Grasshoppers To Swarming Pests

Scientists have uncovered the underlying biological reason why locusts form migrating swarms.

Their findings, reported in today’s edition of Science, could be used in the future to prevent the plagues which devastate crops (notably in developing countries), affecting the livelihood of one in ten people across the globe.

A collaboration between a team of scientists in Cambridge and Oxford, UK and Sydney, Australia has identified an increase in the chemical serotonin in specific parts of the insects’ nervous system as initiating the key changes in behaviour that cause them to swarm.

Desert Locusts are one of the most devastating insect pests, affecting 20% of the world’s land surface. Vast swarms containing billions of locusts stretching over many square kilometres periodically devastated parts of the USA at the time of the settlement of the West, and continue to inflict severe economic hardship on parts of Africa and China. In November 2008 swarms six kilometres (3.7 miles) long plagued Australia.

Locusts belong to the grasshopper family but unlike their harmless relatives they have the unusual ability to live in either a solitary or a gregarious state, with the genetic instructions for both packaged within a single genome.

Locusts originate from barren regions that see only occasional transient rainfalls. While unforgiving conditions prevail, locusts eke out a living as solitary individuals with a strong aversion to mingling with other locusts. When the rains come, the amount and quality of vegetation expands and the locusts can breed in large numbers.

In deserts, however, the rains are not sustained and food soon becomes more and more sparse. Thus large numbers of locusts are funnelled into dwindling patches of remaining vegetation where they are forced into close contact with each other. This crowding triggers a dramatic and rapid change in the locusts’ behaviour: they become very mobile and they actively seek the company of other locusts. This new behaviour keeps the crowd together while the insects acquire distinctly different colours and large muscles that equip them for prolonged flights in swarms.

As Steve Rogers from Cambridge University emphasises: “The gregarious phase is a strategy born of desperation and driven by hunger, and swarming is a response to find pastures new”.

Solitary and gregarious locusts are so different in looks and behaviour that they were thought to be separate species until 1921. But the realisation that crowding triggers swarming posed a new problem: how can the mere presence of other locusts have such a dramatic effect? The new research, which was funded by the Biotechnology and Biological Sciences Research Council, the Natural Sciences and Engineering Research Council of Canada and the Royal Society, solved this 90 year old question by identifying an increase in the chemical serotonin in specific parts of the locust’s nervous system as launching the fundamental changes in behaviour that lead to the gregarious phase.

morevia sciencedaily