History of locust and grasshopper outbreaks in Australia

Locusts as pests
History of Australian plague locust outbreaks
Spur-throated locust outbreaks
Migratory locust outbreaks
Outbreaks of other species
Further reading

Locusts as Pests

The three main pest species of locusts in Australia are the Australian plague locust (Chortoicetes terminifera), the spur-throated locust (Austracris guttulosa) and the migratory locust (Locusta migratoria).

Locusts can cause widespread and severe damage to pastures, cereal crops and forage crops. In closely settled areas they may also cause considerable damage to vegetable and orchard crops. The Australian plague locust is the most serious pest species in Australia due to the frequency of outbreaks and the large areas infested. In 1984, during a major outbreak of the Australian plague locust, estimated crop loss was $5 million. A cost-benefit analysis estimated that without locust control over $100 million of crop losses may have occurred. More recently an economic analysis of APLC locust control during 1999-2004 concluded there was a direct cost-benefit ratio of approximately 1:8.

Dense aggregations of locust nymphs (hoppers) are called bands. Damage by Australian plague locust hopper bands is mainly confined to pasture, and losses have been estimated to be equivalent to about a 10 per cent increase in stocking rate. Hopper bands also damage cereal crops, often causing significant damage along the edges of advanced crops. In dry conditions, however, less advanced and more open crops are highly susceptible to locust infestation. Bands may extend over several kilometres and are often visible from the air.

Aerial photo of damage to a wheat crop caused
by bands of Australian plague locust nymphs

An aggregation of adult locusts is called a swarm. A swarm may contain millions of locusts and can cover an area of several square kilometres. Young winter cereals are very susceptible to damage in the autumn when locust populations usually reach their peak. Winter grain crops have usually hardened off by the time adult Australian plague locusts become active in early summer, but damage to ripening heads or contamination of harvested grain sometimes occurs.

A swarm of migratory locusts in central Queensland

Adults of the Australian plague locust and spur-throated locust are capable of migrating over very large distances. Overnight migrations up to several hundred kilometres are not uncommon and this behaviour can lead to the sudden appearance of large numbers of locusts in previously uninfested areas.

Australian plague locust nymphs can consume between 100 and 450 mg of green vegetation a day. Adult food intake has been estimated to be about 0.2 grams of green vegetation per day on average, a smaller proportion of their bodyweight. With densities of adult in settled swarms ranging from about 4 to 50/m², a swarm covering an area of 1km² could consume over 1000 kg of vegetation a day, depending on density.

Australian Plague Locust

The earliest known record of swarms is from 1844. Geographically extensive outbreaks were reported from the 1870s and occurred with an appearent increase in frequency and intensity after 1900. Inland agricultural areas of New South Wales and Western Australia became prominent during outbreaks in the 1920s. A pattern of high density populations developing in some locations in most years, with less frequent very large 'plague' populations extending across several states for one or two years, has persisted in eastern Australia since that time. In Western Australia outbreaks are generally less frequent. However, major outbreaks have occurred in 1982, 1990-91, 1999-2001, 2004 and 2006.

Graph of the level of outbreaks of the Australian plague locust in eastern Australia between 1934 and 2008.
The years are divided into three seasons: summer, autumn and spring.
Scale of the outbreaks: 0 - very few locusts
1 - background population, a few bands/swarms
2 - outbreak, localised bands/swarms in several areas
3 - major outbreak, many bands/swarms, some dense
4 - plague, several hundred thousand hectares infested by bands/swarms in the agricultural zone
5 - major plague, over 500,000 hectares infested by bands/swarms in the agricultural zone

The scale for the graph above is from estimates of the area of land infested by locusts based on historical reviews by Casimir (1962), Farrow (1977), Key (1938), Magor (1970), Wright (1987) and APLC Locust Bulletins (1977-2008).

