Density-dependent phenotypic plasticity in birdwing grasshoppers, Schistocerca americana (Drury) and S. serialis cubense (Saussure) (Orthoptera: Acrididae: Cyrtacanthacridinae)

The desert locust Schistocerca gregaria (Forskål) is one of the most devastating pest species affecting many countries in North Africa and the Middle East. The desert locust exhibits an extreme form of density-dependent phenotypic plasticity, known as locust phase polyphenism.  At low density, locust nymphs are cryptically colored and shy, while at high density, they become conspicuously colored and highly gregarious. Although the proximate mechanisms of behavioral plasticity have been well characterized in the desert locust, the presence of similar density-dependent phenotypic plasticity in other Schistocerca species has received little attention.  In this study, we quantify the effect of rearing density in the behavior, morphology, and nymphal coloration in two non-swarming species, S. americana (Drury) and S. serialis cubense (Saussure). We show that both species display density-dependent phenotypic plasticity similar to the desert locust and also show species-specific differences in phenotypic plasticity.  In light of this finding, we discuss how phenotypic plasticity might have evolved at a macroevolutionary scale.