Effects of koa moth defoliation on Acacia koa plantations

Project collaborators: Paul Scowcroft, James B. Friday, Faith Inman-Narahari, and Oriana Rueda Krauss

Forest Service Institute of Pacific Islands Forestry, Pacific Southwest Research Station
University of Hawaii Manoa, College of Tropical Agriculture and Human Resources, Department of Natural Resources and Environmental Management

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A major koa defoliation event began on the windward side of the island of Hawaii in January 2013. The defoliation is due to a large increase in the populations of Scotorythra paludicola, a moth endemic to the Hawaiian Islands. Several outbreaks have been recorded over the past 100 years on the islands of Maui and Hawaii, but the January 2013 outbreak is the first reported on Hawaii Island since the 1950's. It is also the most widespread and intense in historical times. Before this, the most recently recorded outbreak in the state was on Maui in 2003 (Haines et al. 2009). Healthy koa forests generally recover after defoliation by the koa moth, but mortality as high as 35% has been documented in forests under stress (Stein and Scowcroft 1984).

By June of 2013, the outbreak reached the uppermost potions of Hakalau Forest National Wildlife Refuge. The extensive corridor and block plantings of koa were impacted, but the effect of the defoliation on survival and growth of the trees is unknown. Our project is examining the extent and impact of koa moth defoliation on the oldest natural and planted stands of koa in the Refuge. Our study takes advantage of measurements made in 2008 when the koa trees were 20 years old.

This study will help us to better understand the effects of the koa moth, how long it takes for a koa stand to fully recover, and to expand the knowledge about koa stand dynamics. Understanding the recovery timeframe will help in planning conservation and restoration efforts, and landowners and managers will have a better idea of what to expect when another koa moth outbreak occurs in the future.

We currently analyzing how the pre-defoliation growth rates and trees sizes relate to the intensity of koa moth defoliation. Preliminary analysis show that larger trees suffered relatively less defoliation than smaller trees. Future measurements will allow us to discover how the amount of koa moth defoliation effects the future growth and survival rates of affected trees.To measure refoliation rate, we are taking hemispherical canopy pictures every two months and analyzing the images to estimate the total leaf area of affected trees. We will then compare the pictures over time to determine how rapidly refoliation occurs.

Citations
Haines, W. P., M. L. Heddle, P. Welton, and D. Rubinoff. 2009. A recent outbreak of the Hawaiian koa moth, Scotorythra paludicola (Lepidoptera: Geometridae), and a review of outbreaks between 1892 and 2003. Pacific Science 63:349–369.

Stein, J.D.; Scowcroft, P.G. 1984. Growth and refoliation of koa trees infested by the koa moth, Scotorythra paludicola; (Lepidoptera: Geometridae). Pacific Science 38(4): 333-339.

Links to more information

Koa moth photo gallery
https://www.flickr.com/groups/koamoth/

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