By COLIN M. STEWART
Tribune-Herald staff writer
A voracious outbreak of koa moths has continued its spread across Hawaii Island, having now defoliated 50,000 acres of the rare, native Acacia koa trees.
But, scientists said this week that a glimmer of hope may be on the horizon: The moths could be eating so much so quickly that they will exhaust their food supply and fall back under the control of native predators.
The outbreak of the native pest was first detected in January, and has now impacted every major koa forest on the island, according to Will Haines, a University of Hawaii at Manoa entomologist who is assisting the state Department of Land and Natural Resources with monitoring the situation. While the pest could always experience a resurgence in some areas, Haines argued that it may have depleted most of its food source, effectively halting its own population explosion.
“It’s already hit almost all the main koa areas on the island,” he said via phone from Oahu on Friday. “It was just reported having spread to South Kona last week, the only area we hadn’t seen it yet. They have a lot of caterpillars there now. Once they’ve eaten all the leaves, they’ll run out of places to disperse to.”
Areas that were hit earlier this year, such as the Hilo and Hamakua areas, have had a chance now to recover, with trees beginning to sprout new leaves, he said, leaving the possibility that the moth could recolonize. But, “I think the trees haven’t produced enough leaves to sustain the same kind of population. So, it’s my guess, and my hope, that it will run out of steam,” he said.
Haines said he spent last week on Hawaii Island traveling to various locations to measure to progress of the moth as it has multiplied and spread. He explained that as a native insect, the koa moth, aka scotorythra paludicola, has its share of natural predators that help to hold it in check, including various bacteria, viruses, birds, and even a parasitic wasp which lays its eggs on the pest.
But sometimes, for reasons not yet understood, that delicate balancing act can topple, allowing the moth population to snowball and spread, Haines said, with the moth’s caterpillar stage chomping on every green koa leaf in sight. Their numbers multiply so rapidly that their predators don’t have a chance to catch up. Then, once they’ve stripped the trees, they move on to another area to start the process again.
“When we were in Pu‘u Wa‘a Wa‘a, they were just dripping off the trees,” he said of the caterpillars. “It was amongst the highest populations I’ve ever seen.”
As for any lasting impact on the koa trees, much remains to be seen, said J.B. Friday, a Hilo-based extension forester with the College of Tropical Agriculture and Human Resources at UH-Manoa.
“The trees are leafing out again, which everybody expected. The big question is, are we going to see subsequent outbreaks? … Nobody really knows the dynamics of an outbreak,” he said.
Friday explained that this is the largest recorded outbreak, so there isn’t much history to rely on.
“It’s unprecedented. We don’t know, we’re just waiting to see if we get a subsequent outbreak, or if it has run its course like a wildfire,” he said.
In one previous outbreak in Maui in the 1980s, up to 35 percent of the trees that were defoliated died. But, Friday said, those trees had withstood an outbreak the previous year. So there’s no telling how many of the trees impacted on Hawaii Island will survive.
“The one thing I’m concerned about are the young trees, like those in Hakalau Wildlife Refuge where they plant the new trees. We don’t know fully, what is the effect on young trees. The bigger, older trees, I’m not so worried about.”
Larger trees have enough energy stored up in their trunks to weather being completely defoliated, he said, but with newer seedlings, their leaves make up a much greater percentage of their entire mass, and therefore defoliation can be much more detrimental.
“A larger plant will have more reserves,” he said.
Another environmental difference that may affect the mortality rate of the trees is the fact that they are currently under siege by a second, unrelated pest: An invasive “psyllid,” an insect which sucks the trees’ juices. The insect was first detected in 1966 and was not present during previous outbreaks, so what effect it could have, if any, remains to be seen.
“What that interaction is between the koa moth and the psyllid we don’t know,” Friday said.
One thing this current outbreak has been good for is study, added Haines.
“We’ve learned so much. We’re learning about the moths’ dispersal biology and reproductive biology. When they emerge from the cocoon, females appear to mate right away, and then you see these swarms of moths. People have described ‘blizzards of moths crossing Saddle Road,’” he said.
These swarms may be following pheromones, or scents, left by other moths to find their way to new food sources. And, Haines added, this might open the door for a method of controlling future koa moth outbreaks.
“The only really viable option to controlling them at this point might be using pheromones, to confuse the moths,” he said. “If you detect an outbreak, you may be able to prevent them from going into new areas by using pheromone in traps, instead of allowing them to disperse all over the island.”
So far, the DLNR has mapped the current outbreak as defoliating forest in East Hawaii from Laupahoehoe to Upper Waiakea. Meanwhile, moths and caterpillars, as well as initial signs of defoliation, have been reported in Kilauea, Keauhou, and Ka‘u regions, and the Pu‘u Wa‘a Wa‘a area of West Hawaii.
Such outbreaks have happened several times before, dating back to the first recorded account in 1892. Oral accounts by Hawaiians go back even further.
“There are currently no tools for slowing or stopping the infestation,” reads a Thursday DLNR release. “Aerial spraying of insecticides would harm other forest organisms and is 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 there is no outside source for predators or parasites that would be specific to the moth.”
For more information, visit the DLNR website at www.hawaii.gov/dlnr/dofaw.
Email Colin M. Stewart at cstewart@hawaiitribune-herald.com.
