Thresholds
In barley
Late maturing barley crops need to have three large caterpillars or more per square metre, before insecticide spraying is considered.
If the barley crop is commencing to hay-off, continue checking daily until fully mature, as caterpiller head-lopping can be unpredictable. Seven large heads or 14 small heads lopped per square metre justify the cost of treatment. This assumes an on-farm return of $150 per tonne and a control cost of $11 per hectare.
In oats and wheat
The threshold for wheat or oats is much higher as only grains are consumed and heads are very rarely dropped. The threshold of oats will be the same as for wheat depending on the price of grain.
Laboratory trials have shown that the yield loss in wheat per hectare per grub per square metre equals 5.4 kilograms per hectare. Given a wheat price of $180 per tonne and the cost of insecticide and its application of $10 per hectare, the threshold number of grubs, above which spraying is warranted on economic grounds, is approximately 10 per square metre.
Control
On the south coast spraying is unnecessary in most years, as natural events can control the pest or allow the crop to mature without damage. The presence of large larvae in spring should not prompt treatment automatically.
The most serious situation is the presence of many large caterpillars coinciding with the maturing of the crop. Usually, little damage occurs in the leafy stages, but it is advisable to check crops regularly after the flag leaf appears.
Weather is the most important factor determining the size and stage of the pest population. Outbreaks appear in spring, following successful preceding generations. The weather is also important when the crop is maturing, as an extended ripening allows the pest more time to develop and damage the crop.
Biological control agents can be important in some years. These include parasitic flies and wasps, predatory beetles and diseases.
Chemical control may finally be necessary as the crop begins to ripen. A number of effective synthetic pyrethroid insecticides are registered for the control of armyworm. 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.
Monitor after spraying
Crops should be check after spraying to ensure that the application is effective. Consideration of insecticide withholding periods is important in late sprayed crops.
Acknowledgments
Yield loss data and supporting information was provided by past department entomologists Kevin Walden and Mike Grimm.