Terms and Descriptions used in the Locust Bulletin

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About the Locust Bulletin

The Australian Plague Locust Commission (APLC) produces a monthly Locust Bulletin during times of locust activity (spring, summer and autumn). The Bulletin reports the current and previous month's locust situation and weather events of potential significance to locust developments. The Bulletin also gives an outlook of likely developments in the following three months for Australian plague locust - Chortoicetes terminifera, spur-throated locust - Austracris guttulosa, and the migratory locust - Locusta migratoria.

The terms used in the Bulletin to describe the biology, behaviour and population density have defined meanings to increase accuracy and usefulness. The forecasts are dependent on current locust population distribution information, and weather and environmental conditions. Risk statements address both the probability and the potential consequences of an occurrence.

The majority of information documented in the Bulletin comes from regular surveys by the APLC. Additional information comes from landholders and the public, state primary industries departments and biosecurity agencies.

Locust population densities

A characteristic of locusts is gregarious behaviour and the formation of high density population units known as Bands of nymphs and Swarms of adults. Where higher densities occur, a large proportion of the regional population is concentrated in small areas occupied by these units and lower densities occur elsewhere. Therefore the high densities cannot be extrapolated to the area of an entire habitat area or region. Typically a range of density classes is found within surveyed regions with higher densities where habitat conditions are favourable. Where specific terms are used for density classes, the word is capitalised (Scattered or Concentration), while more general regional density descriptions (such as low or high) apply to a range of specific density categories.

The following terms are used to describe different density levels

Density classes for nymphs and adults

Nymph Densities

Number per m2

 

Present (P)

1 - 5

 

Numerous (Num)

6 - 30

 

Sub-band (SB)

31 - 80

 

Band (B)

> 80

 
 
 

Adult Densities

Number per m2

Number per hectare

Isolated (Iso)

- 0.02

< 200

Scattered (Scat)

0.03 - 0.1

>200 - 1000

Numerous (Num)

0.2 - 0.5

>1000 - 5000

Concentration (Conc)

0.6 - 3.0

>5000 - 30,000

Low Density Swarm (LDS)

4.0 - 10

>30,000 - 100,000

Medium Density Swarm (MDS)

11 - 50

>100,000 - 500,000

High Density Swarm (HDS)

> 50

>500,000

General regional density classes

Regional Nymphal density

Survey densities

very low, occasional

Nil-Present

low

Present-Numerous

medium

Numerous-Subband

high

Bands

 

Regional Adult Density

Survey densities

very low, occasional

Nil-Isolated

low

Isolated-Scattered

medium

Scattered-Numerous

high

Concentration-Swarm

Bands

A Band is a gregariously behaving group of nymphs at a density greater than 80 locust nymphs per square metre. Bands have well defined fronts and nymphs "march" in the same general direction.
Three parameters are used to define bands of locust nymphs:

  • The size of a band as indicated by the length of the band front.
  • The infestation level which is measured by the length of bands per square km2 kilometre in a given area (most often a paddock).
  • The number of bands in a specified area (e.g. the Hillston-Hay area).
Term Band length (m) Term km band front/km2 Term No. of bands


Band Size

Infestation level Number of bands in a specified area
very small<10Light0 - 0.5

One or two

1 - 2
small10 - 100Medium0.6 - 2A few3 - 10
medium101 - 1000Heavy>2A number11 - 50
large1001 - 5000

Many, a large number>50
very large> 5000



Swarms

Swarms are gregariously behaving groups of adult locusts flying together as a unit at a density greater than four individuals/m2.

The size and number of swarms in a specified area are referred to when detailing an infestation.

Term Swarm area (km2) Term Number
Swarm Size Number of swarms
very small<1few, some3 - 5
small1 - 2a number6 - 10
medium3 - 10many11 - 50
large11 - 20very many, a large number, numerous>50
very large>20

Other terms mentioned in the Locust Bulletin

Other terms used in the Bulletin to refer to locust biology and behaviour, rainfall and forecast probabilities. These are defined below.

TermRain falling in one week (mm)
Rainfall
Light0 - < 20
Moderate20 - 40
Heavy41 - 100
Flood

>100

Forecast probabilities

TermChance that event will occur (%)
Unlikely, low probability<30
May, moderate probability30-70
Likely, high probability>70

Locust Biology and Behaviour

Term

Definition

Adult

A fully winged, mature locust capable of breeding and migrating.

Day flight

Short distance (up to 50 km/day) daytime migratory movement of gregariously behaving locusts, generally at low altitude (0 – 300 m), resulting in redistribution of a population.

Development

General term covering all stages of the locust life cycle from egg to adult.

Diapausing
egg

An over-wintering egg that does not develop for several months. Diapause ends by mid June and development resumes as soon as conditions are favourable.

Early instar

First and second instar.

Egg bed

An area containing more than 10 egg pods per square metre.

Emigrants

Locusts leaving an area by migration.

Fledgling

Newly moulted, soft-bodied adult incapable of sustained flight. This stage lasts approximately 5 days.

Gravid

Females with mature eggs of 4-5 mm length.

Immigrants

Locusts which fly into an area.

Instar

Stage of nymphal development separated by a moult. Australian plague and migratory locusts have five nymphal instars while the spur-throated locusts have 6-8 instars.

Late instar

Fourth and fifth instars.

Laying

Females depositing eggs into the ground in egg pods containing up to 50 eggs for Australian plague and migratory locusts and 120 for spur-throated locusts.

Mid instar

Third instar locusts.

Migration

Nocturnal, wind-assisted flight of locusts usually at higher altitudes (up to 1200 m), resulting in population displacement up to several hundred kilometres overnight.

Nymph

Immature locust without wings (though wing buds may be visible) and is therefore unable to fly. This stage in the locust life cycle follows hatching, lasts approximately five weeks and is often referred to as the hopper stage.

Over-wintering eggs

Eggs in an arrested state of development (diapause or quiescence).

Over-wintering nymphs

Nymphs resulting from an autumn egg laying may develop to third instar and persist through winter in that stage. Development resumes in spring.

Over-wintering adults

Locusts that become adult in late autumn but do not mature and develop eggs until early spring.

Quiescent egg

An egg in which development has been arrested by the onset of dry conditions and which will resume development when sufficient rain falls.

Target

An area of band or swarm density locusts at least 1km2 in size.

Test drilling

Female locusts bore into the ground with their abdomens to test the soil but do not lay eggs.

Forecasting districts referred to in the Locust Bulletin

Map of forecast regions with main areas of potential locust habitat shown in yellow

map of bulletin forecasting districts

General regions referred to in the Bulletin


Map showing forecast regions with main areas of potential locust habitat shown in yellow 

Locust Bulletin Subscription

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If you would like to be on the mailing list to receive printed copies of Locust Bulletins, please send your name and postal address to APLC by email or alternatively, provide them by post to the:

Forecasting & Information Officer
Australian Plague Locust Commission
Australian Government Department of Agriculture and Water Resources
GPO Box 858
Canberra ACT 2601

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