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Plague Locusts - Identification and Biology

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Image: Locust
Australian Plague Locust

The Australian Plague Locust,
Chortoicetes terminifera, is a native Australian insect. It occurs naturally in the far north west of New South Wales and adjacent areas of Queensland and South Australia, an area known as the channel country.

It is a pest of pastures, field crops, and vegetables in New South Wales and southern Queensland and infrequently in South Australia and Victoria. It also occurs in Western Australia.

The eggs of the locust are laid in the ground at depths of up to 8cm. They hatch into very small “hoppers” which can move short distances but which cannot fly. Like all insects, the locust develops through discrete stages called instars until it becomes adult. Only the adult can fly.

When the locust is present in small numbers it behaves as a solitary insect and causes little or no damage, feeding primarily on summer grasses. When numbers increase, it can become gregarious and form very dense bands of hoppers or swarms of adults.

Plagues occur when very large numbers of locusts are present. They are particularly obvious when they are adults and form swarms. Swarms initially form in the channel country and generally head south on hot northerly winds. These swarms usually land in central NSW where, given favourable conditions, they can breed giving rise to new swarms which can fly south into the Riverina of NSW or into northern Victoria. Successful breeding in the Riverina can result in swarms crossing the border into Victoria in late spring, summer and autumn infesting crops and pastures from Mildura to Corryong and south to the Dividing Range. Locusts rarely invade southern Victoria. It is possible for swarms to migrate over several days from south west Queensland to Victoria. Such a migration occurred in Autumn 2004, triggering the locust outbreak that persisted until Autumn 2006.

Appearance
The locust is similar in appearance to grasshoppers. It can be identified by the large dark spot on the tip of the hind wings and the distinctive red shanks on the hind leg. The body colour varies; it can be grey, brown or green. Male locusts are 25-30 mm long while females are 30-42 mm long.

Photo: Plague locust adult
Plague locust adult

Photo: Adult locust pinned
Plague locust adult showing the black
spot on the hind wing

Immature plague locusts are referred to as hoppers or nymphs. Their wings are not fully developed and the red colour of the hindleg shanks is less developed than in adults. This makes it hard to distinguish them from immature stages of other locusts and grasshoppers.

Life Cycle

Egg Laying
Australian plague locusts generally mature within two weeks of becoming adult, depending on weather and availability of food. If conditions are favourable, with green feed, females commence egg laying 4-7 days after maturing and any subsequent egg pods are laid at 7 to 10 day intervals. The female drills a hole in the ground using her ovipositor and lays a pod of about 40 banana - shaped eggs, each about 5 mm long. After laying, the pod is sealed with a frothy plug to protect the eggs from desiccation. Females usually lay 2-3 egg pods.

Often thousands of females crowd onto a laying site giving rise to an egg bed. Up to 100 pods per square metre have been observed in dense egg beds. When temperatures are below 25�C eggs are usually laid in bare ground, often along a fence line or roadside. Frequently very hard red clay pans are selected. In hot weather (>35�C) eggs are preferentially laid in areas with some grass cover. On some occasions eggs have been laid into the ground amongst an emerging cereal crop.

Egg Development
Egg development is strongly influenced by temperature and moisture conditions. Eggs can develop immediately or development can be arrested by diapause. Immediate development occurs only in warm moist conditions. Given sufficient moisture and a daily maximum of 35�C, egg development is completed in just over two weeks, with a daily maximum of 25�C; it takes over a month. Egg development does not take place below about 15�C.

Diapause occurs when development is arrested by environmental conditions. The decreasing light and temperature in autumn can induce diapause. Eggs laid in Victoria from late February to early March onwards generally enter diapause. Development resumes when conditions are warm and moist; in Victoria this is usually in the following spring.

Eggs may hatch almost simultaneously in warm conditions, or over several weeks in cool damp conditions.

Hoppers
In mid-summer hoppers take about 20-25 days to complete development. Plague locusts usually have five instars (growth stages) but may have six in dry or cold conditions. The hoppers shed their skins after each instar and before becoming adult.

Photo: Part of an egg bed in grey soil - note holes where eggs laid
Part of an egg bed in grey soil - note holes where
eggs laid


Photo: Plague locust egg pod
Plague locust egg pod dug up and broken open to show eggs

Figure of plague locust hoppers
Drawing of Plague Locust Hoppers

First instar hoppers are about 3mm long, and whitish when first emerged, but rapidly darken to pale brown, dark brown or black. They sometimes have a white stripe along the back of the first body segment just behind the head. Later instars are grey or brown and sometimes have a white stripe along the back. The wing buds gradually develop in the later instars.

First instar locust hopper
Photo: First instar locust hopper
Later instar locust hopper showing wing buds
Later instar locust hopper showing wing buds

Hopper Behaviour
Thousands of young hoppers may emerge from an egg bed and remain in the location for several days before dispersing into the surrounding vegetation. As they develop hoppers tend to form into aggregations called bands. Bands are usually not well developed until the third instar and frequently contain a mixture of instars. Bands may extend over several kilometres and are often visible from the air: Mid - instar bands may move 50 m or less per day but late instar bands can move 500 m. The photo illustrates a band of locusts moving across a paddock. Virtually all of the green vegetation has been consumed behind the band.

