Why locusts become party animals

Locusts are usually shy and retiring creatures who don’t particularly like each other, so what makes them gather together in their millions to wreak havoc on crops and periodically devastate farming communities?

Serotonin is the answer, it seems. Researchers led by Professor Steve Simpson of the University of Sydney have found that when locusts swarm, they have three times more measurable serotonin than the calmer, more solitary version.

The transformation, or phase change, of shy insects displaying mutual aversion to a swarming mob has been studied by Simpson and colleagues for many years. They have previously shown that phase change is caused by two sensory pathways: sensory stimulation of the insects’ hind legs as they crowd together, and the combined sight and smell of other locusts.

The new finding, published in Science, is that it is serotonin that mediates these effects.

“Solitarious locusts acquire full gregarious behavioural characteristics within the first 2 hours of forced crowding …” the researchers write.

“This period coincides with a substantial but transient [less than 24 hours] increase in the amount of serotonin … specifically in one region of the central nervous system (CNS), the thoracic ganglia, but not the brain.”

Thoracic ganglia are nerve cells that play a role in the sympathetic nervous system. The researchers found that swarming stimuli from both of the sensory pathways converge in this area.

To further test their theory, serotonin inhibitors were administered, resulting in little gregarious activity. And administering serotonin receptor agonists increased gregarious behaviour.

They ask the obvious question: could serotonin antagonists be developed to prevent the shy retiring insect from becoming a party animal? With great difficulty, they write, as the exact receptor has yet to be characterised.