Density-dependent phenotypic plasticity in Schistocerca americana (Orthoptera: Acrididae)

The desert locust Schistocerca gregaria (Forskål) is one of the most devastating pest species affecting many countries in Northern Africa and the Middle East. The desert locust exhibits an extreme form of density-dependent phenotypic plasticity known as the locust phase polyphenism in which individuals in low-density populations are cryptically colored and shy while the individuals in high-density populations are conspicuously colored and highly gregarious. Although the mechanisms and expressions of the locust phase polyphenism have been well characterized in the desert locust, the presence of similar density-dependent phenotypic plasticity in other related species in the genus Schistocerca has received relatively little attention. In this study, we quantify the effect of rearing density in the behavior and nymphal coloration of a non-swarming, but closely related species to the desert locust, S. americana. Past studies on S. americana reported conflicting results in terms of the effect of rearing density. In this study, we subjected S. americana nymphs to both isolated and crowded rearing conditions, and quantified their behavior using a behavioral assay developed for the locust research identify trends. We also quantified the pigmentation in different body parts between two rearing density. We demonstrate that S. americana exhibits density-dependent phenotypic plasticity both in behavior and nymphal coloration. We also discuss some evolutionary implications on why S. americana does not form swarm in a similar magnitude as the desert locust despite the fact that they appear to have the genetic and physiological capacity to express phase polyphenism.