Locust plague explained
Evolution has crafted remarkable adaptations to the problem of survival in arid environments. Desert animals save water by becoming active at night. Desert plants develop waxy skin to slow evaporation, spongy tissues to store water, and spines to keep grazers at bay.
Now consider the lowly desert locust Schistocerca gregaria, which whiles away the months as a solitary, insignificant grasshopper. Then, when an isolated rainstorm greens up its desert home, the locust shifts into its gregarious phase, a generous name for a billion-member Biblical eating machine that scours the landscape of anything green.
The solitary and gregarious phases are so different that they were once considered distinct species:
Solitary: A green, hermit-like insect that hides in the vegetation.
Greg: A darker-colored, swarming insect with stronger wings and a more active metabolism; its tissue is poisonous because it eats toxic plants.
Scientists have known for several years that touching a solitary desert locust on the hind legs, or allowing it to see or smell other locusts, is enough to transform it into the gregarious phase. This week, Science magazine published proof that the behavioral and physical makeover is effected by serotonin, a carrier of nerve signals in virtually all animals.
Loco about locusts!
Corresponding author Stephen M. Rogers told us he's "interested in behavior, not necessarily limited to Biblical plagues. I've been working on locusts and the problem of what makes them swarm," for several years as post-doctoral fellow at the University of Cambridge.
Desert locusts are finely tuned to their arid-but-variable environment. Solitary locusts live through dry periods as insignificant grasshoppers, until an occasional rainstorm turns the desert green. "As the good times roll in, the locust population follows the increase in vegetation, there is lots to feed on," Rogers says.
When these locusts come into contact with each other, their behavior suddenly changes, Rogers says. "They are hungry, they need to eat, and the conflict over the vanishing food store triggers a behavioral and physical transformation, which escalates until they look completely different."
With stronger muscles, bigger wings, and a protective coloration that scares predators, the gregarious locusts are able to make a long migration (flying 60 miles in eight hours or less) toward regions with more food. And that's helpful, Rogers says, because "the vegetation shrivels away to nothing, and a vast number of locusts is trying to eke out living off the rapidly disappearing food."
Seeking road food
At that point, the no-longer-solitary insects depart their desert homes in search of whatever grub they can find, Rogers says. "They head out of the very arid regions, where very few people live, into the more-fertile adjacent regions. It's one of nature's cruelest tricks; farmers look forward to the heavy rains, when the crop starts growing well, and a swarm of locusts will eat it down to the ground."
Although insecticides are not perfect, they can shrink a swarm that could number billions of individuals and stretch for hundreds of miles, Rogers says. According to co-author Malcolm Burrows, at the University of Cambridge, locusts are "a current problem. In the last few years, there have been major, devastating swarms in China, Africa, and Australia."
To explore the chemical signals involved in the transformation, Rogers, together with Michael Anstey at the University of Oxford and colleagues in Sydney, Australia, focused on serotonin levels, which they already knew were correlated with the change to the gregarious condition.
What did they find?
Rubbing the insect's leg with a brush, known to initiate the phase change, sparked a rise in serotonin level;
Serotonin injections (without touching the leg) converted solitary insects into migratory ones; and
Preventing the production of serotonin blocked the phase change.
"The more gregarious they are, the more serotonin they produce," says Rogers. "If we blocked serotonin, you can tickle their legs, or crowd them all you want, and they don't change, they remain solitary."
Knowing for sure that serotonin can transform innocuous, solitary locusts into swarms of grazing monsters does not immediately suggest a safe locust-control mechanism, Rogers admits, because almost all animals need serotonin for their nerves and brains.
Users of Prozac and similar anti-depressants may know their drug as a "selective serotonin uptake inhibitor" that reduce anxiety and depression by keeping serotonin around. So nobody is suggesting spraying a serotonin blocker from airplanes as a way to hold locusts in the harmless, solitary stage.
However, serotonin acts by attaching to a protein called the serotonin receptor, and if that protein is unique in the locust, a blocking strategy could work, Rogers says. "If they are specific to locusts, it's possible that we could interfere with them, and not worry about introducing an epidemic of depression."
Any control mechanism that defeats the serotonin signal would need to detect the swarms at the start, Rogers says. "It's really important to know how and why these swarms form, what environmental conditions cause them to gather. If this is going to work, we need to get the chemical to where the swarms start. Once we have 70 billion insects, it's too late to finesse the control."
- David Tenenbaum
• Serotonin Mediates Behavioral Gregarization Underlying Swarm Formation in Desert Locusts, Michael L. Anstey et al, Science, 30 JANUARY 2009.