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 damage vegetable and orchard crops. The Australian plague locust is the most serious pest species in Australia due to the frequency of outbreaks (gregarious population increases) 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 benefit-cost ratio of approximately 8:1.
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 are estimated to consume 0.04 gm of green vegetation per day, averaged across all development stages. Average adult food intake has been estimated to be about 0.2 grams of green vegetation per day. With densities of adults in settled swarms ranging from about 4 to 50/m2, a swarm covering an area of 1km2 could consume over 1000 kg of green vegetation a day, depending on density.
Australian Plague Locust
Geographically extensive outbreaks were first reported in the 1870s and affected inland agricultural areas of New South Wales, South Australia and Victoria. 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 were less frequent, but in recent decades major outbreaks have occurred in 1999-2001, 2004, 2006, 2009 and 2012.
Graph of the level of outbreaks of the Australian plague locust in eastern Australia between 1934 and 2012.
The years are divided into three seasons: summer, autumn and spring.
Scale of the outbreaks: 0 - very few locusts
- background population, a few bands/swarms
- outbreak, localised bands/swarms in several areas
- major outbreaks, many bands/swarms, some dense
- plague, several hundred thousand hectares infested by bands/swarms in the agricultural zone
- major plague, over 500,000 hectares infested by bands/swarms in the agricultural zone
The scale for the graph above is taken 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 (1977-2012).
The Australian plague locust can reach plague proportions within a year if a sequence of widespread heavy rains occurs 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 in 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.
A plague of spur-throated locusts occurred 1973-75 and caused extensive damage to crops in Queensland. Control was also conducted on outbreaks in 1994-1997, 2000-2001 and 2010, 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 level.
This species is a major pest in many parts of the world but in Australia outbreaks have been infrequent and largely restricted to central Queensland. Early infestations are not well documented and outbreaks prior to the 1970s are unknown. Widespread clearing of scrub and summer cropping in central Queensland greatly expanded the area of favourable habitat for this species. A major outbreak occurred during the wet years 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).
Infestations of Other Species 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 these species have 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 infestations state authorities may assist landholders with the control of economically damaging populations.
Outbreaks of the small plague grasshopper have been recorded since 1843 and this species caused severe damage to cereal crops in South Australia during the nineteenth century. Infestations also occurred in Western Australia and western Victoria during the 1920s and 1930s. More recent infestations occurred in South Australia in 1997 and 2006 and swarms develop in early summer in other states in some years.
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, probably due to the expansion of improved pastures. Widespread regional infestations of wingless grasshoppers occurred in 1965-66, 1979-82, 1989-91 and 2006-07.
Outbreaks of the yellow-winged locust occurred in eastern Queensland in 1911-16, 1930-35, 1939-47 and 1961-62, while the only recorded outbreak of the eastern plague grasshopper was in the Singleton district, New South Wales, in 1906-7. Locally high numbers of both species are often recorded in parts of Queensland and New South Wales.
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.
Key, K. H. L. (1938) The regional and seasonal incidence of grasshopper plagues in Australia. CSIR Bulletin No. 117 (CSIR, Melbourne).
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 Memoir No. 11 (ALRC, London).
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.