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Grey Gentry wrote:

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Do locusts travel at night? Craig...where's Craig? He could answer that

In short - yes they do - if the conditions are right. See below.

Adults

The final moult to the winged adult is called a fledging. Development from egg laying to this stage usually takes 6-8 weeks (for a flow diagram see pathway of development from egg to adult).

The adult usually goes through three stages of development: growth during which the wing muscles are developed and the exoskeleton hardens; fat accumulation; and 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 laying.

If pasture is green to drying off, the adults grow, accumulate fat and migrate. Fat is needed as fuel for long distance night flight and if the locusts locate areas of recent rain they lay soon after arrival. Egg development is delayed if immigrant locusts invade areas that are relatively dry.

When conditions dry off at fledging, locusts persist. They do not accumulate fat or migrate and do not develop oocytes unless substantial rain falls (approximately 40 mm or more). If rain does not occur, numbers decline and 6-8 weeks after fledging few adults remain. 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 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 plague locust adults move only short distances by daytime flight. At densities of less than 1/m2 only a small proportion of the adults make short, 1-5m flights at no more than a metre above the ground. In gregarious populations with a density greater than 5/m2, the majority of adults fly spontaneously for periods of 6-20 s at 2-5 m height. Nevertheless, even in gregarious populations, sustained flight only occurs in adults over 4-5 days old.

Newly fledged adults often continue to behave as nymphs and march within bands or make very brief low level flights. 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 frequently at less than 3m and often appear to roll across the countryside.

The airspeed of freely flying individuals is 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 while it is often as little as 3 km/day 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 1,500 and 3,000 metres. Such behaviour normally occurs with highly gregarious, very dense populations.

While most locust species form well defined, persistent, discrete swarms with no scattered locusts in between, the Australian plague locust is less gregarious. In any one area plague locusts can occur in a continuum from dense swarms through lower density swarms to concentrations, numerous adults and scattered adults.

Swarms may persist for many days, but often individual swarms cannot be tracked; it is likely that they 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 at less than high swarm density 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.

Above: locusts taking off at sunset

Migration

In the inland where the Australian plague locust is common, rain is often localised. To increase the chance of finding localised rain, this species undertakes long distance migrations. Migrations of several hundred kilometres occurs at night and often takes place 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 fronts or lows are in the area there is mass take off in groups. The association of mass take off and long distance migration with fronts or lows may increase the chance of locusts reaching rain areas but does not necessarily lead to locusts arriving in areas (where there is rainfall) suitable for successful breeding (see When locust migration goes wrong).

A few plague locusts take off individually from all adult infestations shortly after sunset if the surface temperature is about 20°C, though mass takeoff has only been observed when the surface temperature is above 25°C and a front or low is in the area.

Night take off is probably stimulated by the decrease in light intensity which is most rapid 20-30 minutes after sunset; this 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. During the day, flight is inhibited in winds >3m/second but after sunset locusts take-off even in strong winds.

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. Under optimum conditions adults will be ready for migration about a week after fledging and migration will occur on the first suitable night thereafter.

The percentage of a population that is ready to migrate can be readily assessed by visually estimating the amount of fat present in the abdomen of several small samples of locusts (10) and determining whether the cuticle is soft (newly fledged) or hard.

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° (the flight threshold temperature). At such heights the direction and rate of displacement is determined by the upper level wind flow, and the total distance covered 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 sunset, giving a maximum of 9-10 hours displacement. Locusts have been reported "raining" on rooftops in the early hours of the morning.

In the inland locusts usually have to migrate to locate areas of recent rain. During autumn, when rain is decreasing in Queensland and increasing in New South Wales and South Australia, locusts often migrate over several nights until they reach rain areas in the south. This results in the common autumn invasion of the agricultural zone.

In late spring, locusts in the agricultural zone can migrate in any direction but those that migrate north where rain is increasing have a greater chance of finding rain and breeding successfully.

Good luck on the ride guys. Cheers, Craig

Ah, they scrape off Peter

Take care on the ride.

Cheers, Craig