Armyworm caterpillars eat the leaves of cane. Bare leaf midribs are all that remain after heavy attack. A single stripping of leaves in spring or early summer rarely causes any crop loss. A wave if attacks in December-February can cause crop loss. Severe attacks weaken shoots under stress, and give weeds a chance. In summer day-feeding armyworms infest well grown cane. In autumn and spring, night feeding armyworms can infest young plant can and ratoon shoots. Armyworm infestations occur only now and then. Trash-blanketed ratoon fields are often attacked. Where trash blanketing has only recently been tried damage seems to be worse than in fields where the practise has been ongoing. This may be because predators and diseases of armyworms have built up with longer-term trash-blanketing.
Night-feeding armyworms hide in rolled spindle leaves or under leaf litter during the day, and feed mostly at night. Caterpillars of sugar cane, common and northern armyworm occur in similar areas and are hard to tell apart. They range from pale olive-green to red/brown with stripes of white, red-brown and black running down the body. The head is a mottled tan without any white markings. Fully grown caterpillars are over 40mm long. Pupae form under trash or 1cm underground in small chambers
Young caterpillars are bright green with tan heads. When about 20mm long, they have dark green, almost black stripes down the body. On the front of the head is an upside down Y-shaped white line. Fully grown caterpillars are 25-35mm long. Caterpillars move about during daylight and feed on leaves. Pupae are found in small chambers just under the soil surface. The moth is small, about 14mm long, and has dark-coloured forewings with small white lines in the central area. The hindwings are pale with a light pink tint and a dark edge.
In a field, there is usually only one major infestation per season. Sometimes a second wave of attack can follow the first, but this is rare.
Between December and February large numbers start off in grass, weedy canefields, lawns, headlands and creek banks, before moving into cane. It takes 4 weeks to go from egg to caterpillar to moth in mid-summer.
Spraying armyworms is not usually done. Each field will normally receive only one major infestation in a year. One attack on a healthy crop early in spring will not cause yield reduction. Most of the damage has already been done before spraying is done anyway. Early spraying of the first wave of armyworms also kills parasites and reduces the natural build-up of diseases. A series of attacks in early to mid-summer, particularly in damaged, weedy or weak crops may cause enough leaf loss to warrant spraying. Always identify caterpillars found under the trash before deciding to spray.