How social distancing for locusts could save crops

There are few more awesome – and perhaps fearsome – sights in nature than a swarm of billions of locusts devouring all in their path.

Amazing though such mega-sized swarms may be, they are the stuff of nightmares for farmers, whose crops may be devastated before the swarm moves on and lays waste to another area.

The Middle East, of course, knows this all too well, with swarms of desert locusts seen in Abu Dhabi and Dubai in May this year as well as on several previous occasions.

When conditions are favourable and numbers multiply, the normally solitary desert locust switches to its gregarious phase and forms swarms that migrate from one area to the next in search of sustenance. Locust swarms this year have infested 23 countries across East Africa, the Middle East and South Asia, with Kenya having its biggest outbreak in 70 years, according to the World Bank.

Given the damage that can result, scientists have long been interested in discovering the chemicals involved in swarming behaviour.

Now researchers in China have found that members of another species, the migratory locust, produce a pheromone called 4-Vinylanisole (4VA) when they find themselves in groups.

In the journal Nature, the scientists described how 4VA is strongly attractive to migratory locusts, so its production strengthens the integrity of the swarm.

Dr Steve Rogers, a University of Cambridge research associate who has published studies on locust swarming, but who was not connected to this latest paper, indicated that the new findings were significant.

“What this paper has achieved – which no one else has done before – is find a chemical signal between locusts that keeps them together in a group,” said Dr Rogers, who is based at the university's department of zoology.

“What is most interesting about the substance is the more they’re in the group, the more each individual produces. It has this positive feedback effect.”

The identification of 4VA’s role in maintaining swarms could lead to new ways to stop the destructive behaviour, methods more environmentally sensitive than spraying vast amounts of insecticide.

For example, the scientists in China have suggested that locusts genetically engineered not to respond to 4VA could be released, as they might struggle to sustain swarming behaviour.

Such genetically engineered locusts were actually produced as part of the new study.

In their paper, the scientists described how they discovered a 4VA-specific smell receptor called OR35 in the migratory locust.

Using genetic editing tools, they created migratory locusts where the OR35 receptor did not work and found that these individuals did not respond to 4VA.

Dr Rogers thinks that the numbers of genetically engineered individuals that would need to be released to combat swarming are so large that this might not be an ideal tactic. Other methods might, however, prove effective.

“The single most useful thing that comes out of [this research] is that, because you have this smell, you can go out to the field and set traps,” he said.

“In terms of locust control, you’re looking at where the numbers are building up before the populations reach these large levels. Once you have billions of individuals, all you can do is drench everything in insecticide.”

So if 4VA was used to lure migratory locusts into traps, it would be easier to prevent vast swarms from forming in the first place.

For farmers in this region, plagued as they are by the desert locust and not the migratory locust, the question is what – if anything – all this means for them.

Described by the World Bank as “the most destructive migratory pest in the world”, it is the desert locust, also found in Africa and parts of Asia, that has been causing this year's havoc.

Heavier than normal rains in parts of the species’ range have contributed to an outbreak that has affected around two dozen countries.

Unfortunately, the new findings may be cold comfort to agriculturalists in Africa, the Middle East and Asia suffering at the hands of this voracious insect.

“Exciting though the finding of 4-Vinylanisole as an aggregation pheromone undoubtedly is, there is currently

no evidence that it has the same effect in desert locusts,” said Dr Rogers.

Swarming is thought to have evolved independently in the two locust species, and earlier research has shown that, when it comes to aggregation, there are important chemical differences between them.

Dr Rogers and colleagues have previously highlighted the importance of a neurotransmitter called serotonin in causing desert locusts to switch in the first instance from the solitary phase to the gregarious phase.

With the migratory locust, however, this process is thought to be promoted by a different neurotransmitter, dopamine.

Yet even if the two species use different aggregation pheromones, the discovery of 4VA’s importance in maintaining swarm integrity in migratory locusts will focus attention on finding the equivalent chemical in desert locusts.

When this is achieved, farmers in this region may be able to employ new, more effective ways to prevent the migratory locust from decimating their valuable harvests.