Australian Plague Locusts Identification and Biology

Australian Plague Locusts July 2010 Identification and Biology Australian Plague Locust from south west Queensland to Victoria. Such a migration occurred in Autumn 2004, triggering the locust outbreak that persisted until Autumn 2006. Plague locust adult The Australian Plague Locust, Chortoicetes terminifera, is a Plague locust adult showing the black spot on the hind wing native Australian insect. It occurs naturally in the far north west of New South Wales and adjacent areas of Queensland and South Appearance Australia, an area known as the channel country. The locust is similar in appearance to grasshoppers. It can be It is a pest of pastures, field crops, and vegetables in New South identified by the large dark spot on the tip of the hind wings Wales and southern Queensland and infrequently in South and the distinctive red shanks on the hind leg. The body colour Australia and Victoria. It also occurs in Western Australia. varies; it can be grey, brown or green. Male locusts are 25-30 The eggs of the locust are laid in the ground at depths of up mm long while females are 30-42 mm long. to 8cm. They hatch into very small “hoppers” which can move Immature plague locusts are referred to as hoppers or nymphs. short distances but which cannot fly. Like all insects, the locust Their wings are not fully developed and the red colour of the develops through discrete stages called instars until it becomes hindleg shanks is less developed than in adults. This makes it adult. Only the adult can fly. hard to distinguish them from immature stages of other locusts When the locust is present in small numbers it behaves as a and grasshoppers. solitary insect and causes little or no damage, feeding primarily on summer grasses. When numbers increase, it can become Life Cycle 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 Part of an egg bed in grey soil – note holes where eggs laid 10/LO/0012 Egg Laying Hoppers Australian plague locusts generally mature within two weeks In mid-summer hoppers take about 20-25 days to complete of becoming adult, depending on weather and availability of development. Plague locusts usually have five instars (growth food. If conditions are favourable, with green feed, females stages) but may have six in dry or cold conditions. The hoppers commence egg laying 4-7 days after maturing and any shed their skins after each instar and before becoming adult. subsequent egg pods are laid at 7 to 10 day intervals. The Egg laying 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 Fledging 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 5th instar Adult maturation observed in dense egg beds. When temperatures are below 25°C (about 2 weeks) eggs are usually laid in bare ground, often along a fence line or roadside. Frequently very hard red clay pans are selected. In hot Egg incubation Hopper (about 2 weeks) weather (>35°C) eggs are preferentially laid in areas with some 4th instar development grass cover. On some occasions eggs have been laid into the (about 5 weeks) ground amongst an emerging cereal crop. Egg pods Egg Development 3th instar 1st instar 2nd instar Target these stages with insecticide application Figure 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. Plague locust egg pod dug up and broken open to show eggs 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 First instar locust hopper 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. Later instar locust hopper showing wing buds Hopper Behaviour 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. Bands form a dark stain on the ground Australian Plague Locusts’ eggs require damp soil to hatch Thousands of young hoppers may emerge from an egg bed and and the newly emerged hatchlings require fresh green grass remain in the location for several days before dispersing into to feed and survive. Hoppers require a source of green the surrounding vegetation. As they develop hoppers tend to feed throughout their development. Adults can survive on form into aggregations called bands. Bands are usually not poor quality feed but require good feed to develop eggs. well developed until the third instar and frequently contain a Eggs are normally laid in damp bare ground. This means mixture of instars. Bands may extend over several kilometres that Australian Plague Locusts will thrive and increase in and are often visible from the air: Mid – instar bands may move abundance only where there is sufficient summer rainfall to 50 m or less per day but late instar bands can move 500 m. The promote grass growth and keep it growing. photo illustrates a band of locusts moving across a paddock. Virtually all of the green vegetation has been consumed behind Locusts are generally confined to areas with summer rainfall the band. in northern NSW and southern Queensland and this is why the Australian Plague Locust does not persist in Victoria. 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 Migration moving carpet of locusts. Often all green grass in the path of Migratory flights of up to 700 km occur only at night, generally the band will be covered with hoppers that are actually eating on strong warm winds associated with fronts or low pressure the grass. systems. Mass take-off after sunset usually only occurs when the surface temperature is above 25°C. The locusts can fly at Adults heights of up to 3000m. Migration flight is usually, but not The final moult to the winged adult is called fledging. exclusively, to the south or south east. Migration flight causes Development from egg laying to this stage usually takes 6-8 locusts, in very large numbers, to appear literally overnight in weeks. The adult goes through a growth stage, which usually locations that were previously free from locusts. Migration is lasts about a week, during which wing muscles are developed important both in the origin of outbreaks, and often in their and the exoskeleton hardens. Adults then accumulate fat and collapse as it does not necessarily lead to locusts arriving in develop eggs. Development can be suppressed and may be areas (where there is rainfall) suitable for successful breeding. permanently impaired by dry conditions. If green pasture is There are reports that locust swarms have overshot continental available the adults grow, accumulate fat, mate, migrate and Australia and drowned in Bass Strait. lay. Fat is needed as fuel for long distance night flight and the locusts lay soon after arrival, even if conditions are dry. Damage 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.

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