Biology and behaviour of the Australian plague locust
Date: 14 Jan 2014 Reviewed by: Vanessa Pastega
In New South Wales, three species of locust are declared pest insects under the Rural Lands Protection Act 1998. 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 is the most recurrent — it causes almost all of the locust damage in New South Wales.
The adult Australian plague locust has characteristics which make it relatively easy to identify. 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). 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 and 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 simultaneously or progressively over several weeks if conditions for hatching are marginal. Hatching normally occurs from spring through to autumn, depending on conditions, 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, until they harden after about a week, sustained flight is not possible. 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 New South Wales, 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 this generation of adults will hatch in late December and throughout January, producing a second generation of adults in February and March.
These adults may lay eggs and, depending on conditions, these may hatch before winter in April and May or lay dormant until the following September–October. Throughout this period of spring to autumn, migration of locusts from other areas may occur and, conversely, a generation of adults may migrate from the area.
When the population is large and at the second and third instar stage, nymphs 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 may sometimes be invaded. Advanced crops are not favoured and bands will rarely penetrate them; however, peripheral damage may be inflicted.
If numbers are sufficient, adult locusts may concentrate into dense groups called swarms. These 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, to cereal crops such as wheat and oats, and to summer forage crops such as sorghum and lucerne. In closely settled districts, vegetables and even orchard trees can be badly damaged. 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.
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 and may not result in the locusts reaching an area with ideal conditions. 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.
Further information on locust biology can be found at: http://www.dpi.nsw.gov.au/agriculture/pests-weeds/insects/locusts/biology
Much of the above information was sourced from documents produced by the Australian Plague Locust Commission (APLC). Further information is available from the following APLC publications:
- Spur-throated locust (leaflet)
- Migratory locust (leaflet)
- Field guide to the locusts and related grasshoppers of Australia.
This information was reviewed by Karen Roberts, Deputy State Emergency Coordinator, NSW DPI, Orange.