Armyworms

Agronomist, David Strahorn (Furneys CRT), has reported armyworm caterpillars in a barley crop near Gilgandra, in the Central West Slopes and Plains district of New South Wales. The crop is approaching head emergence and the larvae are small to medium in size, approximately 10-20mm long. David says the caterpillars have caused some leaf feeding damage. Armyworms have also been reported between Moree and Goondawindi, in the North West Slopes and Plains district of New South Wales. In one barley crop, very high densities of caterpillars of up to 30-50 per m² have been observed.

It is probable that armyworms will pose a serious issue for a number of growers this year. Reports from southern Queensland to South Australia indicate that armyworms are already damaging crops across a wide area. Armyworm moths are among the better-known migratory species that can invade crops in large numbers and lay eggs in late winter and early spring. Barley is most susceptible, but armyworms also attack wheat, oats, triticale and grass pastures.

Armyworms have the capacity to lop heads, particularly in barley, after the crop has turned. As the crop turns the barley stems dry down. However, the nodes take longer to dry, staying green for longer, providing a focal point for the larger caterpillars. In most other cereals, high densities of armyworms can severely defoliate plants before they turn, causing yield penalties if much of the upper foliage is removed. Occasional head lopping of wheat and oats will also occur but economic impact is less common as they prefer weaker, secondary tillers of wheat or single spikelets of oats.

Timing of armyworm infestations is critical. Before deciding on control options, researcher, Garry McDonald (cesar), says growers should assess the size of larvae and the remaining time till harvest. Armyworms usually have 6 stages (instars) before pupating in the soil. When armyworms infest crops early in the season the caterpillars will likely complete their development stages by the time the crop has starting to dry down and become susceptible to head lopping. In these instances there may be less need to apply insecticides, although the crop should be closely monitored.

Growers are advised to begin monitoring cereal crops (particularly barley) for signs of armyworms. Caterpillars are most active at night, although they can sometimes be seen during the day feeding on the leaves and stems. Look for caterpillars on plants and on the ground, or signs of caterpillar droppings, damage to plant heads or evidence of leaf scalloping. Late in the day, when the larvae are becoming active, use a sweep net to assess if armyworms are present in the crop. Infestations are often patchy, so check a number of sites across the field. The most important time is 3-4 weeks prior to harvest for barley, although all cereal and pasture seed crops should be monitored during tillering.

It is essential to recognise the problem early and be prepared to spray when economic damage is imminent. A barley crop can be decimated by armyworms in just a few days. Whilst large larvae (30-40 mm) do most of the head lopping, controlling smaller larvae that are still leaf feeding may be more achievable. There are several chemicals that 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. If insecticides are required, it is recommended that applications be carried out in the late afternoon or early evening when caterpillars are most active.

Armyworms are sometimes confused with other crop caterpillars, but are most easily recognised by their smooth bodies, three distinct white bands that always start at the ‘collar’ behind their heads and mostly run down their bodies, and their otherwise green-brown colour. Click here for images of armyworms.