Biology and behaviour of the Australian plague locust

Oviposition and Egg Development
Nymphs
Adults
Migration


Oviposition and Egg Development

Adult locusts feeding on green vegetation can lay their first egg pod 10-14 days after fledging and any subsequent pods can be laid at intervals of 5-10 days in summer and 10-14 days in autumn. Before laying a female may excavate one or more test drill holes to assess soil condition. Once a suitable location is found, a pod of typically 30-60 eggs is laid and sealed with a frothy plug for protection. In the field females usually lay from one to three egg pods. In very dry conditions oocyte development in females can be delayed for many weeks.

egg laying locust
egg laying locust


During summer, egg laying usually occurs in areas with some grass cover and egg pods are laid vertically in the soil at a depth of 6-8 cm. In the autumn eggs are often laid in bare soil (frequently on claypans or hard ground along tracks and fences) and many are laid obliquely to the ground surface at a depth of 3-5 cm.  Gregarious oviposition often takes place to form dense egg beds, where adult females crowd a given area in response to chemical attractants released by laying females and possibly contained in the secreted froth. Egg pods in these egg beds can reach densities over 500/m2 in places.  

egg laying locuct swarm
egg laying swarm (copyright: Paul Zborowski)

Egg development is influenced by both temperature and moisture conditions. Eggs can either develop directly or development can be arrested by quiescence or diapause. Direct development occurs only in warm, moist conditions. The rate of egg development increases with temperature so that complete development can occur in as little as 15 days with a daily maximum of 35oC, while at 25°C it can take a month. Egg development ceases below about 13.5°C and egg death occurs above 38°C.  In saturated soils most eggs can survive at least 14 days, unless soil temperature is ≥ 25°C.

There are several mechanisms which can delay the the hatching of eggs of the Australian plague locust. Eggs can undergo one or more of these dormancies at different Embryonic stages of the Australian plague locust, depending on environmental conditions, leading to several possible development pathways.

Egg Diapause

Diapause is a state where growth is suspended by an indirect effect of environmental change to synchronise development to optimal climatic conditions. Diapause occurs in eggs laid during autumn in response to declining daylength experienced by the maternal generation. Diapause occurs in eggs laid from late February to late April in latitudes south of 30°S, with a rapid rise in proportion entering diapause to almost 100% by mid-March. The proportion declines gradually for eggs laid during April, and by May very few eggs enter diapause. Changing photoperiod is the principal stimulus inducing diapause in offspring eggs, but temperature also mediates the proportion eggs entering diapause.  Low (< 15°C) or high (> 26°C) soil temperatures during pre-diapause embryonic stage inhibits diapause. Diapause intervenes when eggs are ~ 40% developed and ends after 7 - 9 weeks . Embryonic development resumes in moist soils at a development rate dependent on soil temperature, but winter temperatures in southern NSW, SA or Victoria delay hatching until October.

Egg Quiescence

Quiescence is the state where embryonic growth is arrested directly by either cold or dry conditions. In dry conditions eggs enter quiescence at either ~ 35% development if the soil was dry at laying, or at ~ 40%  if the soil becomes dry while the eggs are in diapause. Dry-induced quiescence can occur at any time of the year, however in summer when ambient temperatures are very high and the soil may become very dry (≤ 3% water by weight) most eggs can only survive for 2 – 3 weeks. In cooler periods, most eggs can survive in dry soil for longer due to decreased water loss at lower temperatures. 

Nymphs

It takes about 20-25 days for Australian plague locust nymphs (hoppers) to complete development in mid summer. Plague locusts usually have five instars (growth stages) but may have six in dry or cold conditions. Mortality is usually highest during the first instar. Under very dry conditions during summer, high nymphal mortality will occur at any instar.  Nymphs are able to withstand cold conditions after emerging and typical winter night-time minimum temperatures are not likely to cause high mortality. Hatchling nymphs are also tolerant to very hot conditions (up to 45 - 53°C depending on the exposure duration) provided that suitable vegetation is present for feeding to replenish water reserves and to provide shelter.

