Managing Insect Pests in Winter Cereals - Armyworms

Armyworm Moth

Armyworms

Armyworms can feed on the leaves of all winter cereals, particularly barley and oats, generally during August and October.

Leafy cereal plants can tolerate considerable feeding from moderate numbers of armyworms.

Control in the vegetative stage is seldom warranted unless large numbers of armyworms are well distributed throughout the crop or are moving in a ´front´, destroying young seedlings or completely stripping older plants of leaf.

Infestations are indicated by the eaten-out margins of leaves due to feeding of the older larvae and also by the faecal pellets around the plant base.

The most serious armyworm damage occurs in barley when the older larvae start feeding on the green stem just below the head as the crop matures.

Control is warranted if the armyworm population distributed throughout the crop is likely to cause the loss of 7-15 heads per square metre.

Characteristic armyworm damage in winter cereals

During the vegetative growth phase, plants can tolerate considerable leaf feeding. Leaves may look tattered from the eaten-out leaf margins. Faecal pellets around the base of plants are another indication of armyworm infestation. Armyworm generally do not require control during the vegetative stage.

The most serious armyworm damage in cereal crops occurs when larvae feed on the upper flag leaf and stem node as the crop matures. Larvae target the stem node as the leaves become dry and unpalatable, and the stem is often the last part of the plant to dry. Head cutting begins at this time.

One large larva can sever up to seven heads of barley a day. One larva a square metre can cause a loss of 70 kg/ha grain per day. A larva takes around 8-10 days to develop through the final, most damaging instars, so the crop is susceptible to maximum damage for this period.

Calculating an economic threshold

The table shows the value of yield loss incurred by 1 larva/square m per day, based on approximate current values for wheat and an estimated loss of 70 kg/ha per larva.

Based on these figures, and the relatively low cost of controlling armyworm, populations in ripening crops in excess of 1 large larva per square metre will warrant spraying.

Based on these figures, and the relatively low cost of controlling armyworm, populations in ripening crops in excess of 1 large larva per square metre will warrant spraying.

Monitoring and management strategy

For insecticide application to be economic, check or scout the crop and assess the problem before head cutting starts.

Check for larvae on the plant and in the soil litter under the plant. Late in the day, when the larvae are becoming active, use a sweep net (or swing a bucket through the crop) to make a quick assessment of whether armyworm larvae are present in the crop. Infestations are often patchy, so check a number of sites across the field.

Some judgements will need to be made about how quickly the larvae will reach damaging size and when this will occur in relation to the crop´s development. For example, if the crop is nearing full maturity/harvest, and the grubs are still small, then there is most likely no need to spray. Small larvae take 8-10 days to reach a size capable of head lopping.

The other extreme would be a late crop that is still very green and at early seed fill. In this case, any small larvae present will most likely reach their most damaging size in time to significantly reduce crop yield, and so a spray is more likely to be required.

If you are unable to monitor the crop on a regular (daily) schedule during the critical period of drying down, and armyworm are present, it may be better to spray just in case. This is not the preferred option, but provides peace of mind in a year where armyworm are abundant.

Early recognition

It is essential to recognise the problem early and be prepared to spray when economic damage is imminent. A cereal crop can be almost destroyed by armyworm in just a few days. Whilst large larvae do the head lopping, controlling smaller larvae that are still leaf feeding may be more achievable.

Control

Many chemicals will control armyworms. However their effectiveness is often dependent on good penetration into the crop to get contact with the caterpillars. Control may be more difficult in high-yielding thick canopy crops, particularly when larvae are resting under leaf litter at the base of plants. As larvae are most active at night, spraying in the afternoon or evening may produce the best results.

If applying sprays close to harvest, be aware of relevant Withholding Periods. Always read the label.

Biological control agents may be important in some years. These include parasitic flies and wasps, predatory beetles and diseases.