The desert locust, schistocerca gregaria, is a particularly ugly little insect, only a few inches long and in appearance not dissimilar to our own familiar grasshopper, but capable of wreaking havoc on a gargantuan scale.
The locust problem is at its worst in North Africa. For much of the time the pest is not a major worry, but every few years, conditions are such that locusts assume plague proportions. A migrating swarm of these hungry insects is capable of consuming everything edible in its path, leaving in its wake a starving human population. The last significant occurrence in Africa was about 10 years ago, and on that occasion the origins of the plague could be traced to particularly favourable weather conditions on the shores of the Red Sea in 1987, with the locusts multiplying and migrating across the continent in successive breeding seasons.
The insect's life-cycle lasts about two months, beginning when the eggs are laid a few inches below the ground. Successful hatching occurs only if the soil has been moistened by heavy rain in the recent past, so a plague develops only following the fortuitous occurrence of plentiful rain in the different hatching grounds of a few successive generations. But it takes more than this to make a swarm. In normal conditions, it seems that locusts lead a relatively solitary life. But when food is scarce and concentrated in relatively small areas, the locust population, too, is concentrated in those zones: locusts as it were, bump into one another unexpectedly, and this unpleasant crowding triggers them to fly off together in search of an alternative food source. And thus a swarm occurs.
A small swarm of locusts may contain a mere 10 million insects; a very large one, on the other hand, may occupy an area as large as Dublin county, and contain 100 billion insects with a collective weight approaching 20,000 tonnes. When you consider that a swarming locust likes to eat its own body-weight in food per day, the potential for destruction becomes obvious.
When swarming locusts have exhausted the food supply in a particular area, they take to the air to be carried by the wind to a new and more bountiful location. As a rule they fly by day, travelling perhaps several hundred kilometres, and then roost and feed at night. Accurate wind forecasts for a day or two ahead, therefore, allow the future position of a swarm to be anticipated with considerable accuracy, and resources can be deployed in advance to attempt its eradication with a suitable insecticide.