In January 2003, Haleakalā National Park employees reported defoliation of mature koa trees at Healani. By November, noticeable defoliation affected ridges and valley floors in Kīpahulu Valley about 3 miles away. The following February, a site was discovered on the western slope of Haleakalā where most of the koa trees were about 70 percent defoliated.
In all, about six square miles were affected, including parts of Haleakalā National Park and the Kīpahulu and Makawao Forest Reserves. The culprit was Scotorythra paludicola, the endemic Hawaiian koa moth.
Outbreaks of S. paludicola caterpillars aren’t new—recorded in 1892 and known to Native Hawaiians even earlier. Most of the trees in the recent outbreak soon developed new growth. Little is known about the moth, however, and severe outbreaks have reportedly killed as many as one in three trees.
With the help of colleagues from Haleakalā National Park and Bowling Green State University, University of Hawaiʻi at Mānoa graduate student William Haines and Associate Professor Daniel Rubinoff, of the College of Tropical Agriculture and Human Resources’ Department of Plant and Environmental Protection Sciences, looked for clues in the six outbreaks recorded in the area over the past century.
Neither temperature nor rainfall correlated with known outbreaks, although wind or another climate factor could play a role, they report in the July 2009 issue of Pacific Science.
They reared several parasitic wasp species from the caterpillars, including one observed on koa trees in affected areas, but the researchers say controlled laboratory studies are needed to identify the role that parasitoids or pathogens might play in the occurrence or duration of koa moth outbreaks.
They also recommend long-term monitoring of an accessible site known to experience outbreaks to document activity before defoliation becomes apparent, as well as studies comparing the biology of the koa moth with similar species.
Determining what triggers or tempers outbreaks could suggest management strategies to protect a tree that is both a dominant and defining part of Hawaiʻi’s forest ecosystem and an economically valuable wood crop, they say.