Armyworms

Armyworms have been found by agronomist, Brett Aitken (Landmark), attacking a wheat crop west of Donald, in the Wimmera district of Victoria. They have attacked approximately 5% of the plants across several paddocks, with damage ranging from occasional chew marks to plants being completely chewed off at ground level. Brett says the affected areas will be sprayed to prevent further plant losses. Agronomist, Mike Laidlaw (Harberger Farm Supplies), has also observed armyworms near Donald, causing damage to a barley and vetch crop. The caterpillars were approximately 20 mm in length and were found cutting off barley plants at ground level over the whole paddock however the vetch plants were unaffected. Alpha-cypermethrin is likely to be applied to prevent further damage.

Armyworms owe their name to the observation that larvae sometimes ‘march’ in large numbers away from crowded sites and areas depleted of food. Larvae may attack cereal crops and grass pastures, and are usually most damaging toward the end of spring when crops are close to harvest. However, in some seasons a winter generation occurs which can cause significant damage to young crops. Larvae climb up plant stems and extensively chew the leaves, with larger larvae able to completely defoliate plants or cut through stems. The later spring generation of armyworms can cause severe damage to ripening cereals, with barley crops most susceptible. The larvae chew through the last remaining green part of the plant stems (just below the head), causing the heads to drop off. When crops are at this stage even relatively low numbers of caterpillars can cause significant losses.

There are three armyworm species commonly found in southern Australia: the common armyworm (Leucania convecta), the southern armyworm (Persectania ewingii) and the inland armyworm (Persectania dyscrita). They are difficult to distinguish apart, however correct species identification in the field is not critical as their habits, type of damage and control are similar. All species have three parallel white stripes running from the ‘collar’ behind the head, along the body to the tail end, making them easily distinguishable from cutworms and native budworm. Adult moths are grey-brown in colour and have a stout body with a wingspan of approximately 40 mm.

Armyworms can be difficult to detect as they sometimes shelter on the ground beneath dead leaf material, particularly during the day. At other times they can be easily found on the stems or heads of plants. The first sign of armyworms is often the presence of frass (droppings) at the base of plants. Treatment (if warranted) should be carried out in late afternoon or early evening, as many species are nocturnal feeders.