1. The Problem
The scene: Hilo High School auditorium. The date: an evening in early May. The occasion: After months of rehearsals, students from Hilo and Waiakea high schools perform their hearts out in “Jekyll and Hyde: The Musical.”
Throughout the performance, an unscripted chorus accompanies the dialogue and drowns out all but the most robust vocalists and ensemble numbers. It is the sound of the coqui, a tiny native frog of Puerto Rico that has been invading more and more neighborhoods on the island of Hawai’i. Thousands of the frogs inhabit the brush behind Hilo High, and their noise fills the night air, disrupting evening classes as well as the occasional student play.
Across the islands, such scenes are occurring with increasing frequency. From Cape Kumukahi at the eastern end of the island chain to Limahuli Valley, near the western end, tiny but populous frog cities are popping up, unseen to all but the most vigilant, but audible to nearly everyone. Initially, the soft sounds of the evening and night are pieced by the distinctive two-tone chirp of one or two males of the species Eleutherodactylus coqui (the females are silent). Within a few weeks or months, the numbers of frogs have multiplied and the occasional chirp has grown into the amphibian equivalent of a Rolling Stones concert.
From 1999 to 2001, the number of confirmed frog populations jumped more than tenfold, to 150 by mid-year. Now, no one hazards a guess as to the number. The exploding population prompted the Department of Agriculture to designate the coqui as an agricultural pest; the designation means it is illegal to ship the frogs within the state and to release them deliberately. (They are not a listed noxious species, a designation that allows the DOA to eradicate them on private land without the owner’s permission.)
Where are the infestations now? Kim Tavares, a staffer with the state Department of Agriculture in Hilo, laughs at the question. “It’s easier to say where they are not,” she responds. “They’re moving to new places every day – pretty much all roaded areas.” To her knowledge, the coqui have not strayed too far into the wild, “except where they’ve been released on purpose – Kalopa State Park, Manuka State Park.”
As much as the noise of the coqui is a nuisance in populated areas, some researchers think its presence on the islands poses a potential environmental nightmare of even greater dimensions. Bill Mautz, assistant professor of biology at the University of Hawai’i at Hilo, notes that in Hawai’i, where the frog has no natural enemies, the coqui can achieve densities of 15,000 per acre, many times the densities reported in the frog’s native range.
With these high densities, Mautz notes, the coqui and the smaller (and quieter) greenhouse frog (Eleutherodactylus planirostris) “are eating a huge amount of insects and other crawling bugs and spiders in the ecosystem, and so we expect they’re going to change the community of animals in the forest – insects, spiders, other arthropods – in ways that are hard to predict.” Some researchers have estimated such dense populations of coqui may eat up to 200 pounds of bugs per acre per year.
Do they eat mosquitoes, as some champions of the coqui claim? Only rarely, says Mautz, who has analyzed stomach contents of many coqui. “Coqui tend to reduce the numbers of spiders,” he says. “The coqui are particularly attuned to identify things that crawl in front of them. They’re not major mosquito eaters.”
The fact that the coqui eat bugs has been touted by some as a reason to welcome their growing numbers on the islands. Sydney Ross Singer, head of the Coqui Hawaiian Integration and Reeducation Project (CHIRP) has penned an ode to the coqui that includes the couplet, “We should be giving them big hugs/The coqui frogs eat lots of bugs.”
Fred Kraus, an entomologist at Bishop Museum, scorns such attitudes. “Now in our culture, people are trained not to care about insects, they laugh and think it’s a big joke, and say okay, we’ll have fewer roaches and mosquitoes” with the coqui, he told Environment Hawai’i. “Frankly, it’s kind of an infantile attitude. Part of what characterizes Hawai’i’s uniqueness is the tremendous array of native insects we have like native crickets, native fruit flies. Even if you don’t care about insects in their own right, of course they also serve as the food for a large number of our native birds. Virtually all our native birds require insects at least to feed their young, even if the adults don’t eat insects themselves. So the potential ramifications for the food web are large.”
Mautz suggests the dispersal of the frogs in Hawai’i may have been facilitated by previous invasions. “When you have an isolated island, sensitive to invasion by exotic plants and animals, the invasion of one species creates a situation where it’s easier for others to invade,” he says, adding: “This is a fairly new idea in ecology.”
“In the case of frogs, we have nitrogen-fixing trees, particularly albizia, that are much more productive than the native `ohi`a dominated forests. There is a lot more chemical energy available for other species to insert into the food chain. The stage is then set for invasion by coqui and other species.”
And the coqui invasion in turn may set the stage for the establishment of other unwanted species. “What worries me absolutely the most about the coqui invasion,” says Mautz, ” is if you have a situation where you have 15,000 frogs per acre, you have set the stage for the colonization of frog-eating snakes, such as the brown tree snake.”
