PestFacts WA

Armyworm update

  • Moora
  • Narrogin
An armyworm caterpillar
A common type of armyworm, Mythimna convecta, on barley. Photo courtesy of DPIRD.

David Cameron (Farmanco) has reported finding armyworm caterpillars in cereal crops around the Moora area over the past month. He noted that generally they were only causing low levels of damage.

An armyworm caterpillar
An armyworm caterpillar. Photo courtesy of Trent Butcher (ConsultAg).

Trent Butcher (ConsultAg) reported recently that armyworm have caused considerable damage to an oat crop south of Narrogin. The larvae were large and few could be found indicating that many of them have likely pupated in the soil already after consuming their full larval diet.

Armyworm caterpillars are often unpredictable as the moths migrate and fly into crops at night and lay eggs during winter and spring. The caterpillars are fat and smooth and may be distinguished by the three parallel white stripes on the collar just behind the head.

Growers need to be mindful of correctly distinguishing armyworm caterpillars from the recently introduced fall armyworm caterpillar and from native budworm caterpillars.

Armyworm caterpillars are most damaging in barley crops close to harvest so monitor crops now. When barley crops are maturing in spring, large armyworm caterpillars climb plants and can chew through the stems, causing the heads to fall to the ground. Damage to wheat and oat crops occurs less frequently and is usually minor compared to damage in barley because the stems are thicker and leaf defoliation does not usually result in yield loss.

The economic level for spraying armyworm in mature barley is about three large armyworm grubs per square metre of crop. The threshold for wheat or oats is much higher as only grains are consumed and heads are very rarely dropped. Spray thresholds in these crops are more like 10 grubs per square metre of crop. If applying insecticide be mindful of harvest chemical withholding periods (WHP) and to check chemical labels before spraying.

A number of effective insecticides are registered for the control of armyworm if required (see DPIRD’s 2022 Winter Spring Insecticide Guide).

To read about previous armyworm activity reported this season refer to the 2022 PestFacts WA Issue 13 article Identifying caterpillars in cereal crops.

For further information on armyworm, refer to DPIRD’s Diagnosing armyworm and Management of armyworm in cereal crops pages.

For more information, contact Technical Officer Alan Lord, South Perth on +61 (0)8 9368 3758 or Research Scientist Svetlana Micic, Albany on +61 (0)8 9892 8591.



Article authors: Cindy Webster (DPIRD Narrogin) and Dustin Severtson (DPIRD Northam).