When Hoppers Start Flying; the amazing plague locust story

Locust plagues have been reported - and feared - since at least Biblical times. What we have not done, until very recently, is understand them. In recent decades that has changed, due in large part to the CSIRO (the Australian national science institution, for those reading from elsewhere).

'Locust' is a curious term referring not to a species, but to an even more curious life form; some grasshoppers can be locusts sometimes. In Australia just four arid land grasshopper species (plus others elsewhere in the world) can perform the trick. They are normally solitary and sedentary, but in some specialised conditions they suddenly increase dramatically in number, start to move in vast numbers, eating hundreds of tons of vegetation per day. Individuals change in colour, size and behaviour when in high densities compared with when they are solitary; the term is kentromorphism!

Spur-throated Locust, Austracris guttulosa, south-west Queensland;
all the following photos were taken in this general area.

It is all to do with the uncertainties of living in deserts, especially those ruled by El Niño. Normally the female lays about 40 eggs in a hole drilled into hard open ground between patches of vegetation, scattered over vast areas. In drought they are forced into smaller and smaller moist areas eg at the foot of dunes, in soaks etc. Eggs are laid closer and closer together, until there may be 3,000 holes per square metre, ie there may be a thousand million eggs per hectare – that’s a lot of little grasshoppers!

Spur-throated Locusts.

What’s more, they only hatch in favourable conditions (no point in emerging in a drought), so they may sit in the ground for years until the right conditions come along, then all hatch simultaneously. When such large numbers hatch at once, a behavioural change occurs, apparently induced by developing in close proximity to lots of other little grasshoppers. In such circumstances they seem to need company, which they don’t if they’ve hatched separately. They also look different – darker pigments form in the skin, so they absorb more of the sun’s heat, so are more active, so in turn must eat more. All this leads towards a swarm. 

Locust swarm; in fact this was just a tiny part of a swarm that we drove through for hours.

Having unusually high internal energy levels from the sun, and from increased eating, forces them to keep moving, so they must keep eating. Under normal circumstances these great swarms are driven by the winds into south-eastern Australia; when the wind speed exceeds 11kph, they move with it. As they move into cooler areas, they stop moving and eventually all die. Meantime they’ve been laying eggs as they go, which until the next drought changes things, will hatch as 'normal' grasshoppers.

For obvious reasons the swarms are dreaded by anyone relying on growing plants, but there are always good reasons for understanding a phenomenon. This one is an adaptation to the 'boom and bust' nature of desert living - spread when conditions are good, contract and survive when they’re not. It is a perfectly natural event, and as wonderful a survival mechanism as any in all of nature.