Kraus dismisses those champions of the coqui who say there is no hard evidence of the damage they do. “People are saying, well, there’s no evidence that these things have been a problem here,” he says. “We don’t wait until we see a house burning to gather around, discuss among ourselves the likelihood of that fire inflicting significant damage before we construct a fire department. We try to be proactive. We anticipate the problems and try and head them off and stop them at the earliest possible time.
“It’s the same exact thing we’re trying to do with the frogs and other pests in the island. We’re trying to put this fire out as soon as possible.”
2. The Origins
Until the mid-1990s, the coqui and the greenhouse frog were not known to exist in Hawai’i. They were introduced as accidental tourists on plants imported to Hawai’i, probably from Florida or the Caribbean. The earliest infestations were found in nurseries and adjoining areas. The frogs, continuing their hitchhiking adventures from wholesaler to retailer to home garden, eventually settled in other areas. Usually the dispersal was not intentional, but in at least one case that Kraus has reported, a nursery owner allowed frog aficionados to capture animals on his grounds, with the intention of establishing frogs at their homes.
The coqui have an unusual reproductive cycle, which avoids the tadpole stage altogether. A female frog can lay up to six clutches of eggs a year, with an average of 28 eggs per clutch. From the eggs hatch tiny, fully formed froglets. With no predators, and without the need for bodies of water to support a tadpole stage, the coqui in Hawai’i is a virtual replicating machine. Just one pair of coqui of average fecundity can have 20,000 descendants in two years.
Dispersal of coqui and greenhouse frogs through plant sales remains one of the chief means by which they spread from neighborhood to neighborhood and even island to island. Other means that the DOA’s Tavares has identified include deliberate introduction and the unwitting transportation of frogs in wheel wells of vehicles parked in infested areas.
In March, the U.S. Department of Agriculture announced it had rewritten its eradication plan. While still working to eradicate the frogs on Maui, Kaua’i, and O’ahu, the USDA would no longer have eradication as its goal on the Big Island, said Mike Pitzler, head of the USDA’s Wildlife Services branch. The Big Island’s Coqui Frog Working Group has not yet given up, although each day the odds against eradication seem to grow higher.
3. The Solutions?
Since the coqui began to make a nuisance of itself in Hawai’i, the state has looked at several control measures. One of the earliest was a hot bath for landscape plants. Water at about 113 degrees Fahrenheit drives the frogs off plants and allows them to be captured more easily than simple unassisted hand capture. This approach has obvious limitations, however. It does not work well on areas that are heavily infested or out of reach of a source of warm water.
More controversial was the identification of caffeine as a means of killing the frogs. Although application of a caffeine spray to infested areas received emergency approval from the Environmental Protection Agency, it was not used owing to concerns over possible water quality impacts. EPA approval has since lapsed, although some at the Department of Agriculture are still exploring ways in which caffeine might be useful in the future.
Most recently, a 16 percent solution of citric acid has been identified as a “frog-icide.” The acid kills frogs on contact, but, if it is not washed off within an hour of application, sensitive plants can be damaged. Because of this, some nurseries have been reluctant to employ citric acid against coqui infestations. Citric acid also has the benefit of killing frog eggs. In laboratory experiments, almost all eggs were killed when sprayed with citric acid that was allowed to remain on the eggs. When the acid was washed off within an hour, about a quarter of the eggs survived.
Applying hydrated lime to frog-infested areas has also been reported to be effective in reducing their numbers. But since the lime hasn’t been approved for this use by the EPA, the Department of Agriculture is not actively endorsing it. In addition, the lime can burn plants. (Because citric acid is a food product, it does not need to have EPA approval.)
The option of biocontrol – using one organism to fight another – is also being explored by the DOA. A fungal disease, chytridiomycosis, that has devastated vulnerable frog populations across the world is one agent under consideration. Arnold Hara of the College of Tropical Agriculture and Human Resources told Environment Hawai’i that another might be a phorid fly that eats coqui eggs in Puerto Rico. “This species is already in Hawai’i,” Hara says, “and may only need some time to begin infesting coqui eggs. Also, there may be another fungus that attacks coqui eggs in Puerto Rico. We will be identifying this and investigating the possibility of using it as a biocontrol agent, too.”
CTAHR has prepared a brochure for homeowners who want to discourage coqui from settling in their yards. Measures they can take include preventing the introduction of hitchhiking coqui on new landscape plants; capture of coqui by hand; spraying with citric acid or giving plants a hot-water bath; and eliminating thick understory growth or leaf litter, areas of prime frog habitat. (For a copy of the brochure, call CTAHR in Hilo at 808 974-4105.) Meg Jones, CTAHR’s public outreach officer on the coqui, suggests that, as with any problem, action be taken as early as possible. “The way the frogs reproduce, a small problem can grow into massive one in a short time,” she says.
— Patricia Tummons
Volume 13, Number 12 June 2003
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