Host: Shamini Bundell
Over the past few months, plagues of locusts have been devastating crops across Africa. The sheer size of the record-breaking swarms is leaving farmers fearful for their livelihoods and communities with dwindling food supplies. And yet, locusts aren’t always so devastating. In fact, they spend most of their lives as solitary creatures. What triggers their swarming behaviour has been a long-standing mystery until now. A group from the Chinese Academy of Sciences in Beijing has discovered a pheromone which they believe is responsible for the change – the chemical signal that turns Jekyll into Hyde. We couldn’t get hold of the authors for this podcast, but reporter Geoff Marsh spoke to entomologist Leslie Vosshall from the Rockerfeller University in New York.
[Locust sound]
Interviewee: Leslie Vosshall
These insects are crazy. So, they start out as solitary, peaceable insects that nibble on food now and again, but once you get them together into a group of more than a small number, they turn into a mob. They change how they look. So, they change from this nice green grasshopper look to this menacing brown look, and then they take flight and migrate in enormous numbers and settle into agricultural fields and strip them of all crops. And then they get up and they fly to the next field, strip that crop, and keep going, and it has an unbelievably devastating effect on human agriculture and life. I’m Leslie Vosshall, professor at Rockerfeller University in New York, and I am a molecular neurobiologist and I really care about how humans smell smells in the environment and how insects smell smells in the environment and how they smell humans.
Interviewer: Geoff Marsh
A slight tangent then for you – we’re here to talk about how locusts smell locusts, essentially. There’s a real sort of Jekyll and Hyde vibe to their lifestyle, isn’t there?
Interviewee: Leslie Vosshall
Absolutely. They become completely different creatures. There’s almost no precedent in biology for their behaviour.
Interviewer: Geoff Marsh
And am I right in saying that it’s been long-suspected that there’s some sort of molecule or pheromone that forms a sort of master switch from that calm, solitary form into these devastating swarms?
Interviewee: Leslie Vosshall
Absolutely, and people have been searching for that pheromone for a very long time because it’s an interesting biological puzzle, number one, but also, number two, if you found that pheromone, you could of course use it to lure solitary locusts, trap them, and prevent the riotous mob from forming.
Interviewer: Geoff Marsh
Well, perhaps we should sort of cut to the chase. The authors have, it seems, fairly conclusively nailed their suspect and identified this pheromone as something called 4-Vinylanisole, which probably means very little to most people, but perhaps we could discuss how they got to that result.
Interviewee: Leslie Vosshall
This is an amazing series of experiments. When I was sent this manuscript, I just gasped. I thought, this is unbelievable. They』ve finally solved this long-standing, centuries old problem, and they did everything that one would expect of a pheromone hunt.
Interviewer: Geoff Marsh
What’s your checklist? How do you provide robust, gold standard evidence that this is the pheromone creating the switch?
Interviewee: Leslie Vosshall
You do that by sampling the air around the locust and finding the kinds of smell molecules that are in the air, do the analytical chemistry to generate the list of molecules, go down the list to find out which one causes them to aggregate. Then the key experiment, you chemically synthesise that molecule, so you make an artificial version of whatever the locust was producing and then you ask, 『Does the chemical synthetic variant of what you think the pheromone is have the same effect?』
Interviewer: Geoff Marsh
And then the authors extend that, don’t they, and they go on to show specifically which receptor type and where the receptor type is. It sounds seriously involved, the research.
Interviewee: Leslie Vosshall
That’s right, so this paper compresses what would be decades of work into a single paper. So, not only identifying the molecule, showing that the synthetic variant has the same activity as the stuff that the locusts make. Then they ask this question of how are the locusts smelling it, and so they do recordings of individual cells on the antenna – there are thousands of different cells – and then they puff over these individual compounds, 4-Vinylanisole, and identify cells that respond to this punitive pheromone. They then have a long list of odorant receptor genes that could be detecting the pheromone, so these would be the proteins sitting in the neurons in the locust antennas that are actually binding to the pheromone, and in these really beautiful experiments, they put these genes into cells, force those cells to express this locust receptor, so you have tissue culture cells smelling the punitive pheromone and then they make a match. This particular receptor is the closest match to what the locust is actually using to smell the compounds.
Interviewer: Geoff Marsh
And then as if that wasn’t enough evidence, they then, in a world where CRISPR gene editing is more easily applied, they knocked out that specific receptor and lo and behold, the behaviour was stopped.
Interviewee: Leslie Vosshall
That’s right, so a key prediction, if you have the right receptor, if you make an animal that lacks the receptor, it should no longer respond to the pheromone. The mutant retains a little bit of activity, probably because there’s additional chemical components that have a little bit of activity, but the mutant is very, very defective, so that was a beautiful genetic validation using modern techniques to make a locust that’s impervious to the pheromone.
Interviewer: Geoff Marsh
Obviously, it’s very cool that we』ve now got the specific identity of this pheromone that we』ve been searching for for such a long time, but beyond that pure science factoid, how can this new knowledge be useful in terms of stopping these devastating swarms?
Interviewee: Leslie Vosshall
The authors have already shown the way toward the deployment of this molecule. So, you can put the molecule out on bait traps, as they』ve shown in their paper, and you will attract locusts. I think that the key is to come up with something that has the same activity as 4-Vinylanisole but is amped up, so it’s 100 times more potent, and then use that as a super juiced-up pheromone version.
Interviewer: Geoff Marsh
As you said, you come from a background of mosquito biology. I suppose an idea could be borrowed from that field in terms of yeah, releasing knockout locusts into the wild that don’t respond to this pheromone.
Interviewee: Leslie Vosshall
Yeah, this is another idea, again, that could be borrowed from insect friends, the mosquitoes, that are probably an even more intractable problem, so the authors in this paper have generated mutants that don’t respond to the pheromone. You could, in principle, generate a whole race of locusts that take over the populations and now they will neither produce the pheromone nor smell it, would be the prediction.
Host: Shamini Bundell
That was Leslie Vosshall. You can find a News and Views article she’s written about this, along with the paper, in the show notes. Also, thanks to Baudewijn Ode for the migratory locust field recordings.