2020 not only brought a worldwide pandemic with Covid-19 and severe forest fires in Australia, it also brought unusually large locust swarms in Africa and the middle East in regions where food is already scarce at the best of times. In particular East Africa saw its worst locust plague in decades. These swarms consisting of billions of locusts (Locusta migratoria) descend onto hundreds of square kilometres at a time, devouring crops, and other vegetation completely.
The only defence is currently aerial spraying of pesticides which can be hit and miss as the swarms move fast and unpredictably. In addition, the chemicals can harm beneficial insects as well livestock and the environment.
Researchers from China find the way forward
But new research shows that there is a light at the end of a very long tunnel. It has long been assumed that the swarms congregate due aggregation pheromone excretion that attracts more and more locusts. A Chinese research team headed by Xianhui Wang and Le Kang of the Chinese Academy of Sciences has discovered the key compound 4VA (4-vinylanisole), a molecule emitted by gregarious locusts. This compound is a volatile odorant and smells sweet to humans.
Locusts are solitary by nature and averse to socialising. But something happens when four to five locusts congregate. They will start to produce 4VA which will in turn attract more locusts and transform them into their gregarious state. 4VA attracts locusts of all sexes and ages whether they are in their solitary or gregarious state. As the group grows, more and more 4VA is emitted attracting locusts from ever larger distances contributing to the exponential growth of the swarm into millions and billions of individuals. The compound also contributes to the cohesion of the swarm over a longer period of time.
The researchers suggest using the molecule to lure the locusts away from crops and trap them. So far traps laced with 4VA have only had limited success. It is assumed that a larger concentration of the compound may be needed. But another factor could play an important role. It is also thought that 4VA is not the only chemical causing this mass congregation but that there are other background odours causing the behaviour by amplifying the swarming signal. More research is still needed on this.
The Chinese researchers also discovered that the locusts use the OR35 olfactory receptor of their antennae to detect 4-vinylanisole. They used the gene editing technique CRISPR-Cas9 to deactivate this particular receptor which in turn led to the locusts to ignore the presence of 4VA. The locusts showed no behavioural changes.
This discovery could lead to further research of chemicals that block the activity of this particular receptor. These chemicals could cause the locusts to stop congregating and turn back to their solitary and more peaceful way of life.
Sources