CONSERVATION BIOLOGY:
Hawaii's Coral Trees Feel the Sting of Foreign Wasps
Carolyn Gramling
Island researchers are desperate to find a natural enemy of the parasitic wasps that are killing a local treasure, the wiliwili
The once-beautiful coral trees on the University of Hawaii's Manoa campus where botanist David Duffy works have deformed lumps where the leaves and flowers should be. "Trees here look like they have been hit by a flamethrower," says Duffy.
The bulbous growths are infested with tiny wasps, a recently identified parasitic species that first appeared in southern Taiwan in 2003. Within a year, the wasps had spread across the island and had also reached Singapore, Reunion, and Mauritius. By 2005, they appeared in Hong Kong and China and were first seen on Hawaii's Oahu island this April. By August, the wasps had invaded every island in the state, threatening the existence of one of Hawaii's most enduring symbols, a native tree locally known as the wiliwili that provides flowers and seeds for leis and bark for canoes.
Threats to Hawaii's native species by foreign invaders are nothing new. Long cut off from the rest of the world, Hawaii's endemic species are particularly vulnerable to invasion by foreign insects, plants, and other organisms, and state officials constantly race to keep up with the latest threat (Science, 2 December, p. 1410). But even the state's weary conservationists have been stunned by the speedy efficiency with which this latest pest has spread from island to island. And now, researchers are struggling to identify any measure, from burning infested trees to chemical or biological defenses, that can halt the wasps' devastation of the native wiliwili and other nonnative species of coral trees. "Either all the trees are going to die, or they'll never be the same again," says botanist Art Medeiros of the U.S. Geological Survey's Haleakala Field Station on Maui.
The wasps, dubbed Quadratichus erythrinae in 2004, lay their eggs in green stems and leaves of the trees, creating outbreaks of tumors that stunt the trees'growth and eventually kill them. The wasps disperse easily as larvae-infested tissue falls off and is scattered with the wind, or as adult wasps emerge to lay more eggs in new growth.
Inside attack. Infestations of parasitic wasps (right) transform Hawaii's cherished coral trees (top) into deformed eyesores (bottom) and threaten their survival.
CREDITS (TOP TO BOTTOM): ART MEDEIROS; RON HEU/WALTER NAGAMINE; MICHELLE TREMBLAY (INSET) |
The Asian-Pacific path of the wasps parallels the habitat of the genus of trees called Erythrina, popularly known as coral trees. Erythrina's 115 species are found around the world in tropical and warm temperate regions, from Southeast Asia to the southeastern United States. With their bright red flowers, they are highly prized as ornamental trees and have figured widely in local mythology. Native and nonnative Erythrina are both extensively cultivated in Hawaii, but the wiliwili is the only species of the tree that is found exclusively in the state. A dominant species in the large dry forests that form on the leeward slopes of many of the islands, wiliwili grow on rocky lava substrates called aa, a forbidding terrain that has helped discourage previous invaders.
"The species has been bulletproof," says Medeiros. But now, he fears, the trees are in danger of extinction.
That danger has resulted in a concerted effort by state and federal officials and university researchers to find an effective remedy. Cutting down infested trees and burning the detritus has proved ineffective, Duffy says: The wasps simply spread too quickly. Another possible solution is injecting the trees with an insecticide, says Anne Marie LaRosa of the U.S. Department of Agriculture's Forest Service in Hilo. However, such a strategy is very expensive, costing nearly $30 per tree, and is likely to prove impractical on trees in the wild. "The only point in treating them chemically is as a stopgap method," LaRosa says. Injections could preserve some trees for a while, giving researchers more time to identify a biological control agent--now considered the only viable long-term solution.
But biological control agents are fraught with their own dangers, as Hawaii well knows. Fifty years ago, a different species of parasitic wasp was brought into Hawaii to repel sugar-cane pests; those wasps now dominate the food web of the Alakai Swamp, a wilderness preserve on Kauai island. Such cautionary tales highlight the need for stringent prerelease testing to ensure that the new agents won't run amok, researchers say (Science, 17 August 2001, p. 1241). "We need to be incredibly sure that whatever we try to introduce will not attack native species in Hawaii," says entomologist Daniel Rubinoff of the University of Hawaii, Manoa.
Because the coral tree wasp is brand-new, adds Rubinoff, he and other researchers seeking a biological control agent have their work cut out for them. Rubinoff, with colleagues Russ Messing and Mark Wright, is working on identifying the origin of the wasp. Africa is the likeliest source, they believe: Scientists in South Africa have seen similar gall-forming parasitoid wasps on Erythrina species in the region. As a result, Hawaiian researchers are soliciting wasp samples from colleagues in Kenya and South Africa, and they are preparing to mount expeditions to other possible hot spots on the continent, hoping to locate a natural enemy that will be specific to the wasp.
The University of Hawaii team will head to South Africa in March 2006, which should coincide with the end of the rainy season there, when the trees will be sporting new growth and infestations will be easier to find. Meanwhile, state of Hawaii entomologist Mohsen Ramadan hopes to leave by the end of 2005 for Tanzania, also to coincide with the rainy season in that country.
Back in Hawaii, scientists are working on a last-ditch solution. Called the Noah of wiliwili," Alvin Yoshinaga, a botanist at the University of Hawaii's Center for Conservation Research and Training in Honolulu, is overseeing a collection of the trees' seeds, harvested by volunteers on all the islands and hoarded against the day that a wasp-control method is found.
"We are trying to gather seeds from as many subpopulations on different islands as possible," Duffy says. Identifying an effective but safe biological control agent could take anywhere from 1 to 50 years, he adds--and the trees almost certainly wouldn't last that long. "We have very little time," Rubinoff agrees. "All of the Erythrina are being hammered."
