Koa Moth Outbreak Could Be Boon for Birds

As they have been in windward sections of Mauna Kea, koa moths are also taking their toll on forests of Mauna Loa.

Large numbers of the koa looper – the moth’s caterpillar stage – have been munching on koa trees in Hawaii Volcanoes National Park, according to Robert Peck, a University of Hawaii at Hilo entomologist writing for the Pacific Island National Parks blog.

But, he said, the outbreak may have its positive side.

In recent months the loopers’ work has been evident in the park’s Mauna Loa Strip Section, Peck said, referring to the experiences of National Park Service worker Melissa Simon.

“While conducting vegetation fieldwork in the Mauna Loa Strip, one week it seemed like it was raining caterpillars,” Simon said. “The next week we walked through clouds of moths.”

Beginning in early May, the caterpillars (Scotorythra paludicola) could easily be spotted feeding on the vegetation of nearly every koa tree between the 5,000-foot and 6,500-foot elevation along the Mauna Loa Strip Road, Peck said.

By the middle of June, most of the loopers were in the pupal stage, with some already emerging from their cocoons as moths.

That left biologists wondering where these new waves of moths might go, and whether it might mean a new round of infestation in koa foliage in lower sections of the park such as along Chain of Craters Road.

Peck noted that under normal conditions, the koa looper is just one of several species of Scotorythra that feeds on koa leaves. But because it usually eats only a small portion of the koa foliage, it generally goes unnoticed.

But as state wildlife officials have noted, this appears to be the biggest outbreak ever observed.

“Why does the koa looper experience occasional outbreaks? No one knows for sure, but it may be due to a relaxation in predation or pathogen pressure, a decrease in chemical defenses within koa foliage, favorable climatic conditions, or some other factor,” Peck wrote.

“And while it is unknown what initiates the outbreak, it is also unknown what causes it to stop.”

Such an outbreak is obviously cause for concern. While most of the trees usually recover and re-sprout foliage, in areas where trees are stressed as much as a third may die.

But there may be positive outcomes.

For example, Peck notes, caterpillars are important food for native birds, particularly during the nesting season which is often when the outbreaks occur.

The massive increase in the number of koa loopers could prove to be beneficial to native bird species. Pacific Islands National Parks blog photo.

“Caterpillar levels not seen for decades such as we are experiencing right now, may result in higher nesting success for native species such as the Hawaii ʻamakihi, ʻapapane and ʻelepaio,” Peck said.

“The full extent of the outbreak will not be known for many months, but it is clear that (Hawaii Volcanoes National Park), like much of the Big Island, is experiencing a spectacular natural phenomenon involving an often overlooked native insect and its more famous host plant,” Peck said.