Lawn armyworm in pastures, lawns and field crops

Female armyworm moths mate the first night after emergence and, two days later, begin laying clusters of eggs covered with fine scales onto the under-surface of non-host vegetation and objects such as walls and under the eaves of buildings. Most eggs are deposited between dark and midnight with individual moths laying an average of 1700. Leaves such as citrus, eucalypt and papaya are favoured backyard oviposition sites. Egg masses occur on upper and lower surfaces of the leaves, usually within a few metres of the ground.

Individual eggs, about 0.5 mm in diameter, are shaped like a flattened globe and are light brown. Up to seven layers of eggs have been recorded in a single egg mass, but three to six layers are common. Although light tan when newly laid, egg colour deepens during embryo development. Shortly before hatching, an outline of the body and dark head capsule of the developing larva can be seen through the semi-transparent egg shell. Infertile eggs become metallic green with a pink spot at the apex.

The larvae pass through seven development stages or instars, each being larger than the previous one. A growth stage is completed by a moulting or shedding of the complete larval skin and head capsule to allow an increase in size to the next instar. Small larvae are initially cream coloured at emergence. They attain a greenish colour after feeding.

As the larvae pass through the various instars they develop thin longitudinal white stripes on the sides and top of the body and the head capsule changes from black to brown. Towards maturity, characteristic black triangular marks become prominent. In the final instar, these triangles are a dominant feature of larval colouration, being superimposed on a background of brownish dorsal and lateral bands.

Fully grown larvae up to 45 mm long and 7 mm wide have an inflated sausage-like appearance with a body width much greater than that of the head capsule. They possess three pairs of walking legs on the thorax and five pairs of fleshy ‘pro-legs’ on the abdomen. If disturbed, they curl into a tight spiral.

The larvae pupate in chambers they construct in the soil. They are deep brown and measure about 15 mm in length and 5 mm in width. They prefer sheltered feeding sites. Feeding starts immediately after hatching and continues at night until larval maturity.

• eaten out margins of leaves due to feeding of the older larvae
• faecal pellets around the base of the plant
• larvae often feed on the leaf blades leaving the midrib and giving the plant a tattered appearance.

Cultural control
The caterpillars feed only on the softer above-ground parts of grasses and the roots and growing points are undamaged. Therefore liberal applications of nitrogen fertiliser and water to lawns and pastures will often assist plant recovery after the armyworms have left or been controlled.

Biological control
The larvae are subject to fungal (Nomuraea rileyi), protozoan (Nosema sp.) and viral diseases, but these normally become widespread only when large populations of larvae occur and act too late to prevent damage by the pest. Larvae affected by nuclear polyhedrosis virus can be readily recognised. They hang in an inverted V from the plant and the skin is fragile, rupturing readily to release the body contents that have liquefied as a result of the disease. Disease and parasites usually ensure that there is only one generation during an outbreak.

There are a number of wasp and fly parasitoids. The wasps include the endoparasite Apanteles sp. that feeds inside the larvae and is seldom seen until the 10 to 30 wasp larvae emerge from the armyworm. They spin their small oblong fluffy white cocoons near the armyworm, often in a cluster. The ectoparasites attach and feed externally. The Ichneumonid wasps are the most common. They can often be seen flying in large numbers over lawns and pastures searching for caterpillars on which to lay eggs. The larvae of the wasp grow up to 10 mm when mature. Wasp cocoons are constructed of tightly woven black fibres and are oblong. They can be found in the cell under the soil constructed for pupation by the armyworm larvae.

The flies belong to the family Tachinidae and are endoparasites. The eggs are laid on the skin of the armyworm larva, and the maggot-like larvae feed internally. Usually only one of these develops to maturity, and the fly pupa is found within the armyworm pupa.

Predators include frogs, cane toads, birds, larger wasps and various species of sucking bugs.

An entomopathogenic nematode (EN) Steinenema carpocapsae is available commercially for use in turf in Australia. This species is an effective biological control agent against armyworm, when treatments are correctly administered. More information is available from the Ecogrow website.