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). The Australian plague locust is the most serious pest and outbreaks requiring control occur about every 2 years.

Generally, locust plagues develop when widespread areas of inland Australia receive good rainfall in successive seasons. High rainfall during summer and the good growth of grass produces ideal conditions for locust breeding. The timing of rainfall events is important for the survival of locust nymphs and the sequence of breeding from one generation to the next. While rainfall is the most important factor in plague development, migration also contributes to the size and location of outbreaks.

Australian plague Locust
Spur-throated Locust 
Migratory Locust
Outbreaks of Other Species (of locusts and grasshoppers in Australia)
Further Reading

Australian Plague Locust
The earliest record of swarms of Australian plague locusts is from1844. Further outbreaks were recorded in the late 19^th Century but are not well documented. Since 1933, when more definite records were kept, there have been 12 plagues and five major outbreaks of the Australian plague locust in eastern Australia. In Western Australia (WA) outbreaks are generally less frequent. Major outbreaks have occurred in WA in 1982, 1990-91, 1999-2001, 2004 and 2006.

The above graph shows the level of outbreaks of the Australian plague locust in eastern Australia between 1934 and 2000. The years are divided into three seasons: summer, autumn and spring. Scale of the outbreak: 0 = very low populations; 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 of dense bands/swarms in the agricultural zone; 5 = major plague, over 500,000 hectares of dense bands/swarms in the agricultural zone.

The scale is derived from an estimate of the area of land infested by locusts based on locust reports, principally by Casimir (1962), Farrow (1977), Key (1938), Magor (1970), Wright (1987) and APLC Locust Bulletins (1977-2000).

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 ususally result in a population decline to background levels.

An outbreak cycle may involve exchange migrations between regions of summer and winter rainfall, the maintenance of high density populations, and the invasion of the 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 takes a longer period of time (3-4 years) for this species to develop into plague proportions compared to the Australian plague locust or migratory locust. If good follow-up rain (> 40mm) does not fall within six weeks after spur-throated locust eggs are laid, the vegetation will dry off and nymphal mortality will be high. But when more than 50 per cent of the nymphs receive follow-up rain, population increase will be high and there will be a plague after this occurs 3-4 years in a row.

Migratory Locust
This species is a major pest in many parts of the world but in Australia outbreaks are less frequent and 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) but a program of preventive control, initiated by the APLC and Queensland Department of Natural Resources in 1996, has kept populations below plague proportions.

Outbreaks of the migratory locust occur if good rain falls over two consecutive years, allowing 3-5 generations to develop in each year. Good winter rain is of particular 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 off vegetation. If the eggs are laid in dry soil and no follow-up rain occurs then high mortality results. In the Central Highlands of Queensland where migratory locusts are a frequent pest there is much 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 locust (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, hence the Australian Plague Locust Commission is 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. A major outbreak occurred in South Australia in 1998 and 1999 that required control by the State government.

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 500mm. 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 5 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, most recently during the summer and autumn of 2006. 

Further Reading
Baker, G. L. (1993). Locusts and grasshoppers of the Australian region. D9E. The field guides to the most serious pest locust and grasshopper pests of the world. The Orthopterists' Society Series of Field Guides.

Casimir, M. (1962). History of outbreaks of the Australian plague locust, Chortoicetes terminifera (Walk.), between 1933 and 1959 and analyses of the influence of rainfall in these outbreaks. Aust. J. Agric. Res. 13: 670-700.

Farrow, R. A. (1977). Origin and decline of the 1973 plague locust outbreak in Central Western New South Wales. Aust. J. Zool. 25: 455-89.

Hunter, D. M. and Elder, R. J. (1999). Rainfall sequences leading to population increases of Austracris guttulosa (Walker) (Orthoptera: Acrididae) in arid north-eastern Australia. Aust. J. Ent. 38: 204-218. (Reprint available from: aplc@daff.gov.au)

Key, K. H. L. (1938). The regional and seasonal incidence of grasshopper plagues in Australia. Bull. Coun. sci. ind. Res. Melb. No. 117.

Magor, J. I. (1970). Outbreaks of the Australian plague locust (Chortoicetes terminifera Walk.) in New South Wales during the period 1937-1962 particularly in relation to rainfall. Anti-Locust Mem. No. 11.

Wright, D. E. (1987). Analysis of the development of major plagues of the Australian plague locust Chortoicetes terminifera (Walker) using a simulation model. Aust. J. Ecol. 12: 423-437.