Researchers find a chemical that makes locusts swarm

Image of a person fleeing from a cloud of locust.

The year 2020 may be one for the record books in terms of apocalyptic tidings. In addition to the usual background of fires, floods, and earthquakes, the plague is still around. And you might have heard something about a pandemic. But what really nails down the apocalyptic vibe is the fact that the year has seen swarms of locusts causing the sorts of problems they’re famous for.

In a tiny bit of good news, the same sort of research that may bail us out with therapies and a vaccine for SARS-CoV-2 could potentially help us out against future locust swarms. That’s because a team of biologists based in China has now identified the chemical that calls locusts to swarm and shown that genetic engineering can eliminate the response.

A lot of evidence

There’s nothing especially exciting about any single aspect of the research here. Instead, the researchers simply put together techniques from a variety of specializations and then applied them to the topic of locust swarms. Locusts are normally solitary animals, but they become immensely destructive when conditions induce them to form massive swarms that are big enough to be picked up by radar. In addition to the altered behavior, swarming locusts actually look physically different, indicating that the decision to swarm involves widespread changes to a locust’s biology.

As in many things insect, researchers have long suspected that the changes of that nature were probably induced by a pheromone. These small molecules generally behave a bit like a mixture of odorants—they’re small molecules that diffuse freely through the air—and a hormone that induces changes in animals that are exposed to them. In the past, researchers involved in this kind of work searched for small chemicals made by locusts that diffuse into the air. They ended up finding 35 of them, and of those, six were more prevalent in swarming locusts.

So the researchers in the new team put individual locusts in an enclosure with two chambers, one with a chemical present and one without. They then measured how much time a locust spent in each of the chambers. One of the six chemicals, called phenylacetonitrile, could be useful, in that it seemed to repel locusts; another named guaiacol seemed to suppress some of the behaviors associated with swarming locusts. But the work focused on something called 4-vinylanisole (4VA), which was the only chemical to attract swarming locusts. Critically, it would also attract solitary locusts, suggesting that it could help draw them in to the swarm.

The researchers found that 4VA production increased as swarming locust population density went up. Additionally, the researchers could induce production by crowding a lot of solitary locusts into a single cage.

The researchers went on to identify the specific parts of the locust’s sensory organs that responded to 4VA and then screened a panel of odorant receptors to identify the one that responded to 4VA. They then used the CRISPR gene-editing system to delete that gene from locusts, showing that the insects that lacked the gene were no longer attracted to it.

It’s a trap!

Finally, they placed 4VA on sticky boards and placed those outside before releasing locusts in the area. The sticky boards that had the chemical on them trapped an average of 26 locusts; those without the chemical trapped an average of three. 4VA was clearly a major draw to locusts even in a relatively normal environment filled with other aromas.

All of this is persuasive evidence that 4VA is involved in drawing together large swarms of locusts. That doesn’t mean 4VA is the only factor involved or that there might not be other chemicals that alter distinct aspects of swarming behavior. In fact, the researchers identified a couple chemicals that might also influence swarming. But the data clearly suggests that interfering with signaling through 4VA could disrupt the formation of swarms and thus much of the destructive power of locusts.

And the work offers plenty of ways in which that disruption might work. Genetic engineering to delete the gene that encodes the receptor for 4VA blocked the chemical’s ability to attract locusts, so it might be possible to alter swarming behavior by releasing non-swarming insects into the population. 4VA also works to draw the animals to traps, and the authors note it could draw them into areas treated with pesticides, eliminating the need for widespread pesticide use. Finally, having identified the receptor, scientists might one day design a pheromone-like chemical that interferes with 4VA’s ability to activate the receptor.

And all of that is possible thanks to just one chemical. The researchers involved here will likely be looking into a more detailed characterization of the other two chemicals they identified: one that repels locusts and one that alters their behavior.

Nature, 2020. DOI: 10.1038/s41586-020-2610-4  (About DOIs).


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