The state is seeking approval to release a tiny African insect on all islands that is believed to be the best hope in controlling its invasive cousin — the destructive Erythrina gall wasp. Since it was first detected in Manoa two years ago, the wasp has killed thousands of trees from urban Honolulu to remote regions of the Neighbor Islands.

State scientists have spent more than a year studying the predator insect — a parasitic wasp found in Tasmania — and say they are confident it will not have any negative impacts on the environment. Though they have just started the permitting process, which includes a public comment period, they hope to release the African bug by the end of the year.

The gall wasps, also native to Africa, have been one of the most environmentally damaging and costly invasive species to creep into the Islands in decades, scientists say. A plethora of pesticides have done little to stop them and they have no predators in Hawai'i, which has allowed them to spread quickly and relatively unabated.

"It's been one of the worst we've seen in a long time," said Neil Reimer, manager of the plant pest control branch at the state Department of Agriculture. "By the time it was discovered ... it (had) hopped off to the other islands."

The wasps target trees in the genus Erythrina, including the indigenous wiliwili and non-native tropic coral and coral trees.

Conservationists say the insect has depleted already-small populations of wiliwili in native dry forests on Maui and the Big Island. Meanwhile, the city, state and private landowners have had to pull out thousands of tropic coral trees, once one of the most common landscape trees in Ho-nolulu and elsewhere, and the tall, spindly tropic coral trees often used as windbreaks around farms. The city alone has cut down about 1,000 trees from parks, medians and along roadways since late 2006.

"We've had infestations before, but nothing like this," said Stan Oka, chief of the city forestry division.

The last time the state released a biological control for an invasive species was in 1999, when the citrus black fly was damaging crops on the Big Island and Maui, according to the state Department of Agriculture.

About two years after a parasitic wasp from Guatemala was released, the citrus black fly was under control and the Guate-malan wasp was kept at bay by other insects. It was a success story made possible by advances in science since the 1970s, when Hawai'i entomologists started methodically testing not only how effective biological controls are, but how they could affect other species.

In fact, though the history of releasing alien species in the Islands to control pests is littered with bad results — most notably the introduction of day-hunting mongoose to kill nocturnal rats in the 1880s — there have been no major negative effects from introduced biological controls in more than three decades, state Department of Agriculture scientists say.

"When people think of bio-control, they think, 'Oh, the mongoose, that didn't work so well.' We have to remember that was done more than 100 years ago," said Christy Martin, the spokeswoman for the Coordinating Group on Alien Pest Species, a consortium of public and private interests dedicated to protecting Hawai'i from invasive species. "Today, the Department of Agriculture ... conducts very rigorous tests. They will look at any number of ways that a bio-control will go wrong."

Since 1890, more than 850 alien species have been released in Hawai'i for pest control. Of those, more than 80 percent targeted insects, the state Agriculture Department says. Still, scientists and environmentalists point out that releasing an alien species to combat another one is always a last resort.

Currently, the state is working on testing biological controls for two other invasive species — the forest weed miconia and fireweed — but the release of any species is still years away.

"Anytime you fight fire with fire, there's a risk of causing two fires," said Bob Loy, director of environmental programs for The Outdoor Circle. "I don't think anyone takes it lightly. But we would hope that any type of biological control would be properly tested and have a reasonable expectation of success before it's released on the countryside of our state."

It is unclear how long it will take to control the gall wasp population with the biological control insect.

The gall wasp lays its eggs inside the plant tissue of leaves, and its larva form galls, sac-like chambers in which the wasps mature. The galls severely deform leaves, leading to defoliation and, in most cases, the death of the tree. Indigenous wiliwili trees have not been dying at the same rate as the introduced coral and tropic coral trees, which have almost disappeared from the landscape. But they are not faring well.

And some scientists fear they could reach endangered status soon.

Robert Hauff, state forest health coordinator with the Department of Land and Natural Resources, says the biggest concern with the wiliwili is many are not producing seeds, probably because they are so weakened by the gall wasp and other environmental factors, such as years of drought.

Scientists have saved seeds from the wiliwili at Lyon Arboretum, just in case the trees die out completely. But they are optimistic the predator wasp will help the trees get stronger.

"We're definitely hoping the bio-control will allow the trees to reproduce," Hauff said.

The female eurytoma — the wasp the state wants to release — attacks gall wasps by inserting its egg into a gall inhabited with a wasp larva. Once the egg hatches, it feeds on the larva. Neither the eurytoma wasp nor the gall wasp harms other species.

State exploratory entomologist Mohsen Ramadan found the eurytoma wasp when he traveled to Tasmania in January 2006 for the state searching for predators of the gall wasp. Two other insects also were collected, but the eurytoma showed the most promising results.

The state Department of Agriculture has applied for state and federal permits to release the eurytoma wasp.

A draft environmental assessment conducted on the release is available for comment through Oct. 8, at which point Agriculture Department officials will determine whether to issue the permit or call for an environmental impact statement.

If an environmental impact statement is not needed, the state process could be completed within two months.

Officials expect an answer on whether the federal permit has been approved within a few months.

Even as they discuss the release of the parasitic eurytoma wasp, scientists are still trying to determine how the gall wasp got to the Islands — a mystery, they add, that illuminates holes in the protections meant to keep invasive species out of the Islands.

Some suspect it got to O'ahu from Taiwan, perhaps aboard a plane. There is no way the wasp would have been detected, and it would have had an easy supply of food right away — tropic coral trees used to dot Honolulu International Airport.

Reach Mary Vorsino at mvorsino@honoluluadvertiser.com.