Armyworm, Bertha (Mamestra configurata Walker), Lepidoptera

Armyworm, Bertha biology & life cycle

The Bertha armyworm overwinters as a pupae and adults emerge from mid-June until August. Females oviposit on the underside of the leaves and larva hatch and disperse within one week. The larva are not highly mobile. As such, they will only migrate from a field when the food supply is short or the crop is over-ripe. In September the fully grown larvae enter the soil to pupate. There is one generation per year.

Identifying Armyworm, Bertha

Armyworm, Bertha: eggs

Eggs are the size of a pinhead and white in colour when laid. The eggs darken as the larvae develop inside.

Armyworm, Bertha: larvae

Young larvae (3mm) are pale green in colour, while older larvae may remain pale green, but most are brown to black with yellow stripes running the length of the body. Fully grown larvae are 38 mm long an resemble a black caterpillar. The larva molt 5-6 times. Young larvae chew irregular holes in the lower leaves but cause little damage. Mature larvae cause the most damage by defoliatio and seed pod consumption. The 2 last molting stages are the most detrimental to the crop as 80% of the eaten plant tissue is consumed at this time. Crop losses are most severe when there are few leaves on the plant. From a distance, the crop can look white as the pest has eaten off all of the outer layer of the stems and pods.

Armyworm, Bertha: Pupae

5-18 mm long, reddish brown in colour.

Armyworm, Bertha: adult

25 mm long with a wingspan of 40mm. The forewing is mostly grey and marked with patches of black, brown, white and pale green scales.

Armyworm, Bertha: management

Sampled at three locations 50 meters apart. Beat the plants to dislodge larvae and count the number per square meter.