Bands could contain up to 15,000 hoppers per square metre at the front of the band. Hoppers in bands all move in approximately the same direction giving the impression of a moving carpet of locusts. Often all green grass in the path of the band will be covered with hoppers that are actually eating the grass.

Adults

locust hoppers - band
Bands form a dark stain on the ground
The final moult to the winged adult is called fledging. Development from egg laying to this stage usually takes 6-8 weeks. The adult goes through a growth stage, which usually lasts about a week, during which wing muscles are developed and the exoskeleton hardens. Adults then accumulate fat and develop eggs. Development can be suppressed and may be permanently impaired by dry conditions. If green pasture is available the adults grow, accumulate fat, mate, migrate and lay. Fat is needed as fuel for long distance night flight and the locusts lay soon after arrival, even if conditions are dry.

Adults are gregarious. If densities are low, around 1/m2, they take short flights of 1 -5 km. When densities reach 5/m2 they may undertake longer swarm flights which usually only occur in light winds (< 3 m/sec) and at temperatures 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. Swarms usually fly with the wind but can be diverted by small changes in the landscape such as a row of mallee trees. Swarms will often bank up behind such obstacles. They often take the same or similar flight paths as other swarms, probably because of the obstacles that redirect their paths.

Population Increase
The number of Plague Locusts can increase very quickly and spectacularly. In Victoria, evidence suggests a lifecycle of two full generations between spring and autumn. However in northern Australia, four generations are possible in any one year although three generations are more likely.

Factors Affecting Population Increase and Decrease
Australian Plague Locusts breed and develop in the warmer months of each year. In Victoria that can mean locusts may be present from early November to late April.

Australian Plague Locusts' eggs require damp soil to hatch and the newly emerged hatchlings require fresh green grass to feed and survive. Hoppers require a source of green feed throughout their development. Adults can survive on poor quality feed but require good feed to develop eggs. Eggs are normally laid in damp bare ground. This means that Australian Plague Locusts will thrive and increase in abundance only where there is sufficient summer rainfall to promote grass growth and keep it growing.

Locusts are generally confined to areas with summer rainfall in northern NSW and southern Queensland and this is why the Australian Plague Locust does not persist in Victoria.

Migration
Migratory flights of up to 700 km occur only at night, generally on strong warm winds associated with fronts or low pressure systems. Mass take-off after sunset usually only occurs when the surface temperature is above 25�C. The locusts can fly at heights of up to 3000m. Migration flight is usually, but not exclusively, to the south or south east. Migration flight causes locusts, in very large numbers, to appear literally overnight in locations that were previously free from locusts. Migration is important both in the origin of outbreaks, and often in their collapse as it does not necessarily lead to locusts arriving in areas (where there is rainfall) suitable for successful breeding. There are reports that locust swarms have overshot continental Australia and drowned in Bass Strait.

Damage
Swarms can cause severe damage to crops where they land. Grain crops, if still just green, can have the upper node beneath the head nibbled causing the head to fall. Young vegetable crops can be decimated and vines can be stripped of their leaves.

Adult locusts can cause significant damage to the heads of wheat crops by chewing off the awns and the bracts surrounding the developing grain. In some cases the head will be nearly or fully severed and the developing grains completely removed.

In the worst case, a large swarm can cause up to 10% loss of crop. However that will not be a general 10% loss across an area but rather a total loss of crop or pasture on 10% of the area. Often the locust swarm will land during the night and will have eaten out a crop or pasture by daylight the following morning

Victorian Government Policy

Membership of the Australian Plague Locust Commission
The Victorian Government makes an annual contribution of 10% of the budget of the Australian Plague Locust Commission (APLC), which has responsibility for control over 2 million square kilometres of eastern Australia including north western Victoria. This policy is designed to assist the APLC to control locusts in Queensland, New South Wales and South Australia to prevent swarms of locusts from entering Victoria.

Periodically exceptionally favourable weather conditions enable locust populations to develop to plague proportions putting all eastern States of Australia at risk. The last such occurrence was in 2004. When locust plagues threaten, Victoria Government policy is reviewed to see if additional resources need to be provided to minimise the risk of locust damage to the State.

Control of Locusts in Victoria
The control of locusts in Victoria is the responsibility of, and at the cost of, the land manager. There is no legal requirement to control locusts in Victoria.

Much of the information and images on this site were provided by the Australian Plague Locust Commission (external link). We acknowledge their generosity.

Photo: Locust band at the edge of a crop
Locusts can occur throughout a
standing cereal crop and on occasion
form bands at the edge of the crop.

Photo: Damaged wheat head
Significant damage to the heads of wheat crops.



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