Garren Knell (ConsultAg) reports finding up to 6 relatively large armyworm caterpillars on ryegrass plants within a lupin crop near Narrogin. This indicates that the moths have been in the area for many weeks. The caterpillar characteristics visible in photos submitted indicate that they are more likely to be a common armyworm species (Mythimna sp.) and not the new fall armyworm (Spodoptera frugiperda).
Growers are encouraged to monitor cereal crops for armyworm as they can defoliate plants during vegetative stages and damage heads (especially barley) later in the season during grain fill stages.
Armyworm caterpillars are fat and smooth and may be distinguished by the three parallel white stripes on the collar just behind the head. The first visible sign of armyworm caterpillars is often their green to straw-coloured droppings, about the size of a match head, found on the ground between the cereal rows.
Armyworm caterpillars are most damaging in barley crops close to harvest. 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. Armyworms are seldom a serious problem in pastures.
Assessing the numbers of armyworm in a cereal crop can be difficult, as their movements will 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.
Growers should be mindful to distinguish armyworm caterpillars from native budworm caterpillars given the incidence of budworm in cereal crops this season and from the new fall armyworm.
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 likely 10 grubs per square metre of crop. For more armyworm threshold information refer to DPIRD’s Management of armyworm in cereal crops page.
A number of effective insecticides are registered for the control of armyworm if required (see DPIRD’s 2020 Winter Spring Insecticide Guide). However, their effectiveness is often dependent on good penetration into the crop. This can sometimes 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 armyworms are predominately night feeders. Spray withholding periods also need to be observed.
For further information on armyworm refer to DPIRD’s Diagnosing armyworm and Management of armyworm in cereal crops pages.
For more information contact Alan Lord, Technical Officer, South Perth on +61 (0)8 9368 3758 or Svetlana Micic, Research Scientist, Albany on +61 (0)8 9892 8591.
Article authors: Cindy Webster (DPIRD Narrogin) and Dustin Severtson (DPIRD Northam).