Hawaii gained a new tool last week in its battle to reduce burgeoning populations of the tiny, screeching Puerto Rican coqui frog.
overcomes coqui frogs
State workers employ citric acid
on Oahu and the Big Island
By Diana Leone
A 16 percent solution of citric acid in water -- about double the strength of lime juice -- was OK'd by the state Department of Agriculture as a safe way for homeowners and nurseries to kill the alien pest.
Since the concoction is made with food-grade citric acid used in canned juice or sodas, the U.S. Environmental Protection Agency doesn't regulate it, or consider it a human health or environmental concern. But it wasn't until Thursday that the state released instructions on how to use citric acid to kill coqui (Eleutherodactylus coqui) and their cousins, the greenhouse (Eleutherodactylus planirostris) frogs.
The frogs, which measure less than two inches, are considered invasive species, and the coqui have a piercing call, especially in large numbers.
Workers with the state Agriculture and Land and Natural Resources departments are using the citric acid mixture in two locations: at a small infestation of the coqui frogs at the Iwilei Home Depot store in Honolulu and at the largest infestation in the state, Lava Tree State Park on the Big Island.
In both cases the citric acid was applied after plants where the nocturnal frogs hide during the day were cleared away. At Home Depot that meant removing a thick groundcover near the store's parking garage. At least 16 coqui frogs and more than 100 greenhouse frogs were captured there, many of the coqui caught alive as the plant cover was removed and before the citric acid was sprayed Thursday and Friday, said Mindy Wilkinson, invasive species coordinator for the state Division of Forestry and Wildlife.
The Home Depot frogs had been living in the isolated patch of greenery for at least a year and a half, but attempts to hand capture them in the past were foiled by the thick plant growth, Wilkinson said.
State wildlife workers will continue to monitor the area, which could become the first place a known breeding population of the frogs is eradicated.
At Lava Tree State Park, where the cumulative sound of hundreds of male coqui calling in the evenings has been at times so loud you can't carry on a conversation there, at least 100 adult coqui are presumed dead in a one-acre area, based on a rapid survey after the first citric acid spraying Thursday, said Arnold Hara, a scientist with the University of Hawaii's College of Tropical Agriculture and Human Resources and leader of the Big Island's Coqui Frog Working Group.
Workers from several state and federal agencies and inmates from the Hawaii Community Corrections Center cleared nonnative plants and a few nonnative trees from about an acre of the park, said Larry Nakahara, with the state Agriculture Department.
They are removing plants where the frogs can easily hide and will replace them with native plants that aren't as hospitable to them, Nakahara said. Big Island residents are being asked to help with the native replanting, which should improve the park, he said.
"This is a demonstration that you can manipulate the habitat and control the frogs," Nakahara said. "If you can do it there in Lava Tree State Park, where conditions are the worst, you can do it in a yard, or anywhere." The park's cleanup and replanting should be complete by April, he said.
"Non-target" creatures including earthworms, slugs, moths, snails, ants and mosquitoes were observed in the citric acid sprayed area the next morning, Hara said, which is a good thing.
Pro-coqui Big Island resident Sydney Ross Singer, who objects to killing the frogs on the grounds they aren't a serious problem, complained in a release Friday that the Lava Tree operation was "defoliating" the park and "drenching" it with citric acid.
Not so, Nakahara and Hara said.
There is a downside of using citric acid to fight the frogs, Nakahara said. It can "burn" sensitive nursery plants, discoloring their leaves and in some cases killing them.
But apparently a number of Big Island nursery growers are willing to give it a try. BEI Hawaii, a wholesaler of fertilizer and agricultural chemicals, shipped a ton of the stuff to Hilo Thursday. Friday it was all gone and BEI was ordering more, said sales director Carolyn Ambrose.
BEI Hawaii sells citric acid in 50-pound bags, but some garden stores may repackage it into 5- and 10-pound bags, Ambrose said. Department of Agriculture instructions will be included with each sale, she said.
Based on the recommended ratio of 1.28 pounds of citric acid to a gallon of water, 10 pounds should be plenty for an average-size yard, Nakahara said.
"I think initially you'll see quite a bit of people using it, especially where frogs have been abundant for a while like in East Hawaii," Nakahara said.
Scientists have said for some time that there is probably no single "silver bullet" for reducing the coqui's numbers in the islands. A caffeine solution proved effective in trials, but cannot be used again unless approved by the EPA.
Other techniques being used at the nursery level include steam-heating potted plants, chilling them to kill off frogs and their eggs or killing frogs with hydrated lime, a common soil supplement.
Officials hope they can eradicate the coqui on Oahu and Kauai, which have smaller populations. They admit that it may be impossible to get them off the Big Island and Maui, where they are more established.
It's believed the frogs hitched a ride to Hawaii on greenhouse plants shipped to the state in the late 1980s or early 1990s. Since 1998 the expansion of the frogs has been exponential, mostly via shipments of nursery plants interisland, but in a few cases because of deliberate actions by a few individuals, according to scientists Fred Kraus, with the Bishop Museum, and Earl Campbell, with the U.S. Fish & Wildlife Service.
The frogs are invasive because they can effect both the economy and the ecology of Hawaii. Their screeching calls bother most people to the point that coqui infestations could lower property values or hurt tourism. And scientists say the frogs eat insects that native birds could feed on and could be required to pollinate native plants. Finally, the frogs themselves could be a bountiful food source for non-native snakes, which also are invasives.
"It's exciting to finally have a tool available where homeowners can go out and control problem areas in their yard," said Christy Martin, spokeswoman for the Hawaii Coordinating Group on Alien Pest Species.
Department of Agriculture
Department of Land & Natural Resources
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