The Effect of Humidity and the Role of Visual Cues During Feeding on Green/Brown Color Polyphenism in Locusta migratoria

The regulation of body color in locusts comprises phase polyphenism, green/brown polyphenism, and homochromy. Humidity was believed to control green/brown color polyphenism in locust species. However, recent findings indicate that humidity has minimal influence on this polyphenism in Schistocerca gregaria nymphs reared in isolation. This study investigated the effects of humidity and visual stimuli on Locusta migratoria nymphs reared in isolation. When L. migratoria nymphs were fed either Bromus catharticus or Dactylis glomerata leaves only during the dark period, the proportions of green nymphs at the 5th instar were comparable. Increased humidity (through addition of moist cotton) induced more green morphs under a 16-h dark/8-h light cycle, suggesting that humidity is involved in green/brown polyphenism in L. migratoria. However, these nymphs were not sensitive to humidity regarding this polyphenism under a 23-h dark/1-h light cycle, suggesting that sensitivity is related to visual cues. The daily overlap between food availability and light periods for 12 h resulted in a higher proportion of green morphs than a daily overlap for only 4 h, suggesting that such an overlap affects green/brown polyphenism in this species. Although we were unable to assess the effect of the total food availability period on polyphenism in this study, we confirmed that at least (1) humidity and (2) visual cues during feeding are associated with green/brown polyphenism in L. migratoria. These regulations may have ecological significance for this species, allowing them to phenotypically adjust to the seasonal and spatial variation in their environment, increasing their chances of survival.