Sizing up armyworm

Armyworm larvae can cause economic damage to cereals during autumn and winter – but they are also of concern in maturing cereals as they graze cereal heads, and (at their worst) lop barley tops right off when larger larvae chew through the last green material left on the plant stem, just below the head (wheat stems are thicker than barley stems and are less susceptible to head-lopping).

Armyworm larvae have been reported in cereals in several areas of south-eastern Australia (and northern regions), and in some cases, damage has been reportedly heavy and required intervention.

But one of the biggest questions is, will current armyworm populations still be around in cereals near maturity in south-eastern Australia?

What’s in a name?

Armyworm is a catch-all common name for several crop and pasture moth larvae spanning temperate, sub-tropical and tropical species.

In south-eastern Australian winter cereals, armyworm populations are likely to be native species; either the common armyworm (Leucania convecta), the southern armyworm (Persectania ewingii) or the inland armyworm (Persectania dyscrita).

At the larval stage, these species are difficult to tell apart and correct species identification in the field is generally not critical because their habits, type of damage and control are similar.

Forecasting armyworm development

Fortunately, we have a model that can simulate the development times of the common armyworm and the southern armyworm using 15-year averages of temperature data.

Using this model, we have constructed a table of estimated dates of when you could expect current 4th instar populations to stop eating crops and pupate in key cereal growing regions of southern NSW and Victoria (see below).

Armyworm go through 5-6 larval stages (‘instars’) before pupating in a cocoon and emerging as a moth.

You can estimate the life-stage of armyworm larvae with a ruler and by following the size guide in the table.

RegionCommon armyworm expected pupation dateSouthern armyworm expected pupation date
Dubbo/Griffiths NSWSep 19thSep 14th
Parkes NSWSep 22ndSep 22nd
Wagga Wagga NSWSep 27thSep 20th
Deniliquin NSWSep 23rdSep 17th
Swan Hill VICSep 19thSep 14th
Yarrawonga VICSep 28thSep 21st
Horsham VICSep 29thSep 18th
Elmore VICSep 30thSep 22nd
Hamilton VICSep 27thSep 11th
Predicted pupation dates of the common armyworm and southern armyworm at 4th instar on Monday 5th August 2019, based on 15-year averages of temperature data for locations in southern NSW and Victoria. 

Management considerations

Remember, these armyworm species are highly migratory. When current populations reach maturity, they are more likely to fly away rather than stick around and generate more grubs in the same location.

If armyworm continue to be reported in crops as we move into spring, we will provide further updates on the predicted development windows in future articles. 

Don’t forget that correct management starts with correct identification. There are other moth larvae species about at the moment, including herringbone caterpillars.

When in doubt, get in contact with us for assistance.

For further information on armyworm, including identification, thresholds and management information, visit our comprehensive PestNote.


Field observations

Rohan Brill, NSW DPI (Riverina NSW)

Josh Buerckner, @buerckner_josh via twitter (Riverina NSW)

Chris Dunn, Landmark (Northern Country VIC)

Aaron Hutchison, Grower (Riverina NSW)

Hayden Lunn, Landmark (Riverina NSW)

Luke Maher, AGRIvision (Mallee VIC)

Simon Mock, Clovercrest Consulting (Wimmera VIC)

Cover image: Photo by Julia Severi, Cesar Australia

What is Pestfacts south-eastern?

PestFacts south-eastern keeps growers and advisers informed about invertebrate pests and beneficials in broadacre crops and pastures during the winter-cropping season in Victoria and southern New South Wales.


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Since 2019 PestFacts south-eastern has been running through IPMforGrains: Best Practice Insect Pest Management, a project delivered by the National Pest Information Network (Cesar Australia, DPIRD, QDAF, NSW DPI, and SARDI). This project aims to provide grain growers and advisors with information on invertebrate grain pest occurrence and equip industry with the knowledge needed to implement integrated pest management practices. This initiative is a GRDC investment and includes in-kind contributions from all project partner organisations.

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