Spur-throated Locust

Austracris guttulosa

Family: Acrididae

Order: Orthoptera

Other Names:

Locusts

Summary:

Spur-throated locusts are larger than most other species and have a spur, or throat-peg, between the front legs. The hind legs bear 2 rows of dark tipped white spines.

Description:

Adults

Colour - Adults are pale brown with white stripes and dark markings on the thorax.

Body - Adult males are 50-65 mm long. Females are 70-80 mm long. The body has white and dark markings. The front segment of thorax has a strong shield (pronotum) with a spur. The body is slender compared to most other species.

Wings - Has wings and flies. The hind-wings are colourless or have a bluish tinge. The front wings are spotted.

Legs - Strong rear legs for jumping. The shanks of the hind legs are straw or mauve coloured and have two rows of dark-tipped white spines.

Mouthparts - Chewing

Nymphs

Young nymphs are bright green. Older nymphs have a distinct pale or dark stripe down the middle of their back and often change in colour from green to straw coloured. Nymphs also have a spur like the adults.

Eggs - In pods of up to 130 eggs

Key Characters:

Nymphs and adults have a conspicuous spur between the front legs.

Nymphs are green on hatching, developing a black stripe down the middle of their back. As they mature, colour may change to light brown with a white stripe.

Adults are up to 50-80 mm in length with slim pale brown bodies and a long white stripe.

It displays a strong darting flight that ends with the locust plunging into the grass.

Hind wings are clear or with a slight blue tinge. Family characters

End section of leg (Tarsi) has 3 segments.

Abdomen has 11 segments. Spiracles are on segments 1 to 8.

Antennae are less than half the length of the body.

Antennae have 7 or more segments.

Biology:

Locusts can eat their own weight of green food each day.

They tend to gather to mate and lay eggs.

Life Cycle:

Spur throated locusts only live for 10-12 months, from the autumn of one year until the following summer. They have little ability to survive extended dry spells.

Eggs laid in moist soil take 20 to 30 days to develop with temperatures around 30-35°C. At 22°C it can take up to 62 days. Eggs laid in dry soil enter a quiescent (resting) state and do not hatch until further rain falls. The eggs are not able to stay in quiescence for prolonged periods and many die after a month with no moisture. An immature locust is called a nymph or hopper. Nymphs emerge from summer to autumn and go through six to eight nymphal growth stages called instars, moulting at each stage. Early instar nymphs require suitable green plant material to survive and develop into adults.

From the third instar onwards nymphs are quite large and can damage crops.

Nymphs are usually present from December--March, but if hatching is late, nymphs can be present well into autumn.

It takes 10 weeks to reach the fledgling adult stage. The developing wings become more noticeable at each stage until the locust becomes a fledgling adult and then an immature adult capable of sustained flight.

The colour of nymphs is variable. Early instars tend to be green, with variations in later instars of yellow and straw through to grey. Like the adults, later instars tend to have a distinct pale or dark stripe down the middle of their back.

Fledging (development of nymphs into winged adults) usually occurs during March--April but can occur as early as January when eggs have been laid early. Newly fledged adults enter a sexual diapause stage, where reproductive development is delayed. During winter these adults tend to move in to timbered areas, forming very dense roosting swarms. They may move down to feed on the vegetation below the trees during the warmer part of the day. The overwintering state is maintained until increasing photoperiod and temperatures trigger migration behaviour. Sexual diapause is broken after spring rainfall and it takes 15 days for male and 20 days for female adults to reach sexual maturity (this may be longer in cooler temperatures).

If spring conditions are favourable egg maturation inside female locusts begins and eggs are laid from late October. This is typically seven to nine months after fledging and usually lasts for two to three months. If there is little spring rainfall, the adult locusts may migrate or delay egg laying until there is rainfall which can be as late as February or March. The females die after laying eggs. The eggs are laid in up to four pods or batches with up to 130 eggs in each pod. The pods tend to be laid near short grass over a wide area rather than in egg beds on bare soil like the Australian plague locust

At normal summer temperatures (28-33 °C), the minimum life cycle is: egg (18 days) ==> hopper (65 days) ==> laying adult (220 days).

Habitats:

Nymphs do not form high density bands although they can be found in high densities in preferred grasses and herbage. Young nymphs are most common in short grasses and ephemeral plants while older nymphs are found in taller vegetation

Migration

Sexually mature spur-throated locusts can undertake nocturnal, wind-assisted, long distance migrations from between October and January, often associated with the passage of tropical depressions. Migrations of 700-1000km have been reported. These migrations can result in major redistributions of population.

During the autumn and winter period, fledging, young adults form swarms that migrate in daylight over shorter distances (up to 20km per day). This flight activity may result in only a short migration or populations may move substantial distances during the autumn and winter months. Swarms may occur from April to September

The preferred food is green grass but any green material will be consumed

Origin and History:

Native to Australia.

