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.
|Region||Common armyworm expected pupation date||Southern armyworm expected pupation date|
|Dubbo/Griffiths NSW||Sep 19th||Sep 14th|
|Parkes NSW||Sep 22nd||Sep 22nd|
|Wagga Wagga NSW||Sep 27th||Sep 20th|
|Deniliquin NSW||Sep 23rd||Sep 17th|
|Swan Hill VIC||Sep 19th||Sep 14th|
|Yarrawonga VIC||Sep 28th||Sep 21st|
|Horsham VIC||Sep 29th||Sep 18th|
|Elmore VIC||Sep 30th||Sep 22nd|
|Hamilton VIC||Sep 27th||Sep 11th|
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.
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)