Outbreaks of the Hawaiian geometrid caterpillar Scotorythra paludicola Butler, and defoliation of koa forests (Acacia koa): A historical perspective

The koa moth, Scotorythra paludicola (Lepidoptera: Geometridae) is an endemic moth that undergoes sporadic outbreaks in Hawaii, sometimes causing vast defoliations of its host plant Acacia koa. We present observations from a recent outbreak that occurred on East Maui in 2003, in which approximately 16 km² of forest were severely defoliated in Kipahulu Valley and Makawao Forest Reserve. We visted defoliated sites, collected adult moths and larvae, and recorded size class distribution of larvae. Larvae at a given site tended to be of a similar size class, suggesting that outbreaks were synchronous, and mean development time from first instar to adulthood was 42 days under laboratory conditions. Mortality of field- collected, lab-reared larvae due to disease was high (80%), making it impossible to quantify meaningful parasitism rates, but three non-native hymenopteran primary parasitoids were reared: the braconids Meteorus laphygmae and Cotesia marginiventris, and the ichneumonid Hyposoter exiguae. One ichneumonid hyperparasitoid, Gelis sp., was also reared. No native parasitoids emerged from larvae. We examined long-term weather data between 1920 and 2006, to see whether outbreaks could be explained by precipitation, temperature, or the interaction between them. In logistic regression analyses, none of these climate variables were significant, begging the question: what really triggers Scotorythra outbreaks?