Dynamics of a Koa Looper Moth Outbreak and Response by the Native Forest Community

Science Center Objects

A massive outbreak of the native koa looper moth (Scotorythra paludicola; Geometridae) defoliated more than a third of the koa (Acacia koa) forest on Hawai‘i Island during 2013–2014. Our objective was to record the dynamics of the koa looper (Scotorythra paludicola) outbreak and evaluate the response to the outbreak by the forest ecosystem generally as well as select native and invasive species and communities at Hakalau Forest National Wildlife Refuge.

Koa looper moth caterpillars on Koa leaves

Koa phyllodes being devoured by Scotorythra paludicola caterpillars during the largest outbreak ever recorded in 2013. Photo: R. Peck

Overview:

A massive outbreak of the native koa looper moth (Scotorythra paludicola; Geometridae) defoliated more than a third of the koa (Acacia koa) forest on Hawai‘i Island during 2013–2014. Although outbreaks of the koa looper are a natural, if rare, phenomenon, the scale and severity of the 2013–2014 outbreak far exceeded what has been described historically.

Biologists assemble an aerial malaise trap

Drs. Paul Banko and Eben Paxton assemble an aerial malaise trap. A ground malaise trap can be seen in the background. Photo: S. Yelenik

Project Objectives:

Our objective was to record the dynamics of the koa looper (Scotorythra paludicola) outbreak and evaluate the response to the outbreak by the forest ecosystem generally as well as select native and invasive species and communities at Hakalau Forest National Wildlife Refuge. Specifically, we sought to evaluate changes in demographic parameters of the koa looper and identify temporal and geographical patterns of defoliation in native koa forest and forest restoration sites. We evaluated the response to the outbreak by introduced and native birds, including endangered species. The abundance of bats was determined during the outbreak and compared with data from previous, “normal” years. Changes in populations of predatory and parasitoid wasps were documented, and rates of caterpillar parasitism established to assess the role of parasitoids during the outbreak. Changes in soil and foliar nutrients resulting from the deposition of caterpillar frass were evaluated to understand how invasive grasses and other plants may responded to a sudden increase in nitrogen and phosphorous. 

A primary goal of the research was to provide the scientific basis for designing strategies to protect, sustain, and restore the ecosystem functions and services of Hakalau Forest National Wildlife Refuge, a landscape that as demonstrated resilience in recovering from many decades of degradation by invasive ungulates and other alien species.

Dense cloud of koa moths swarm a koa tree at Hakalau Forest National Wildlife Refuge

Dense clouds of native Scotorythra paludicola moths swarm after emerging from larval stages that defoliated large tracts of native Acacia koa forest. Photo: S. Yelenik

Highlights and Key Findings:

The massive 2013−2014 outbreak of the koa moth (Scotorythra paludicola) defoliated more than a third of the koa (Acacia koa) forest on Hawai‘i Island during 2013−2014. This was the largest koa moth outbreak ever recorded and the first on the island since 1953. The outbreak spread to sites distributed widely around the island between 800−2,000 m elevation and in wet rainforest to dry woodland habitats. We monitored the outbreak at two windward forest sites (Laupahoehoe and Kipuka) and one leeward forest site (Kona), and we studied the dynamics of the outbreak and its impacts on the forest ecosystem at Hakalau Forest National Wildlife Refuge, our higher elevation windward site. Study sites at Hakalau included two stands of koa that were planted (reforestation stands) in former cattle pastureland about 20 years earlier and two stands of koa that were dominated by ‘ōhi‘a (Metrosideros polymorpha) and that were naturally recovering from cattle grazing (forest stands).

Koa tree defoliated by koa loopers at Hakalau Forest National Wildlife Refuge

Acacia koa trees denuded of foliage by native Scotorythra paludicola caterpillars at Hakalau Forest National Wildlife Refuge. Some forest tracts below Hakalau were defoliated three times by successive waves of caterpillars. Photo: R. Peck

Aerial malaise trap collects moths and other flying insects in the forest canopy

Aerial malaise trap collects moths and other flying insects in the forest canopy. Photo: S. Yelenik

We observed one outbreak at Hakalau, multiple outbreaks at the two other windward sites, but no outbreak at the leeward site. Caterpillars at Hakalau reached peak estimated abundances of more than 250,000 per tree and 18,000,000 per hectare, and they removed between 64−93% of the koa canopy in managed forest stands. Defoliation was more extensive in naturally recovering forest, where ohia dominated and koa was less abundant, compared to the planted stands, where koa density was high. Koa trees were still growing new foliage six months after being defoliated, and leaves were produced in greater proportion to phyllodes, especially by small koa (≤8 cm dbh) and by larger trees in forest stands, where light levels may have remained relatively low after defoliation due to the high cover of ohia. Small branches of many trees apparently died, and canopy regrowth was absent or low in 9% of koa trees and seedlings, which indicates the likely level of mortality. Between 2,000−5,000 kg/ha of frass fell during the defoliation event, resulting in the deposition of up to 200 kg/ha of highly labile nitrogen on the forest floor in less than two months. The deposition of nitrogen was detected as pulses in resin-available nitrogen in the top 5−10 cm of soil at two of three sites. These sites showed elevated soil nitrogen for about seven months. Nitrogen content of understory plant foliage, which is indicative of nitrogen uptake, suggested weak and variable effects of nitrogen deposition in the soil. Foliar nitrogen increased slightly in alien pasture grasses four months after the deposition of frass, although distinctive increases were not detected in native woody species.

Birds responded to the abundance of caterpillars by increasing their activity in koa during the buildup of caterpillars and decreasing their use of koa after defoliation. During the outbreak, caterpillars increased in the diets of the two generalist insectivores we examined, and nearly all species gained weight. Bats responded to the abundance of moths by compression of active foraging into the first three hours of darkness each night after presumably having reached a digestive bottleneck. Reduced foraging activity by bats also resulted in lower indices of detectability based upon acoustic monitoring when compared to non-outbreak years. Parasitoid wasps tracked caterpillar abundance, but the low rate at which they attacked caterpillars suggests that they had little influence on the population. The predatory yellowjacket (Vespula pensylvanica) did not respond to the outbreak. Although a single, protracted outbreak occurred at Hakalau, multiple outbreaks and defoliations occurred at lower elevations. Our results provide a broad foundation for evaluating the dynamics and impacts of future Scotorythra outbreaks.

Progress:

A final technical report is available.

Sampling caterpillar densities by branch clipping

Sampling caterpillar densities on Acacia koa by clipping branch tips into bags for later sorting. Photo: S. Yelenik