Biology and behaviour of the Australian plague locust
Date: 11 May 2015
In New South Wales, three species of locust are declared pest insects under the Local Land Services Act 2013. These are:
- Australian plague locust (Chortoicetes terminifera)
- migratory locust (Locusta migratoria)
- spur-throated locust (Austracris guttulosa).
All three of these species can cause significant damage to pastures and crops, but the Australian plague locust occurs the most frequently - it causes almost all of the locust damage in New South Wales.
The adult Australian plague locust has three characteristics which makes it easy to identify from other species. There is a dark spot at the tip of the hindwings and red shanks on the hind legs. A dark 'X' mark is also present on top of the thorax (neck). Adult body colour can be grey, brown or green.
The adult female locust is approximately 4 cm long and the male is usually shorter, about 3 cm long.
The immature locusts called nymphs or hoppers are more difficult to identify. If a large mass (band) of nymphs is found, it is more likely that they are Australian plague locust nymphs than another type of nymph.
Female locusts lay eggs in batches, called pods, in the soil, usually at a depth of 2–10 cm. Each pod will contain 30–60 pale yellow banana-shaped eggs 5–6 mm long. An individual female may lay up to four pods. Each pod is sealed with a froth plug which protects the eggs from extreme temperature and ensures adequate moisture is available for development.
A collection of egg pods laid by a number of locusts is termed an egg bed. Egg beds occur typically in bare patches of compact soil, as distinct from self-mulching soils. Egg beds may vary from a few square metres to several hundred square metres and be scattered irregularly throughout a region.
Eggs need warmth and moisture to develop and will suspend development if these needs are not met. In summer, eggs may hatch within 14–16 days, while eggs laid in autumn will probably remain dormant (diapause) through winter and resume development and hatch the following spring. These egg pods will normally be laid closer to the surface than summer-laid eggs (non-diapause).
Eggs in a single egg bed may hatch together or over several weeks if conditions for hatching are marginal. Hatching normally occurs from spring through to autumn, with two to three generations hatching through that period if conditions are favourable.
An immature locust is called a nymph or hopper. After hatching from the egg, a locust goes through five growth stages called instars, moulting at each stage. The developing wings become more noticeable at each stage until the locust becomes a fledgling adult and then a mature adult capable of sustained flight. Normally, the nymphs take 4–8 weeks to complete this development.
After the final moult, the adult locust emerges with fully formed wings. At first the body and wings of the locust are still soft and they can't sustain flight for about a week until the wings harden.
Green feed is required to provide fuel for flight and for egg development. Depending on conditions an adult locust — once sufficient fuel is available for flight — may leave or migrate from an area. If conditions are ideal, however, the adult female may lay eggs in the same area in which she developed.
Locust occurrence depends entirely on weather and feed conditions. In NSW, hatching typically begins in September in the north through to October in the south, with maturity of locusts occurring in November and December. Eggs laid by the first generation of adults will hatch in late December and throughout January, producing a second generation of adults in February and March.
The second generation of adults may lay eggs which may hatch before winter in April and May or lay dormant until the following September–October, depending on conditions.
Throughout spring to autumn, migration of adult locusts may occur into or out of a particular area.
When the population is large, nymphs at the second and third instar will often concentrate into dense aggregations called bands. These bands will vary in size but can extend over several kilometres.
A band can move over 1 km from the egg bed before the nymphs fledge and can consume or damage all vegetation in its path.
Damage by nymph bands is confined mainly to pasture, although crops are sometimes invaded. Advanced crops are rarely entered, however peripheral damage may be inflicted.
If numbers are sufficient, adult locusts may concentrate into dense groups called swarms. The swarms infest areas that are usually less than 5 km2 but which can be up to 50 km2. During outbreaks, locust swarms can do widespread and severe damage to pasture, cereal crops such as wheat and oats, and summer forage crops such as sorghum and lucerne. Winter grain crops have usually hardened off by the time adult locusts are active in early summer. However, in dry weather less advanced and more open crops are highly susceptible to attack. Vegetables and even orchard trees can also be badly damaged.
If sufficient green feed is available to enable flight but conditions are drying off, migration of locusts may occur. The direction and distance covered during migration depends on temperature, wind-speed and wind direction. This means that locusts cannot fully control their direction and they may end up reaching an area that is also not ideal. Migrations of 500–600 km overnight are not uncommon and this behaviour can lead to the sudden appearance of large numbers of locusts in areas previously uninfested.
Much of the above information was sourced from documents produced by the Australian Plague Locust Commission (APLC). Further information about the biology of Australian locusts is available from the following APLC website.