Environment: Wiliwili Trees Under Attack

A mysterious wasp is spreading through Hawai‘i.

The damage (main) and the culprit (inset). photo: courtesy Hawai‘i Department of Agriculture

It may only be a millimeter long, yet the invasive gall wasp has managed to infest nearly every tree of the Erythrina genus in Hawai‘i—including the native wiliwili tree, and non-native coral trees.

Discovered in Manoa in April 2005, the wasp had spread across the Islands by the following October, destroying trees by laying eggs in the tissue of young leaves, shoots and flowers. The trees react by creating knobs, or galls, around the eggs (see left), causing the leaves to wither and fall. Severe infestation can defoliate and even kill a tree. While the insect isn’t harmful to humans, their effects have prompted the city to remove affected dead and potentially hazardous trees from the civic center grounds, Salt Lake District Park, Ke‘ehi Lagoon Park and Magic Island.

“We know little about the insect in general; it was only discovered by science in 2004,” says Robert Hauff, forest health coordinator for the Department of Land and Natural Resources. Its introduction to Hawai‘i is also a mystery.

What to do about the wasp? The answer may be more wasps, specifically, three species of parasitic wasps, now under quarantine with the Hawai‘i Department of Agriculture. Mark Wright, assistant professor of entomology at UH, explains, “The female parasitic wasp lays eggs inside the gall, and parasitic larvae then emerge and feed upon the gall wasp.” As unpleasant as that might sound, this may be the only long-term solution.

Introducing a new species to Hawai‘i now requires extensive testing—enter thoughts of the mynah bird and mongoose mishaps—so it may take up to five years for the parasites to be released, although Wright hopes this can happen sooner.

Until then, chemical and pesticide trials—in which a tree receives an injection of insecticide into its vascular system—are underway. “Because this is expensive, and we don’t know how long the injection will last, this would only be used in special situations, such as in a restoration site where the tree is of importance to the ecosystem, or if there is a population that is genetically significant,” says Hauff.

Last fall, volunteers and resource managers, fearing the worst for the wiliwili, collected more than 100,000 seeds from across the Islands and deposited them in a seed bank at Lyon Arboretum. “The hope is that, some day, a biological control for the wasps will be available, and, if all the trees are wiped out, we would be able to grow seedlings and reintroduce them,” says Hauff.

Some wiliwili populations are holding on despite last year’s grim outlook, possibly due to the winter’s heavy rains, and other factors. This fall, Hauff and other researchers will monitor wiliwili trees to see if they’re strong enough to produce seedpods and flowers. With any luck, these efforts will take the sting out of the gall-wasp invasion.