Armyworms

Agronomist, Matthew Shephard (IMAG Consulting), has reported armyworms attacking canola crops in the Forbes-West Wyalong region of southern NSW. Matthew says 2-3 caterpillars were observed feeding on young canola plants. Typical feeding damage of ‘scallop’ marks on the leaf edges was noticeable in many areas of the affected crops.

Armyworms are caterpillar pests of pastures and cereal crops. Armyworms mostly feed on leaves, but under certain circumstances will feed on the seed stem, resulting in plant head loss. The change in feeding habit is caused by depletion of green leaf material or crowding. Matthew says the outbreaks were relatively localized, with the affected paddocks having a history of significant armyworm pressure last spring. It is suspected that these armyworms are a carryover from then.

Armyworm caterpillars may be confused with loppers, budworm larvae, cabbage moth larvae and cutworms. Mature armyworm caterpillars are 30-40 mm long. They have three prominent white or cream stripes running down the back and sides of their bodies. These are most obvious where they start on the thoracic segment (‘collar’) immediately behind the head. They have no obvious hairs, are smooth to touch and curl-up when disturbed.

There are 3 common species of armyworm found in southern Australia: the common armyworm (Mythimna convecta); the southern armyworm (Persectania ewingii); and the inland armyworm (Persectania dyscrita). For accurate identification, they must be reared through to the adult (moth) stage.

Given the timing, it is likely that these are either the southern or inland armyworm, which can be found in autumn and winter throughout the grasslands and crops of southeastern Australia. These species have 2 generations per year. Most eggs are laid either in autumn, often immediately after rain, or in late winter. The young caterpillars cause little damage while feeding on leaf surfaces. The older larvae can defoliate the plant.

During the day, search under clods of soil and at the base of plants. Look for faecal pellets (resembling small green-brown hay bales) on the ground. Thresholds should be used in light of proximity to harvest, available green matter, expected crop return and stage of larval development. Sweep sampling is not reliable for armyworm; it is best to rely on ground searches.

Matthew reports that the affected paddocks did require spraying. A synthetic pyrethroid was employed and achieved adequate control. Native parasites can exercise good control and spraying is not normally required every year. Several wasp parasites including Apanteles ruficrus have been released to increase biological control. Windrowing may be an alternative to spraying.