Do desert locust hoppers develop gregarious characteristics by watching a video?

Various sensory stimuli have been suggested to induce gregarious body coloration in locusts, but most previous studies ignored the importance of substrate color. This study tested the effects of visual, olfactory and tactile stimuli from other locusts on the induction of gregarious body coloration in single (isolated-reared) Schistocerca gregaria nymphs housed in yellow–green cups. Odor from gregarious (crowd-reared) locusts, which is believed to induce black patterns in single locusts, had little effect when applied to visually isolated nymphs at the 2nd stadium onward, and all test nymphs remained green without black patterns at the last stadium, as in controls reared without odor and visual stimuli. Visual stimuli alone induced black patterns when a single solitarious nymph was allowed to see other locusts in another cup. The degree of black patterns increased as the number of locusts shown increased, and some test nymphs developed body coloration typically observed in gregarious forms. A classical morphometric ratio (hind femur length/head width) shifted toward the value typical of gregarious forms when the single nymphs were allowed to see 5 or 10 locusts. Single nymphs also developed black patterns when presented green conspecific nymphs and adults of two hemipteran species kept in another cup. No synergetic effects of visual and odor stimuli were detected. Movies of locusts, crickets and tadpoles were found effective in inducing black patterns in single locusts. Ontogenetic variation in the sensitivity to crowding and experimental methodology might be responsible for some discrepancies in the conclusions among different researchers.