Herbivores tend to be nitrogen (N)-limited and enriching host-plant N often improves the performance of insect herbivores. N-enrichment of host plants reduces the frequency of migratory morphs in aphids, suggesting that resources interact with population density to control insect migration. We tested this hypothesis in Oedaleus asiaticus, a dominant locust across the Asian Steppe, that exhibits brown and green morphs. During migratory outbreaks and in areas of high population density, the brown forms dominate.
We tested the effects of an N-fertilized plant diet (0 or 60 kg N ha−1 yr−1), and two levels of population density, on the growth, viability, and frequency of brown individuals in both field and lab cages. In support of our hypothesis, the proportion of brown O. a. decreased at low densities only when locusts were reared on N-fertilized plants in field cages. However, contrary to our predictions, brown morphs were larger regardless of diet, and N-enrichment decreased size and viability for both morphs in both lab and field cages. Oedaleus population density and percent brown generally increased with livestock grazing intensity. Fields with heavy livestock grazing are dominated by Stipa grandis (O. a.’s favored host-plant) that has a lower nitrogen content than Stipa found in ungrazed fields. These data also suggest that O. a. perform optimally on a relatively low N content diet (high carbon:nitrogen ratio) and may explain their dominance in heavily grazed areas where wind erosion has led to a decrease in organic N available to plants.