Dynamics of a koa moth (Geometridae: Scotorythra paludicola) outbreak at Hakalau Forest National Wildlife Refuge, Hawaiʻi

Irruptions of the endemic koa moth (Geometridae: Scotorythra paludicola) have defoliated stands of the endemic koa (Acacia koa) numerous times over the past 125 years, but the last outbreak on Hawaiʻi Island was reported in 1953. In January 2013, the largest recorded outbreak was first observed along the windward slope of Mauna Kea Volcano on Hawaiʻi Island, and by April it had spread to Hakalau Forest National Wildlife Refuge. The outbreak potentially impacts important koa-ʻōhiʻa (Metrosideros polymorpha) habitat for endangered forest birds and more than 20 years of koa restoration efforts at Hakalau. We monitored larval and adult koa moths and their natural enemies at old growth and koa restoration sites (1600–1920 m elevation) during April−December 2013 to understand the dynamics of this unusual interaction between native species and the potential consequences for forest restoration. We clipped branch tips to evaluate caterpillar abundance and development; used malaise traps to track koa moths and parasitoid wasps; used heptyl butyrate traps to monitor western yellowjacket (Vespula pensylvanica) abundances; and reared koa moth caterpillars to determine parasitism rates. The outbreak peaked during 16 May–20 June when 4.4−18.9 million caterpillars/ha were recorded. Moths peaked during 19 July–11 September. Parasitoids tracked the abundance of caterpillars, but the low (<5%) rate of attack suggested little influence on outbreak dynamics. Western yellowjackets showed no response to caterpillar abundance. A subsequent outbreak following the regrowth of foliage was not observed at Hakalau, as it was elsewhere on windward Mauna Kea.