Many, many moths | Scotorythra paludicola are feasting on Koa
HILO — East Hawaii’s precious koa trees are under siege.
“The trees, they were all brown. There’s no leaves. They’re dead,” said a man who called the Tribune-Herald on Thursday afternoon. He was driving over Saddle Road, at about mile marker 13, when he came upon the scene of the crime.
Scotorythra paludicola, also known as the koa moth, is a native insect that usually manages to find an equilibrium with the environment, says University of Hawaii College of Tropical Agriculture and Human Resources Extension forester J.B. Friday. But this year, for mysterious reasons, the moth is breeding like wildfire, defoliating koa trees throughout the Hilo and Hamakua regions, up to elevations of 3,700 feet.
“There was an outbreak in late December, early January, along the Hamakua Coast from Saddle Road to Ookala and Kukaiau (Ranch),” he said. “It’s pretty much completely defoliated the trees in the area. One hundred percent.”
When the state Department of Land and Natural Resources’ Division of Forestry and Wildlife first documented the outbreak, nearly 25,000 acres of koa had been defoliated, making it the largest koa defoliation ever recorded. Since then, the area affected has grown to “well over 30,000 acres,” Friday said.
Luckily, he said, the damage looks much worse than it may actually be.
“You first see it and you think ‘Oh my God, this is a disaster.’ I know it’s certainly concerning to see it. … When I first saw it, I practically fell out of my truck. It’s shocking,” Friday said. “But, it’s important to note that the trees aren’t dead. Certainly not now. Some of the smaller, weaker, more stressed trees, they may die, but the bigger, older ones, they should be OK.”
The moths have continued to spread through the area since the start of the outbreak, largely unseen, he said. That is, until they hit Saddle Road. About three weeks ago, the moths reached the road, and have generated calls to the forestry office.
“You can see clouds of moths around the trees, on the ferns at the base of the trees,” Friday said. “They like koa as long as there is koa, but apparently they will chew on other things, but we don’t know if they can complete their life cycle if they eat other things.”
Eventually, he said, either predators or a sheer depletion of their food sources causes outbreaks like this to end. Such outbreaks are a relatively regular occurrence on Maui, happening every 5 or 10 years. But here on the Big Island, there haven’t been any koa moth outbreaks for 50 years.
“We have the largest areas of koa in the state, for sure,” he said. “But it just doesn’t happen here as often. We don’t know why. It could be that we had a very dry December. It’s a mystery.”
According to a fact sheet prepared by the DLNR, koa moths are endemic to Hawaii, Maui and Oahu.
With a wingspan of between 1.5 and 2 inches, the moth varies in color from pale to dark brown. The larvae are referred to as “looper” caterpillars, referring to their typical “inchworm” movement. They start out tiny and black, but grow to about 1 inch long, and can vary in color and pattern, from gray to brown or green.
The caterpillar stage lasts about a month, followed by a pupation stage of 10-12 days. Adults live for weeks, are usually active at night, and are believed to lay their eggs under bark and in patches of moss on koa trunks. The full life cycle lasts about two months.
The first documented outbreak in Hawaii was in 1892, but oral accounts indicate older such incidents. The last outbreak on Hawaii Island was in the 1950s, while the most recent one in Maui was 2009.
“Healthy koa forests generally recover after defoliation by the koa moth, but mortality as high as 35 percent has been documented in forests under stress,” reads the DLNR information sheet.
Friday said not much can be done about the current outbreak, except to observe and compile data.
“In the old days, they might have gone out there with a crop duster and dumped insecticide on it. But not now. Basically, we are tracking it.”
The affected area is the largest of three regions on the island where koa grows. The others are in Ka‘u, in the forest above Pahala, and Kona, at elevations above the coffee belt.
To report koa defoliation and increased caterpillar or moth abundance if observed outside of the Hilo and Hamakua regions, contact the Department of Land and Natural Resources at 587-0166, or email email@example.com.