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) for Australian plague locust - Chortoicetes terminifera, spur-throated locust - Austracris guttulosa, and migratory locust - Locusta migratoria. The Bulletin reports the 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.
The terms used in the Bulletin to describe the biology, ecology and behaviour have defined meanings to increase accuracy and usefulness. The forecasts are dependent on current locust population structure and 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 used in APLC

Nymph Density

Number per m2

Present (P)

1 – 5

Numerous (Num)

6 – 30

Sub-Band (SB)

31 – 80

Band (B)

81 – 500

Dense Band (DB) > 500
 
Adult Density Category Number per m2 Number per 250 m2

Isolated (Iso)

< 0.02

< 5

Scattered (Scat)

0.024 – 0.1

5 – 25

Numerous (Num)

0.104 – 0.5

26 – 125

Concentration (Conc)

0.504 – 3

126 – 750

Low Density Swarm (LDS)

4 – 10

751 – 2,500

Medium Density Swarm (MDS)

11 – 50

2,501 – 12,500

High Density Swarm (HDS)

> 50

>12,500

General terms density classes in comparison with APLC definitions

General nymphal density

APLC report densities

very low, occasional

Nil – Present

low

Present – Numerous

medium

Numerous – Sub-band

high

Bands

 
General adult density APLC report densities

very low, occasional

Nil – Isolated

low

Isolated – Scattered

medium

Scattered – Numerous

high

Concentration – Swarm

Bands

A Band is a cohesive mass of nymphs that persists and moves as a unit. The Australian plague locust bands have well defined fronts and nymphs "march" in the same general direction.

Two 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 accumulated length of bands per square kilometre (km2) in a given area (most often a paddock).
 

Band Size (length of band front, metres)

Infestation Level (length totals of band front, km/km2)
very small <10 Light 0 – 0.5
small 10 – 100 Medium 0.6 – 2
medium 101 – 1000 Heavy >2
large 1001 – 5000    
very large > 5000    

Swarms

Swarms are gregariously behaving groups of adult locusts flying together as a cohesive unit. Swarms are usually described by their densities and size.

Swarm Size (area of swarm, km2) Note
very small <1  
small 1 – 2 minimum target size for aerial application (>MDS)
medium 3 – 10  
large 11 – 20 minimum infestation area for control campaign
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.

Weekly Rainfall Total (mm)
Light 0 – 25
Moderate 25 – 50
Heavy 50 – 100
Very Heavy

>100

Forecast probabilities

Chance that event will occur (%)
Unlikely, low probability <30
May, moderate probability 30 – 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 dispersal movement of gregariously behaving locusts, generally at low altitude (0 – 300 m), resulting in redistribution of a population.

Development

The progressive changes in shape, size and function from egg to adult.

Diapause egg

An over-wintering egg that suspends development for a period even under temporarily favourable conditions.

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 between 5th instar nymph and mature adult stages.

Gravid

Females with mature eggs of 4-5 mm length.

Immigrants

Locusts flying into an area by migration.

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, or slow development).

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 one km2 in size suitable for aerial application. A total of at least 10 km2 of treatable targets must be present in an area for APLC to consider commencing aerial control.

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 green

map of bulletin forecasting districts

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Last reviewed: 10 March 2021
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