Plant carbohydrate content limits performance and lipid accumulation of an outbreaking herbivore.

Locusts are major intermittent threats to food security and the ecological factors determining where and when these occur remain poorly understood. For many herbivores, obtaining adequate protein from plants is a key challenge. We tested how the dietary protein : non-structural carbohydrate ratio (p : c) affects the developmental and physiological performance of 4th-5th instar nymphs of the South American locust, Schistocerca cancellata, which has recently resurged in Argentina, Bolivia and Paraguay. Field marching locusts preferred to feed on high carbohydrate foods. Field-collected juveniles transferred to the laboratory selected artificial diets or local plants with low p : c. On single artificial diets, survival rate increased as foods became more carbohydrate-biased. On single local plants, growth only occurred on the plant with the lowest p : c. Most local plants had p : c ratios substantially higher than optimal, demonstrating that field marching locusts must search for adequate carbohydrate or their survival and growth will be carbohydrate-limited. Total body lipids increased as dietary p : c decreased on both artificial and plant diets, and the low lipid contents of field-collected nymphs suggest that obtaining adequate carbohydrate may pose a strong limitation on migration for S. cancellata. Anthropogenic influences such as conversions of forests to pastures, may increase carbohydrate availability and promote outbreaks and migration of some locusts.