Insects being released to attack gall wasps

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A tiny insect that attacks the Erythrina gall wasp that has devastated Hawaii's wiliwili trees is being released in the islands by the state Department of Agriculture in an effort to prevent further decimation of the native wiliwili.

The new insect, Eurytoma erythinae, is a small black wasp, slightly larger than the gall wasp. The gall wasp's larvae burrow into the leaves and stems of the wiliwili and heavy infestations eventually kill the plant.

The Eurytoma larvae attack the gall wasp larvae. An initial release of 500 Eurytoma wasps was conducted this week at the Liliuokalani Botanical Gardens in Honolulu. A release of several thousand Eurytoma wasps is planned on Maui in the second week of December in sites in Kula, in South Maui, and around Lahainaluna High School, state agriculture officials said.

Eurytoma's release follows more than three years of research into the insect's potential as a biocontrol agent and studies on whether it could become a pest itself if released in Hawaii.

Neil Reimer, manager of the state Plant Pest Control Branch, said the studies established that the Eurytoma erythinae is a predator specific to the gall wasp and will not attack any of Hawaii's native insects or other beneficial insects.

"It's a small black wasp, probably twice the size of the gall wasp, but still minute, about the size of a gnat," he said.

"It will land on the plant and search for the gall in which the gall wasp larvae is feeding. It has an ovipositor that can probe the plant and lay its eggs next to the larvae of the gall wasp.

"It lays multiple eggs. When they hatch, its larvae feed on the larvae of the gall wasp."

He said the Eurytoma female on average will lay on average 160 eggs, and can lay up to 300. As the numbers of the Eurytoma increase, the predator is expected to bring the population of the gall wasp under control, giving the wiliwili trees a chance to survive.

"They will not get all of the gall wasp. With any biocontrol, you won't exterminate the pest species, but you can control the pest so it's no longer a problem," Reimer said.

The gall wasp, Quadrastichhus erythrinae Kim, was first discovered in Hawaii in April 2005. By then it had already spread across Oahu and quickly moved to the Neighbor Islands where it attacked not only the native wiliwili, Erythrina sandwicensis, but several related varieties of Erythrina, including the coral tree and a Fijian species, E. variegata, that was planted as a windbreak around farms on Maui.

The gall wasp lays its eggs in leaves and young stems, where the larvae form galls, or lumps, in the plant. With heavy infestations, the growing shoots and leaves become so severely deformed that they die and the plant itself eventually will die.

When the gall wasp was first discovered, agricultural specialists tested different types of chemical insecticides to treat infested trees, with limited success. Since the native wiliwili grows wild in coastal drylands and along leeward slopes of all of the islands, chemical control was not considered a viable method of controlling the gall wasp.

The Eurytoma wasp was discovered in Tanzania in 2005, shortly after the gall wasp appeared in Hawaii, by a Department of Agriculture entomologist, Mohsen Ramadan. But it was not approved for release by the state Board of Agriculture until last year, and a permit for release from the U.S. Department of Agriculture was not granted until this month.

Reimer said state enotomologists have been producing Eurytoma wasps in an insectory since 2006 and expects to be able to release 1,000 of the predator wasps at a site.

In Tanzania, the gall wasp is a native species, he said, but it is not a significant problem to the Erythrina trees in the region. There may be trees with a few galls developing after the Eurytoma wasp is released, but the population of gall wasps should no longer be a threat to the trees, he said.

"If it works to our expectations, the gall wasp may be even difficult to find on the trees," he said.

Edwin Tanji can be reached at citydesk@mauinews.com.