The Australian plague locust can reach plague proportions within a year if a sequence of widespread heavy rains occur in inland areas, particularly during summer, allowing them to complete several generations of increase. Less regular rains, falling in both the interior and in the agricultural zone of eastern Australia, can maintain high density gregarious populations for several years, and continue a plague cycle. Prolonged dry periods usually result in a population decline to background levels.

An outbreak cycle may involve exchange migrations between regions of summer and winter rainfall, and the persistence of high density populations of agricultural regions of inland southeastern Australia. Heavy summer rainfalls in western Queensland often lead to large population increases and subsequent southward migrations in late summer and autumn. This pattern has characterised several of the recorded major pest outbreaks, or plagues.

Spur-throated Locust

A major plague of spur-throated locusts occurred 1973-75 and caused extensive damage to crops in Queensland. Control was also required in 1994-1997 and 2000-2001, mainly in cropping areas of central Queensland. Because the spur-throated locust has only one generation per year, it may take several years for populations of this species to develop to plague proportions.

Migratory Locust

This species is a major pest in many parts of the world but in Australia outbreaks are less frequent and are largely restricted to central Queensland. Early records of infestations are not well documented but outbreaks prior to the 1970s were unknown. Widespread clearing of scrub in central Queensland greatly expanded the area of favourable habitat for this species and above average rainfall during the 1970s allowed populations to reach outbreak levels. A major plague occurred between 1973 and 1975 causing severe damage to crops. Since then, smaller outbreaks requiring control have occurred intermittently (1992-93, 1996-97, 1998-1999 and 2000-2001) and a program of preventive control was initiated by the APLC and Queensland Department of Natural Resources in 1996.

Outbreaks of the migratory locust can occur if good rain falls over two consecutive years, allowing 3-5 generations to develop in each year. Winter rain is of importance to migratory locusts, as the eggs do not enter diapause and they cannot survive prolonged dry periods. Unlike the Australian plague locust and spur-throated locust, which normally mature eggs only after rain has produced green vegetation, migratory locusts mature their eggs even on drying vegetation. If the eggs are laid in dry soil and no follow-up rain occurs high mortality occurs. In the Central Highlands of Queensland where migratory locusts are a frequent pest there is much usually less rain during winter than in summer, so egg mortality is more common at this time of the year.

Outbreaks of Other Species of Locusts and Grasshoppers in Australia

Other species of locusts and grasshoppers that can reach outbreak numbers and occasionally cause economic damage include the small plague grasshopper (Austroicetes cruciata), wingless grasshopper (Phaulacridium vittatum), yellow winged locust (Gastrimargus musicus) and eastern plague grasshopper (Oedaleus australis). As all of these species have no or a very low migratory capability, outbreaks are usually localised and do not pose an interstate threat to agriculture. The APLC is therefore not responsible for their control. During severe outbreaks state authorities may assist landholders with the control of economically damaging populations.

Outbreaks of the small plague grasshopper have been recorded since the late 19^th century and this species caused severe damage to winter cereal crops in South Australia and Western Australia during the 1930s and 1940s. Outbreaks requiring control by the state government occurred in South Australia in 1998-1999 and 2005-2006.

The wingless grasshopper is mainly a pest of improved pasture in the tablelands and western slopes of New South Wales, where average annual rainfall is above 500 mm. Outbreaks have been recorded in New South Wales since 1935 but have become more severe over the last five decades largely due to the expansion of improved pasture which provides the wingless grasshopper with favourable food plants and habitats (Baker 1993). Localised outbreaks of wingless grasshoppers occur every five years or so on the western slopes and every 8-10 years in the tablelands. Widespread regional outbreaks of wingless grasshoppers occurred in 1965-66, 1979-82 and 1989-91.

Outbreaks of the yellow-winged locust occurred in Queensland in 1911-16, 1930-35, 1939-47 and 1961-62 (Baker 1993) while the only known outbreak of the eastern plague grasshopper was in the Singleton district, New South Wales, in 1906-7 (Key 1938). High numbers of both species have been recorded infrequently in parts of Queensland and New South Wales in other years.

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