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Common, northern and sugarcane armyworms

  • Photo of common armyworm larva
    Photo of common armyworm larva
  • Northern armyworm larvae.
    Northern armyworm larvae.

General information

The common armyworm (Leucania convecta) is important in southern Queensland where it attacks winter cereals, particularly barley and oats, in September and October. Larvae appear in plague proportions in some years, and are patchy in others. Conditions leading to outbreaks are not yet fully understood. Leafy cereal plants can tolerate considerable feeding from moderate numbers of armyworms. Control in the vegetative stage is seldom warranted unless large numbers of armyworms are well distributed throughout the crop and are destroying young seedlings or the younger two or three leaves from older plants.

Scientific name

Leucania convecta - Common armyworm.

Leucania separata - Northern armyworm.

Leucania stenographa - Sugarcane armyworm.

Description

Common armyworm: First-instar larvae are about 1 mm long. From the second instar, stripes develop along the top and sides of the larva and become more distinct as the larva grows. Crowded larvae are usually darker than those uncrowded. The mature larva grows up to 40 mm in length and has three characteristic pale stripes on the head, collar (segment behind the head) and tail segment. They are smooth-bodied with no distinct hairs. The body of the larva also has lateral stripes. The forewings of the moth have a wingspan of about 40 mm and are fawn or buff coloured.

Northern armyworm: Larvae and adults are very similar to the common armyworm.

Sugarcane armyworm: Moths have pale forewings with a dark line running the length of the forewing.

Similar species

The adults of the common and northern armyworms may be confused. Genitalia dissections by a specialist are required to separate the species. The larval stages likely to be encountered in cereals are all similar in appearance.

Distribution

Common armyworm: Native Australian species, recorded in New South Wales, Queensland, South Australia, Tasmania, Victoria and Western Australia.

Northern armyworm: Throughout South-East Asia, New Zealand and in Australia, where it occurs in all states except Tasmania.

Sugarcane armyworm: Recorded from Asia and Australia, where it occurs throughout drier parts of the mainland and as an occasional vagrant to Tasmania.

Crops attacked

Common armyworm: Damages barley, oats, wheat, native pasture grasses and perennial grass seed crops.

Northern armyworm: In Queensland, recorded as damaging sorghum, maize, barley, wheat and rice.

Sugarcane armyworm: A minor pest, occasionally damaging some WA grain crops.

Life cycle

Common armyworms have three generations per year. The winter and spring generations damage cereals. Moths fly into cereal crops and lay their eggs in the folds of dried or drying leaves on grasses or cereals. Females lay up to 1000 eggs in irregularly-shaped masses, cemented in tight folds of foliage. Eggs hatch in as little as 3-4 days after laying and young larvae, with the assistance of wind, disperse through the crop on fine silken threads. The larvae feed on leaves and stems. Larvae usually develop through six instars but sometimes seven. Indicative development times at constant temperature are: egg-laying to hatch, 7 days at 20ºC and 2.5 days at 30ºC; larval stages (including pre-pupal stage) 34.2 days at 20ºC and 17.2 days at 30ºC. Larvae pupate in the soil. The pupal stage lasts 20.1 days at 20ºC and 10.1 days at 30ºC. Development time from neonate to adult emergence is 61 days at 20ºC and 41 days at 30ºC (Smith 1984).

Northern and sugarcane armyworms - similar to common armyworm.

Risk period and damage

Risk period: The greatest risk to cereals is spring. Moth flights occur in September and October, and the later-stage larvae damage cereals often in the weeks prior to harvest. The mature larval stages of the winter generation will sometimes march in cereal crops in late winter and cause serious damage to crops, particularly on the edges of paddocks. Crops directly seeded into standing stubbles are susceptible to severe defoliation during the vegetative stage as the winter generation matures.

Damage: There are two distinct periods for economic damage. The first, defoliation during early vegetative development, is less common than the second through ripening. Ripening barley is most susceptible to armyworm damage because the last part of the head to dry off is the green tissue just below the head. Mature larvae feed on that area and thereby sever the head of the cereal which falls to the ground. One larva can lop many heads very quickly causing large grain losses. Oats are also damaged but the less compact seed head means less damage. In northern Australia, wheat can also be damaged, but in the south the wheat head stays green later and armyworms feed along the heads and damage grain rather than excise the whole head.

Monitoring and action level

Large numbers of armyworm moths are attracted to farm lights on warm nights in September and October. This provides the first warning of potential problems in cereals. Armyworm larvae are difficult to find in cereals crops as they hide at the base of plants or under clods of soil during the day. Search at the base of plants and under clods of soil to estimate the number of larvae per square metre. The presence of green-yellow pellet-shaped droppings of the larvae on the ground are usually a reliable sign of larvae. Monitor for larvae at dusk with a sweep net; sweep netting during the day can be unreliable.

Two larvae square metre for barley. Other cereals are likely to tolerate slightly higher numbers.

Control

Chemical control: A range of insecticides is registered for armyworm control in cereals. Insecticides should target larvae 10 to 20 mm long. Larvae larger than 20 mm long can be difficult to kill and may require higher rates of insecticide. If possible spray late in the day as larvae are active at night. See Pest Genie or APVMA for current control options.

Cultural control: Windrowed or swathed crops dry out rapidly, rendering them unattractive to the feeding of armyworm larvae. They are also less susceptible to wind damage (head shattering).

Natural enemies

Armyworm larvae are attacked by a number of parasitoids that may be important in reducing the intensity of outbreaks. However, when armyworms are in numbers likely to cause damage, parasitoids are unlikely to give timely control. Predators include green carab beetles, populations of which increase dramatically in inland Australia in response to abundant noctuid larvae induced by favourable seasons. Other predators include the predatory shield bugs and perhaps common brown earwigs. Fungal diseases are recorded as causing mortality of armyworm.

Further information

  • Pests of Field Crops and Pastures: Identification and Control. Editor: P.T. Bailey