Because of their vast numbers and their great appetite for vegetable food. When a swarm of locusts alights in a district the insects may, however, not do much damage before moving on.
But they leave behind them millions of eggs, laid in the ground. These soon hatch out, and the ground becomes alive with tiny locust larvae, 1 inch long.
By the end of three or four weeks they have grown sufficiently to be able to hop (at this stage they have no wings) and then begin to march steadily forward, like a vast army, in one direction. It is these hungry "hoppers" which do the worst damage. They eat every vegetable thing, even the bark of trees, that comes in their way, leaving devastation behind them. Fortunately, their inability to fly makes it possible to collect and destroy huge numbers by poisoning or burning them.