The other day I was doing a quiz about biblical plagues, and so of course locusts came up. I was looking for an insect to blog about, which got me thinking: why not locusts? No animal is quite so impressive or quite so devastating as a locust. Well, that’s not true. Locusts on their own aren’t so bad, it’s when they swarm that the real trouble starts.

All species of locust are actually types of grasshopper; the main categorical difference between what we call grasshoppers and locusts is the ability of locust species to form massive swarms when conditions are right. The species of locust that is most responsible for destructive swarms is the desert locust. It has a huge range, from west Africa to northwestern India. They need warm, dry climates to survive, so the desert locust is confined to its range by mountain ranges and rainforests.

The difference between a solitary and gregarious desert locust nymph.  Photo source: Wikipedia
The difference between a solitary and gregarious desert locust nymph.
Image by Compton Tucker, NASA GSFC, Public domain, via Wikimedia Commons

Desert locusts weigh about two grams, and can range from 1.2 to 7 cm in length. Adult locusts are winged and can fly around. The nymphs are flightless, and thus are usually known as hoppers. Locust colour depends on what stage of life the insects are in, as well as whether they are with other locusts or not. Solitary nymphs are green, while gregarious nymphs are yellow and black. The adults change from a boring brown colour to red and yellow as they mature.

Swarming behaviour in locusts only occurs in certain circumstances. There has to be enough vegetation to support the nymphs, as well as sufficient rain to make many eggs hatch at once. When this happens, the locusts can’t help but bump into each other, something which has a huge impact on the fate of the locusts. The contact between the nymphs causes the locusts to change from their solitary to gregarious form. A pheromone is released that causes the hoppers to be attracted to one another, making them congregate into a band. When the nymphs mature into adults, a swarm is made.

A swarm of locusts.
Image by CSIRO, CC BY 3.0, via Wikimedia Commons

Locust swarms are incredibly destructive and quite terrifying. They ride the winds, which means they can cover a lot of ground in one day: up to 200 km. And while they swarm, they eat. A locust can eat its body weight in food each day, which is a lot of food for a swarm, which usually number in the billions. The largest swarm ever recorded contained 40 billion locusts. I can see why locusts are famous as a plague. If I was caught in a 40 billion strong cloud of insects, I’d think it was a plague too.

Controlling locusts is hard, but it needs to be done, because locust swarms can cause terrible damage to crops. Most often insecticides are used to try and kill bands of hoppers before they develop into a swarm. There has been some research into using pheromones to try and disperse bands of hoppers. When adult locust pheromones are given to the nymphs, they become confused and disband after a few days. I guess they really don’t like the smell of old locusts.

Cover image by Dick Culbert from Gibsons, B.C., Canada, CC BY 2.0, via Wikimedia Commons