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Locusts are known for their striking demonstration of the power of epigenetics. Changes in population density cue a sudden shift in their physical form between a smaller, greener, solitary phase and a larger, darker, gregarious phase by changing the expression of their genes. When the population shifts to gregarious phase, vast swarms of locusts can strip entire swaths of vegetation bare, making the control of these phase shifts of interest from both a physiological and an economic standpoint.

In the desert locust Schistocerca gregaria, this change can also be initiated between generations when a female locust is stimulated by crowding to produce larger gregarious phase eggs. However, how locusts control this phase switch is unclear. Koutaro Maeno from Tsukuba, Japan, and Seiji Tanaka from Nouakchott, Mauritania, recently showed in a publication in Physiological Entomology that solitary locusts require light and chemical cues in order to produce gregarious offspring.

Knowing that touching the chemosensitive antennae, rather than knocking the legs, causes females to lay large gregarious phase eggs, Maeno and Tanaka thought that a chemical cue might be responsible. To test this idea, they blindfolded females with white-out and black nail polish, to prevent them from picking up visual cues. Then they washed the head and thorax of old male locusts – which are known to induce the gregarizing response – in hexane, before brushing the washed and unwashed males against the females' antennae. The duo found that the females had to be touched by the unwashed males to lay gregarious eggs. Similarly, cotton balls soaked in the males' hexane wash induced a similar response, suggesting that a chemical found on the bodies of the males was responsible for the shift, rather than a purely mechanical stimulus. And when the duo repeated the experiment by touching females with either female locusts or several other insect species, Maeno and Tanaka found that the species that were more closely related to S. gregaria (such as other locusts or female S. gregaria) induced more gregarization response in female S. gregaria than more distantly related species (such as beetles and cockroaches). In addition, only older adult S. gregaria seemed to induce the response, while nymphs or sexually immature S. gregaria did not.

As the females' eyes were covered with paint in the initial experiments, the authors went on to test the effect of light and visual cues on the females' ability to receive a gregarization signal. Maeno and Tanaka analysed the size of the eggs produced by crowded non-blindfolded females in bright and dark conditions, and were surprised to find that crowding only induced gregarization (large eggs) in the light. Finally, to exclude the possibility that natural light intensity changes during the day caused the change, and to track down the part of the body responsible for receiving the signal, the scientists painted the female locusts with phosphorescent paint, so that they glowed in the dark. Then they tested the locusts' receptivity to the crowding cue and found that the females with phosphorescent heads produced large gregarious eggs while non-glowing females did not. Taken together, the authors suggest that a light-sensitive organ on the head must also be responsible for receiving the cue.

So, adult S. gregaria females produce gregarious offspring in response to chemical and light cues. As there must be both light and chemical signals to induce gregarization between generations, the authors suggest further research into how light perception influences processing and integration of chemical cues in insects in the hope that we might one day be able to control biblical locust plagues.

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