Koa Defoliation Spreading, But Some Trees Starting to Recover

State wildlife officials say they are continuing to monitor the insect outbreak that has been defoliating large sections of koa forest in East Hawaii.

The result is one of those good-news, bad-news sort of things.

The bad news is that the estimated area being affected has roughly doubled to 50,000 acres.

And the 25,000 acres of forest from an assessment done in February was already the largest koa defoliation event on record.

Also, according to the state Department of Land and Natural Resources, there have been reports of moths, caterpillars and initial signs of defoliation in the areas of Volcano and Ka`u and at Pu`u Wa`a Wa`a in West Hawaii.

The good news is that trees defoliated earlier in the outbreak are already sprouting new leaves which means that the forest is recovering.

The culprit is the koa moth, a native insect. The looper, as it is known in its caterpillar stage, eats away at the leaves which eventually die and fall.

A koa moth looper doing its thing. Photo by William Haines, College of Tropical Agriculture and Human Resources, UH-Manoa.

Koa defoliation events have been documented since 1892, and there are oral accounts of earlier outbreaks.

However, researchers believe they play an important ecological role by eliminating unhealthy trees and thinning dense young koa stand, and also provide an influx of nutrients into the forest system.

Scientists say little is known about the phenomenon or its causes.

Based on previous outbreaks, they believe that many if not most of the affected trees are expected to survive.

However, an invasive psyllid insect first detected in Hawaii in 1966 was not present during previous moth outbreaks, and researchers are not sure what impact that  insect, a type of jumping plant louse, might have on the new shoots of recovering trees.

DLNR officials say they currently have no means of slowing or stopping the moth infestation.

Aerial spraying of insecticide would harm other forest organisms and is also not feasible on a large scale. Biological control is not possible with a native species because its natural enemies are already present in Hawaii, and as a result there is no outside source for other predators or parasites that would target the moth.

The DLNR is seeking funding to investigate natural controls of the moths using traps or baits and to monitor the phenomenon for information that could be used in future outbreaks.