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What is a locust?
Locusts are a type of insect that can be devastating pests of agriculture due to their ability to form into dense and highly mobile swarms.
In Australia, the three main pest species of locusts are :
- the Australian plague locust (Chortoicetes terminifera)
- the spur-throated locust (Austracris guttulosa), and
- the migratory locust (Locusta migratoria).
All three species are native to Australia. Locusts belong to the same order of insects as grasshoppers, katydids and crickets - the Orthoptera (derived from the Greek words orthos meaning straight or rigid and ptera meaning wing).
Insects belonging to the order the Orthoptera can be readily identified
by their large back legs which enable them to hop or jump.
What is the difference between a locust and a grasshopper?
Locusts and grasshoppers are identical in appearance - how they differ is in their behaviour. Locusts can exist in two different behavioural states (solitary and gregarious) whereas grasshoppers generally do not. When the population density is low, locusts behave as individuals, much like grasshoppers. However, when locust population density is high they form into gregariously behaving bands of nymphs or swarms of adults. It is this change from one behavioural state to another, known as phase change, that makes locusts such devastating pests.
In addition to changes in behaviour, phase change may be accompanied by changes in body shape and colour, and in fertility, survival and migratory behaviour. These changes are so dramatic in many species that the swarming and non-swarming forms were once considered to be different species.
However, that the distinction between locusts and grasshoppers is not clear-cut. The migratory locust has all of the features associated with phase change: changes in body shapes and colour, in fertility and in forming dense bands and swarms. The Australian plague locust has all of the locust features except there is little change in body colour while the spur-throated locust rarely forms bands though does form dense swarms and migrates.
Some species of grasshoppers (eg Austroicetes cruciata, Oedaleus australis and Peakesia spp.) can behave gregariously but these species do not tend to undergo long distance migration as dense swarms like true locusts do.