Winged menace. A solitary migratory locust rests on a twig.

Winged menace. A solitary migratory locust rests on a twig.

Quartl/Creative Commons

Gut Parasite May Keep Locusts From Swarming

By itself, the migratory locust is about as harmless as a grasshopper. But under the right conditions, it can assemble with billions of its comrades into apocalyptic swarms that destroy thousands of hectares of crops in Africa, Asia, Australia, and New Zealand. Now, scientists have discovered that a gut parasite may be key to keeping these insects living the single life.

Researchers first identified the parasite’s swarm-prevention potential in 2004. A team led by entomologist Wangpeng Shi of China Agricultural University in Beijing noticed that migratory locusts (Locusta migratoria manilensis) infected by a microbe known as Paranosema locustae were less likely to aggregate into swarms than were their healthy counterparts.

To uncover how the parasite stemmed the swarms, Shi and colleagues infected healthy locusts with P. locustae. Locusts release chemical signals called pheromones in their scat to notify their neighbors that it’s time to swarm. Healthy locusts placed in chambers containing scat from the infected locusts were significantly less likely to display swarming behavior than were those placed in a chamber with scat from healthy locusts, the researchers report online today in the Proceedings of the National Academy of Sciences. An analysis of the infected locust scat found fewer swarm-inducing pheromones.

Looking into the insects’ guts, the team discovered why. The parasite acidified the locusts’ lower guts, subduing the growth of bacteria responsible for creating the pheromones. Additionally, the researchers found that infected locusts produced lower levels of the neurotransmitters serotonin, which can initiate swarming behavior, and  dopamine, which can sustain the behavior.

Although the researchers had finally discovered the mechanisms P. locustae uses to suppress swarming behavior in its host, they were confused about why the parasite would want to prevent swarms at all. P. locustae spreads from one locust to another through fecal matter consumption, cannibalism, and reproduction—all of which occur more frequently when the locusts swarm.

Regardless of why the parasites prevent swarming, the team says countries fighting locust infestations could make use of P. locustae’s swarm-stopping tricks in place of existing insecticides. “People would like to use something biological that is environmentally safer than chemical insecticides,” says molecular biologist Raymond St. Leger of the University of Maryland, College Park, who co-authored the paper. “The question now is whether we can create an easy chemical way to block these bacteria from producing pheromones or select for parasites that can keep the bacteria from flourishing in the gut.”

Insect pathologist Stefan Jaronski of the U.S. Department of Agriculture in Sidney, Montana, who was not involved with the work, hesitates to call the parasite’s mechanisms a surefire way to fight locust swarms. He notes that locusts also signal one another to swarm by physically touching antennas, a process the parasite doesn’t interfere with. “This isn’t a silver bullet, but it is a big step forward,” he says.