In WA plagues have occurred in 1981/82, 1990/91 and 1995. Prior to this, various plagues have occurred but the particular species involved was not certain.

Distribution:

All states.

They are mainly a tropical species but can move large distances to impact on southern agricultural areas.

The major breeding areas of the spur-throated locust are in the Gulf region of northern Australia in monsoonal tropical grasslands, which includes the northern regions of Western Australia. The climate of the northern region is characterised by a distinct summer wet season and a winter dry season, with breeding extending into southern areas only after successive seasons of summer rainfall.

Their inability to survive extended dry spells limits their distribution to wetter or monsoonal areas.

Distribution of Spur-throated locusts.

Significance:

Beneficial:

It is an Australian native insect.

Detrimental:

Nymphs and adults are voracious eaters. Generally, nymphal densities of 20/m² or greater will pose an economic threat to crops. Adults, 1-3/m², have caused economic damage to sorghum and sunflowers.

Nymphs and adults feed on green vegetation. Nymphs can damage crops in autumn but during autumn-winter, crops are most at risk from fledging adults. The amount of crop damage will depend on numbers of locusts that are present in the vicinity of the crop and on migrations into crops. Adults will feed on wheat and canola as well as various types of trees including natives, ornamentals, citrus and fruit. Damage in spring has been recorded in ripening wheat crops.

The amount of damage the spur-throated locusts causes to crops depends on temperature, as these locusts are less active in cool conditions, however frosts will not cause mortality. Spur-throated locusts are known to feed on crops when temperatures are at or above 15°C; it is also likely that they will be able to feed on crops when temperatures are below 15°C if there is sufficient sunshine.

Body parts of locust may contaminate harvested grain causing downgrading.

Legislation:

It is a declared pest in Queensland.

Management and Control:

In most years control is not economical. When plagues occur, concerted campaigns are mounted involving spraying of egg beds, hoppers, bands and swarms.

Hoppers may be controlled by a number of insecticides very effectively, however flying or swarming locusts can devastate pastures and crops very quickly and randomly which makes control very difficult.

Egg beds should be treated each week with insecticide when young locusts are hatching before they spread.

Control

Nymph control is generally not economically viable except in and around high-value crops. The nymphs do not band and are generally quite scattered, at densities less than 20/m². Nymphs generally move into crop from adjoining pasture or native habitat. Effective nymph control will only be achieved if the crop and adjoining pasture or native habitat is treated. If the adjoining pasture or native habitat is not controlled, reinvasion can occur within days. Lower rates of registered insecticides listed below have efficacy on nymphs and these can be applied using a ground-rig. There are reports that synthetic pyrethroids applied at the highest registered rate will control early instar nymphs.

Adult locusts in medium (11-50/m²) to high (>50/m²) density swarms pose a significant economic threat. The most effective control is achieved by aerial spraying of swarms that are roosting in trees. Spraying crops and pastures with contact insecticides using a ground-rig will provide very limited control of spur-throated locust adults, as adults are very good fliers and are able to escape from ground-rig applications. Spraying with residual insecticides may provide better control but this relies on adult locusts coming into contact with lethal doses of insecticides from the crops they are invading and, therefore, may not prevent crop damage.

A number of pesticides are registered for the control of spur-throated locust. If a pesticide is registered for use in Western Australia on a crop for the control of a pest, an agricultural user may use the pesticide on that crop for the control of any pest provided withholding periods and all other limitations are observed.

Adult Locusts surviving for more than 3 weeks in hot dry weather will probably not lay eggs and will often die without spraying.

Thresholds:

5 adults per square metre are usually worth spraying in pastures.

In cereal crops, spray as soon as damage or head lopping is seen.

Related Species:

Similar Species:

Australian plague locust

Crickets

Grass hoppers

Migratory locust

Sandgropers

Spur-throated locusts are larger than most other species and have a spur, or throat-peg, between the front legs.

Wingless Grasshoppers usually have a light coloured stripe down each side of the body. The wings are usually poorly developed and only occasional individuals can fly. The hind wings never have a black tip or blotch.

Yellow-winged locust

References:

CSIRO. The Insects of Australia. Melbourne University Press. (1991) p70, 71(development diagram), 105, 107, 108, 371(diagram), 377, 391, 392(diagram).

WADA. Insects and Allied Pests of Extensive Farming. Department of Agriculture - Western Australia Bulletin No. 4185. p59.

Farmnote 82/2000: Australian Plague Locusts: broad-scale control

Australian Plague Locust Commission provided some of this information.

Micic, S. (2015) DAFWA entomology pers comm.

Acknowledgments:

Collated by HerbiGuide. Phone 08 98444064 or www.herbiguide.com.au for more information.