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 extremely high nymphal mortality will occur at any instar.
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 dense, late instar bands in can move at 500 metres per day.
The final moult to the winged adult is called fledging. Development from egg laying to this stage usually takes 6-8 weeks. The adult usually 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. Adult female locusts in the growth and fat accumulation stage are immature. 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. Fat is needed as fuel for long distance flight. If conditions are dry at fledging locusts do not accumulate fat or migrate and do not develop oocytes unless substantial rain falls. If rain does not occur, numbers decline and few adults remain after several months.
In some areas (eg the Riverina) conditions may dry off completely during the late instar stage. The locusts generally still fledge but do not complete the somatic growth stage. They remain very ‘papery’ and transparent although the wings and thorax may harden to some degree.
Although they may have fledged 2 or 3 weeks previously, ‘papery’ adults may be confused with recently fledged adults. Adults vary their behaviour to maintain their body temperature within the range 35-40°C which is the optimum for growth.
Adults bask when the ambient temperature is low and climb vegetation or seek shade roosting when it is high. Only when the body temperature has reached as close to the optimum as possible, do the adults undertake other activities such as flight and feeding.
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 high proportion of the adults are always on the ground feeding or basking and the rate of displacement is thus usually far less than the individuals 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 therefore relatively slow and rarely exceeds 20 km/day, such movement 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 between at height 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.
Often locusts in swarms tend to fly into light winds with a southerly component and orientate downwind with northerlies and thus day flight usually produces a steady displacement in a southerly direction.
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.
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.
Migration only occurs with immature individuals which have completed their body growth. If fat body reserves are low due to dry conditions, flight will be fuelled by the locust’s limited carbohydrate reserves and will last only about half an hour. This can lead to short range dispersal.
If green vegetation is available the young adult will accumulate fat and then be capable of long distance migration. Observations of 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 determined by the upper level wind flow, and the distance travelled depends also on the number of hours flown.
The locusts remain aloft as long as the surface temperature remains above the threshold for flight and seem to land before sunrise, giving a maximum of 9-10 hours displacement. Locusts are often reported as “raining” on rooftops at night.
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