dense patches of hau bush and cane grass. 

Having visited this site during daytime hours, the crew are all familiar with the terrain and know exactly where the highest concentration of calling frogs is found. “After getting to see them both at night and in the daylight, you get really comfortable with the area,” Angel explains. “You have certain landmarks that you’re like ‘OK I know where I’m at in this gulch’ to continue moving along.” 

“They love the cane grass,” Danielle Higashi chimes in.

The deeper we get, the louder the mating calls of the male coqui frogs become, until we’re surrounded by dozens of randy, loudly chirping frogs. The sound is constant and pierces the night air. This is the problem the Maui Invasive Species Committee’s Coqui Frog Control team is here to attack, and attack they do. Angel, a recent Colorado State Conservation Biology grad, attaches a long, coiled section of hose to one of the many connectors on the PVC line and opens the adjacent valve. Higashi takes the business end. 

Danielle Higashi sprays for coqui frogs as Margaret Angel holds the fire hose. Photo by Dan Collins

Once in position, Higashi radios to another crew member back at the truck to open the valve and turn on the pump. “I’ll take full pressure,” she says. A few moments later the fire hose stiffens with pressure and Higashi, also a conservation biology grad, opens the nozzle, showering the nearby bushes with citric acid. She feels like she’s found her calling. “I came back home and found this job and I ended up loving it. Something completely different than I imagined myself doing after college.”

The 12-16 percent solution of citric acid and water is mixed back at the MISC baseyard near old Maui High School and transported in one of several tanks, ranging from 400 to 1,000 gallons. Dip a finger into the tank and taste it and your mouth will pucker like you’ve just eaten the sourest lemonhead you can imagine. But the citric acid is food-grade and widely used in disinfectants, sanitizers, and fungicides, as well. 

The solution can damage some delicate plants, but is relatively harmless to humans, unless you get it in your eyes. It’s considered a minimum-risk pesticide by the EPA and is approved for general use by the Hawaiʻi Department of Agriculture, so it is exempt from oversight. The MISC crew wear long pants, long sleeves, and gloves to protect their skin—and safety glasses when actively spraying—but some of the acid mist invariably gets on your lips, making them taste sort of lemony. 

The citric acid solution is easily absorbed through the amphibian’s skin and kills the frogs on contact. It’s also effective at reducing the hatching success of coqui eggs, which the females lay in clusters attached to moist foliage. Because the frogs are nocturnal, and seek shelter under rocks or in thick foliage, it’s best to spray them at night, when they are active and easily located by listening for the “singing” males. 

Sometimes, individual frogs are caught by hand, although locating them can be difficult because their calls echo off nearby vegetation. And, of course, as soon as you get close, they shut up. But it only takes a minute or two for Danielle and Margaret to come back with frogs in hand. They will be bagged up and meet their end in a bucket of citric acid later. While most critters in the jungle are unharmed by the spraying, some non-native invertebrates—like earthworms, slugs, and cane toads—are vulnerable. It’ll also kill the greenhouse frog, another invasive species that gets little attention because it’s far less vocal and fecund. Collateral damage in the war.

As the women work, the sound of the calling frogs around us grows quieter and quieter until only distant voices remain. It’s after 10 p.m. when we make our way back to the baseyard near Hookipa, where the tanks and equipment are cleaned and stored for the next mission. As the team wraps up, Serena Fukushima, MISC’s PR and education specialist, mentions that it’s a great job for surfers or others who like their mornings free. “All our night owls, perfect job for you,” she said. 

Do they sometimes wish they’d chosen an easier career? One that doesn’t involve trudging around the wilderness in the dark hunting frogs? “This is exactly what I wanted to do,” says Angel, who just earned her degree in May. “I find invasive work to be incredibly rewarding, especially on an island with such a unique ecosystem.”

Dan Collins

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