Response of Scotorythra Caterpillars (Geometridae) to Drought Simulation and Nutrient Augmentation of Koa (Fabaceae: Acacia koa)

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2015-05
Authors
Chi, Megan
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Haines, William P.
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Biology
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University of Hawaii at Manoa
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Abstract
This study explored the plant stress hypothesis and the plant vigor hypothesis, two opposing hypotheses regarding how bottom-up factors affect herbivores. Our objective was to provide insight into whether drought may contribute to outbreaks of the specialist koa looper moth (Scotorythra paludicola), which is endemic to Hawaiʻi and causes mass defoliation of the important native koa tree (Acacia koa). However, due to the low abundance of koa moths at field sites at the time of the experiment, we used an endemic generalist Scotorythra rara, which is very closely related to the koa moth, to test caterpillar performance on water-stressed or nutrientenriched potted koa. We found no evidence that drought simulation affected phyllode characteristics. Despite this, caterpillar performance was significantly poorer on drought-stressed koa, supporting the plant vigor hypothesis. Caterpillars subjected to drought-stressed koa took longer to develop, and had lower pupal weights than caterpillars reared on unstressed koa. Furthermore, caterpillar performance was correlated with variation in phyllode physical and nutritional traits. Caterpillars performed better on phyllodes with higher water content, higher nitrogen content, lower thickness, and lower toughness.
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Scotorythra paludicola, Scotorythra rara, insect outbreaks, plant stress hypothesis, plant vigor hypothesis, forest pests, no choice assay, tropical ecosystems, Pacific islands
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61 pages
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