High density nymphs of the Australian plague locust readily form into aggregations called bands. There is coordinated movement of individuals within a band and usually a distinct front develops which can stretch for several kilometres.  Bands are rarely more than a few hundred metres deep and the density can range from 1000-5000/m2 at the front to less than 50/m2 at the rear. Dense bands can often be seen from the air and resemble a tide mark on the shore.

Nymphs rarely form bands in the first and second instars but may form dense aggregations when basking in the morning sun. Bands are usually not well developed until the third instar and tend to disperse at the fifth. There is often a range of instars within a band. The rate of band movement varies with the band density, the instar, the weather and vegetation cover. Mid-instar bands in dense vegetation may move 50 metres or less per day but late instar bands in can move at 500 metres per day.

Adults

The final moult to the winged adult is called fledging. Development from egg laying to this stage usually takes 7-8 weeks in summer.

The young adult goes through three stages of development

  • growth during which the wing muscles are developed and the exoskeleton hardens
  • fat accumulation
  • oocyte (egg) development.

Each stage can be suppressed if conditions are dry. The growth stage usually lasts about a week. Copulation can occur well before the female starts to develop oocytes and is often not associated with egg laying.

After fledging the adults grow, accumulate fat and often migrate. Lipids are needed as fuel for long distance flight and egg production. If conditions are dry at fledging locusts may not migrate or develop oocytes unless substantial rain falls. If rain does not occur, numbers decline and few adults remain after several months. In some areas conditions may dry off completely during the late instar stage. The locusts generally still fledge but can remain 'papery' and transparent.

Adults vary their behaviour to maintain their body temperature within the range 35-40°C which is the optimum for development, oviposition, flight and feeding. Adults bask when the ambient temperature is low and climb vegetation or seek shade when it is high.

When densities are low adult locusts move short distances by daytime flight.  Newly fledged adults often continue to behave as nymphs and move within bands or make very brief low level flights.  In gregarious populations the majority of adults fly spontaneously for periods of up to 20 seconds at 2-5 m height.  Swarm flight usually only occurs in light winds (<3 m/sec) and at temperatures of between 20°C and 35°C. Swarms generally fly within 15 m of the ground and often appear to roll across the countryside.

The airspeed of freely flying individuals is around 3 metres per second. However even in a strongly flying swarm, a proportion of the locusts are always on the ground feeding or basking and the rate of displacement is thus usually less than the flying speed. The speed and direction of swarm displacement is further modified by hills and trees especially along creek lines which often act as barriers due to the low level flight. Swarm displacement is usually <20 km/day, but may continue for a week or more. Occasionally swarms fly at considerable heights (>30 m) during the day and have been observed from aircraft at heights up to 1,000 m. Such behaviour normally occurs with highly gregarious, very dense populations.

Swarms may persist for many days, but individual swarms often disperse and reform. As a rule, swarms are displaced downwind. However, the locusts usually fly in streams within the swarm and these streams may head in any direction.

locusts taking off at sunset
locusts taking off at sunset

Migration

Fledged, pre-reproductive adults often undertake wind-assisted long distance nocturnal migratory flights. Migrations of several hundred kilometres often occur on strong warm winds associated with rain-bearing fronts or low pressure systems. A small proportion of locusts take off individually after sunset on most evenings but when a trough or front is in the area there can be mass take off in groups. The association of mass take off with disturbed weather may increase the chance of locusts reaching rain areas, but does not necessarily result in arrival at destinations suitable for successful breeding (see When locust ​migration goes wrong).

Night take off is probably stimulated by the decrease in light intensity which is most rapid 20-30 minutes after sunset, which coincides with the period when the plague locusts are usually taking to the air. Take off occurs into the wind and the locusts climb steeply to at least 50 metres height. The locusts can remain aloft as long as the surface temperature remains above the threshold for flight and land before sunrise, giving a maximum of 9-10 hours displacement. Locusts are often reported as "raining" on rooftops at night.

Observations of nigth migrations using radar have shown that the locusts usually fly at a height of 300 to 1000 metres. The maximum altitude achieved is probably the height at which the ambient temperature is 20°C (the flight threshold temperature). At such heights the direction and rate of displacement is influenced by the upper level wind flow and the distance travelled depends on the number of hours flown.