Armyworm in cereals
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Boyatup
Quenten Knight (Agronomy Focus) recently reported finding very low numbers of armyworm caterpillars in an Illabo wheat crop near Boyatup. There was no feeding damage in the crop at that stage.
Armyworm caterpillars are most damaging in barley crops close to harvest, so growers are encouraged to monitor crops now. When barley crops are maturing in spring, large armyworm caterpillars climb plants and can chew through the green stems, causing the heads to lop off. Damage to wheat and oat crops occurs less frequently and is usually minor in comparison to barley because the stems are thicker and leaf defoliation does not usually result in yield loss.
The appearance of armyworm caterpillars in crops is unpredictable.
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.
The first visible sign of armyworm caterpillars is often their green or straw-coloured droppings, about the size of a match head, found on the ground between the cereal rows.
Assessing the number of armyworm caterpillars in a cereal crop can be difficult, as their movements vary with weather conditions and feeding preference. Sometimes they are found sheltering on the ground and under leaf litter whilst on other days they will be high up on the plants or on the heads, and easily picked up using sweep nets. Larger caterpillars often prefer to hide during the day and feed at night.
The economic threshold for armyworm in mature barley is about three large caterpillars per square metre of crop. The threshold for wheat or oats is much higher, at about 10 caterpillars per square meter of crop. This is because only grains are consumed and heads are very rarely dropped. If applying insecticide, be mindful of harvest chemical withholding periods and to check chemical labels before spraying.
A number of effective insecticides are registered for the control of armyworm if required (see the Department of Primary Industries and Regional Development’s 2023 Winter Spring Insecticide Guide). However, their effectiveness is dependent on good penetration into the crop. This can be difficult to achieve in high-yielding, thick canopy crops, especially when caterpillars are resting under leaf litter at the base of plants. Spraying late in the afternoon or evening is recommended as armyworm is predominately a night feeder.
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 in South Perth on +61 (0)8 9368 3758 or Research Scientist Svetlana Micic in Albany on +61 (0)8 9892 8591.
Article authors: Cindy Webster (DPIRD Narrogin) and Dustin Severtson (DPIRD Northam).
Article input: Bec Severtson (DPIRD Northam).