Biology and behaviour of the Australian plague locust
Above: egg laying locust
The eggs are laid in the soil. The female drills a hole in the ground using her ovipositor and lays a pod of about 50 eggs though there may be fewer eggs when the vegetation is drying off. After laying, the pod is sealed with a frothy plug to protect them from dehydration.
Females usually lay 2-3 egg pods. If conditions are green, females lay their first pod 5-7 days after maturing and any subsequent egg pods are laid at intervals of 5 days (summer) to 10 days (autumn).
After migration, laying can occur even if the vegetation is drying off but the first laying does not usually occur for 10 days or more.
During hot weather in summer, eggs are usually laid in areas with some grass cover and egg pods are generally laid vertically in the soil at a depth of 6-8 cm. In the autumn, when temperatures are below 25 degrees centigrade, eggs are usually laid in bare soil. Frequently, very hard red clay pans are selected for egg laying. Egg pods may be laid obliquely to the ground surface at a depth of 3-5 cm. This could be to place the eggs closer to the surface which will become warmer earlier in the spring.
Female locusts will often make test drill holes in the soil without laying any eggs. Such behaviour is common in many other insects, eg. beetles to moths and butterflies as well as wasps and flies. Typically, such behaviours are associated with females assessing the suitability of a substrate or host (by means of sensors on the tip of the abdomen and elsewhere) for the development of their eggs and larvae or nymphs. It is likely that test drilling by Australian plague locust females is undertaken for the same purpose. APLC staff intend to research test drilling to determine its implications for egg laying. The phenomenon of test drilling means that all persons concerned with the identification of locust egg beds should dig up clods of soil where locusts are seen drilling or holes are evident and directly inspect the actual density of locust eggs in the soil. The extent of previous layings can be greatly over-estimated if the presence of holes is taken to indicate the presence of eggs.
Above: Egg laying swarm (Photo copyright: Paul Zborowski)
Egg pods are laid at densities ranging from 1/100m2 in dispersed populations up to 100/m2 in dense egg-beds. Egg development is strongly influenced by temperature and moisture conditions.
Eggs can either develop directly or development can be arrested by quiescence or diapause (for a flow diagram see pathway of egg development). Direct development occurs only in warm moist conditions. Given sufficient moisture and at a daily maximum of 35°C, egg development takes just over two weeks, while with a daily maximum of 25°C, it takes over a month. Egg development does not take place below about 15°C (see also embryonic stages of the Australian plague locust).
Diapause is the state where growth is arrested by an indirect effect of the environment. Decreasing light and temperature in the autumn increase the temperature threshold for development from about 15°C to 25-35°C. Eggs enter diapause if temperatures are below this higher threshold (ie. after early March in southern NSW, late March in central NSW, mid April in southern Queensland). Diapause ends after about 2 months, the embryos resuming development when conditions are warm and moist.
Quiescence is the state where growth is arrested directly by either cold or dry conditions. Cold induced quiescence occurs in winter in the southern areas. Dry induced quiescence can occur at any time of the year. It is thought that most eggs can only survive for about six to eight weeks in dry soils when temperatures are very high (about 40°). However, most eggs survive in the winter when the soil is dry. In dry conditions eggs enter quiescence at either 25 per cent development if the soil was dry at laying, or at 45 per cent if the soil became dry while the eggs were in diapause.
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.
Nymphs of the Australian plague locust readily form into aggregations called bands. These are often likely to come directly from one egg bed with several waves of bands resulting from different times of hatching. From the air the eaten vegetation appears like a hand with the bands at the finger tips. Individual bands resemble a tide mark on the shore (for an aerial photo of a band see Locusts as pests).
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 direction of movement of a band is usually consistent but differs between bands in the same area; an obstacle, such as a road, sometimes causes a change in direction. There is usually (but not always) a distinct front which can stretch for several kilometres. The bands are rarely more than a few hundred metres deep and the density can range from 1000-3000/m2 at the front to less than 50/m2 at the rear.
Generally, in drier open country in the west, bands are large and visible from the air with few individuals outside the bands. In the better agricultural country bands tend to be smaller, they may not have distinctive fronts and many cannot be seen from the air. At densities below 30/m2 nymphs move little and show little gregarious behaviour. A t densities of less than 5/m2 they are very difficult to detect.
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
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.
The APLC monitors locust migration by operating a network of 8 light traps across Australia. To find out what a light trap looks like, where they are located and how many locusts were caught in recent seasons, go to the How we monitor locusts page.
The Australian plague locust is the most damaging locust species in Australia due to the area of infestation and frequency of plagues (see Introduction to Locusts for a graph of outbreaks).
Damage by Australian plague locust hopper bands is mainly confined to pasture although crops may sometimes be invaded. Hopper bands do not favour advanced crops and rarely penetrate them to any extent but they may inflict significant damage along the edges.
Winter grain crops have usually hardened off by the time adult Australian plague locusts become active in early summer. However, in dry conditions, less advanced and more open crops are highly susceptible to locust infestation. Young winter cereals are very susceptible in the autumn when locust populations usually reach their peak.
When infestations are high, damage to pasture by hoppers and adults can be equivalent to about a 10 per cent increase in stocking rate. Losses may amount to several millions of dollars during a plague unless effective control action is taken.