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Armyworm

Mythimna convecta, Mythimna loreyimima, Persectania ewingii, Persectania dyscrita

 

 


Armyworm
Agriculture Western Australia


Description
There are four species of armyworm in Western Australia - the common armyworm, the southern armyworm, the inland armyworm and the sugarcane armyworm. Of these, the common armyworm is the most damaging. Moths are stout bodied, grey to cream, with a wingspan of about 40 mm. They fly at night and are strongly attracted to lights. Armyworm caterpillars vary in colour depending on their numbers in a crop. If there are many caterpillars in a crop then they become dark while if the population is low, the caterpillars are much paler. Armyworms are about 40 mm long when fully grown and can be distinguished from cutworms and budworms by their large heads and by three prominent white stripes on the "collar" behind the head. Often the armyworms themselves will not be seen as they feed mainly at night but their droppings, which look like small green "square" hay bales, will be obvious on the ground below the crop canopy. Another indicator that armyworms are present is damage to ryegrass seed heads.

 


Armyworm moth
Agriculture Western Australia


Life cycle
Armyworms have three or four generations per year and on the south coast they survive over summer on self sown cereals and grasses that germinate with summer rains. In spring it takes about three weeks from when the eggs are laid for them to hatch and for the caterpillars to reach head lopping size. Once damage begins many heads can be lopped in a short time.
 
Damage
Armyworms are regular pests of coarse grain crops on the south coast and occasional pests in inland areas. Towards the end of spring, when crops are approaching maturity, large armyworm caterpillars chew through the stem of barley just below the head, causing the head to fall to the ground. If in large numbers earlier in the season, they may eat leaves and be confused with cutworm.
 


Parasitic wasp larvae emerging from armyworm
Agriculture Western Australia


Control
Heatwaves may kill most of the caterpillars. Native parasites can exercise good control and spraying is not normally required every year. Several wasp parasites including Apanteles ruficrus have been released to increase biological control.
 

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