Two years ago, Hawai‘i’s colorful wiliwili trees faced a grim future. In only three years, a tiny but voracious invader called the erythrina gall wasp had nearly decimated the wiliwili and its non-native cousin, the coral tree.
But now the trees are blooming again. Scientists have given the gall wasp what Hawai‘i lacked: a natural predator.
A team of ecologists from the Hawai‘i Department of Agriculture and the University of Hawai‘i pulled off their coup by importing an East African parasitic wasp that feasts on the larvae of the invasive erythrina gall wasps while leaving native wasps alone. Released in 2008, within months the newcomers were wiping out the earlier invaders. Now, only two years later, the team is wrapping up the project, which has turned the gall wasp from a plague to a minor pest.
Researchers are optimistic, though not ready to declare victory just yet. “It’s too soon to really say if this is a complete success or not,” says Leyla Kaufman, an entomologist at University of Hawai‘i. It remains to be seen whether the recovered trees can produce viable seeds, she explains.
The erythrina gall wasp is named after its victims – trees in the Erythrina genus such as the wiliwili and coral tree. The trees are known for their radiant orange and red flowers, and locals cultivate them as ornamental trees and as windbreaks throughout Hawai‘i. Erythrina gall wasps cripple Erythrina trees by laying eggs in young tissue, such as new leaves.
“The larva basically excretes hormones which co-opt the plant’s cells and causes them to reproduce almost like a cancerous growth,” explains Daniel Rubinoff, a University of Hawai‘i entomologist who studies gall wasps. These tumor-like outgrowths, known as galls, then serve as a food source for larval wasps. Because galls concentrate nutrients from the surrounding plant tissue, gall wasps effectively steal a wiliwili tree’s nutrition. As a result, a tree infested with gall wasps starves.
The gall wasp infestation has been costly to the islands. In 2008, Honolulu alone spent over 1 million dollars removing dead wiliwili trees. In contrast, officials estimate that the gall wasp project cost Hawai‘i about 80,000 dollars a year since its inception.
Only identified as a species in 2005, the erythrina gall wasp first arrived in Oahu in April of that year. “Within six weeks it was over all the island,” says Rubinoff. By the end of the year, the wasp had infiltrated all of Hawai‘i’s major islands.
“It got to the point that arborists and botanists were collecting seeds to keep in case [the wiliwili tree] does go extinct,” says Neil Reimer, chief of the plant pest control branch in the Hawai‘i Department of Agriculture.
The pest control team saw one possible hope. Some species of predatory wasps, called parasitoid wasps, feed on gall wasp larvae. However, Hawai‘i didn’t have these predatory wasps. To save the wiliwili, they would have to be introduced — a delicate proposition on an island chain that is already overrun with exotic species, some of which took over after being introduced to control other pests. For example, the mongoose, released to control rats, now targets native birds instead.
To find a predator that would leave native insects alone while feasting on gall wasp larvae, state and university scientists traced the gall wasp’s origin through its source of food: Erythrina trees. East Africa is richest in Erythrina species, and although researchers located an erythrina gall wasp doppelgänger there, the trees had few galls—a sign that predators were keeping the wasp in check. Over the course of several trips in 2005 and 2006, the team identified three parasitoid wasp species preying on East African gall wasps and brought them back to Hawai‘i.
The first candidate species tested, Eurytoma, ignored native gall-forming insects and feasted only on erythrina gall wasps. In 2008, researchers released Eurytoma wasps at specific sites. Within two months, says Kaufman, wiliwili branches were growing new leaves. Within four months, Eurytoma wasps were spreading beyond release sites to invade galls at other locations. Though tests show another candidate wasp also targets gall wasps, Eurytoma wasps have been so successful, says Kaufman, that it is unlikely that the other candidate predators will need to be released.
University researchers are applying for more grants to determine just how robust the wiliwili recovery is, says Kaufman. Gall wasps will never be eliminated, explains Hawaii Department of Agriculture’s Reimer, but Eurytoma are bringing their population into balance, allowing Reimer’s organization to concentrate on other invasive species that still crowd Hawai‘i’s native flora and fauna.
“We’re always putting out fires,” says Reimer. “We’ve collected good data for two years [on the wasps], now we’re ready to move on.”