Do desert locusts gregarize by watching a video?

The desert locust, Schistocerca gregaria, shows phase polyphenism in response to population density. Crowding causes the locusts to “gregarize” and to exhibit density-dependent changes in various traits. The present study analyzed the stimuli controlling phase-dependent darkening, behavior and morphology. Black patterns occur when solitary nymphs are exposed to crowding, and mechanical, olfactory and olfactory + visual stimuli are known to be responsible for this phenomenon. However, black patterns often appear even in solitary (isolated-reared) individuals, causing some controversy among researchers about the stimuli involved in gregarization. By finding conditions producing solitary nymphs with few or no black patterns, the effects of different stimuli including visual, olfactory and tactile, on the induction of black patterns, aggregating behavior and morphometric change in FC ratio (hind femur length / head width) were examined. It was confirmed that solitary (isolated-reared) nymphs developed black patterns most intensively after exposure to crowded conditions. However, unlike previous studies by others, smell from other individuals had little or no effect on any trait. Unexpectedly, visual stimuli from other individuals without tactile and olfactory stimuli were found to induce black patterns, aggregating behavior and a shift in FC ratio. Thus, locusts allowed to watch images of other individuals with the video developed black patterns. Interestingly, black patterns were also induced in solitary locusts that were allowed to see crickets or bugs. Because solitary locusts are repelled, instead of attracted, by other locusts, visual stimuli may play an important role in the initial process leading to gregarization or outbreaks.