Armyworm in winter cereals

armywormWhile individual armyworm species can be difficult to tell apart, they are usually grouped together for management decisions, as their activity, damage and control methods are similar.

  • Common armyworm Leucania convecta
  • Northern armyworm Mythimna separata
  • Lawn armyworm Spodoptera mauritia
Pre-emergence to flowering Grain development and Ripening
Insects and damage Armyworm prefer lush growth that provides good cover and protection. They feed on leaf tissue – leaf margins have tattered/chewed/scalloped appearance; in extreme cases whole leaves may be severed at the stem.

Caterpillars produce green/straw coloured droppings (size of match head). These are visible between the rows. Bare patches adjacent to barley fields or damage to weeds may indicate armyworm presence before it is evident in crops.

In oats, pieces of panicles can be bitten off causing grain to fall to the ground.

Armyworm are most damaging in barley close to harvest. The larvae chew through stems below the head causing heads to drop.

Monitoring
  • Monitor crops regularly at least fortnightly at vegetative stage; increase to weekly if larvae are detected as the crop nears maturity. Note: armyworm are often unevenly distributed or confined to small portions of the crop
  • Use a sweep net, beat tray, or beat sheet. Ground searches during the day are necessary to detect larvae sheltering under leaf litter and soil. As larvae mature they will pupate in the ground under loose soil. Use a trowel to examine the soil between rows.
  • Traps, containing lures (10% port, 15% raw sugar and 75% water), can be used to indicate armyworm presence in the region
  • Check regularly for larvae and the first signs of head lopping. Watch for level of defoliation of flag and flag-1 etc. that may impact grain fill.
  • In late spring, small larvae take 10-14 days to reach a size capable of head lopping. To determine if crop will be susceptible (dry, except for green nodes) when larvae reach a damaging size use the day degree estimates (see the Beatsheet blog) for armyworm development.
Beneficials Around 20 species of predators and parasitoids have been recorded attacking armyworm. Frequently observed predators include predatory shield bugs, ladybeetles, carabid beetles, lacewings and common brown earwigs. Parasitoids include tachnid flies and a number of wasp species (e.g. Netelia, Lissopimpla, Campoletis). Viral and fungal diseases can cause armyworm mortality; such outbreaks are more common at high armyworm densities.
Cultural control
  • Control weeds to remove alternative hosts. Armyworm often feed on ryegrass before moving into cereal crops.
  • Standing stubble from previous crops, dead leaves on crops and grassy weeds are suitable sites for female armyworm to lay eggs.

Larvae can move into cereals if adjacent pastures are chemically fallowed, spray topped or cultivated in spring. Monitor for at least a week post treatment. Damage is generally confined to crop margins.

Windrow or dessicate barley as an alternative to spraying for armyworm if rapid drying conditions are forecast.
Thresholds
  • Barley: 2-3 large armyworm/m2 of crop (based on ground and plant sampling). 1 head of barley/m2 equals 10 kg grain/ha. 1 larvae/m2 can cause a loss of 70 kg/ha grain/day.
  • Wheat and oats: Economic threshold estimated at 10 grubs/m2 (higher than barley because heads are rarely lopped).
Pesticides
  • Effectiveness requires good coverage to get contact with the caterpillars. Control is more difficult in high-yielding crops with thick canopies where larvae are resting under leaf litter at the base of plants.
  • Armyworm are active at night – spray late afternoon or early evening to maximum likelihood of contact.
  • Be aware of withholding periods when chemical control is used close to harvest.
Multi-pest considerations Control of armyworm in early crop development can have off-target impact on other cereal pests e.g. helicoverpa, aphids and mites. Exposure to synthetic pyrethroids in these pests will contribute to selection pressure for SP resistance. This is a consideration for insecticide use and selection, particularly in the northern region where H. armigera is prevalent and pests may move from winter crops to subsequent summer crops. In the southern and western regions, SP use in winter cereals for armyworm may hasten or exacerbate SP resistance in earth mite species.
Communication Good communication and sharing information with neighbours through area wide management may provide first indication of army worm presence

Industry publications provide up to date information about regional pest issues

Further information

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