Native armyworms are examples of migratory moth species that are well adapted to breed on native and introduced grasses in production and more arid inland regions.
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Rainfall events can trigger breeding of these armyworm populations, which can quickly build in numbers on new plant growth.
When adults of these populations take flight, armyworms can seem to appear in crops virtually overnight as a result of air currents carrying moths into a region.
Three key species that can occur during the south-eastern winter cropping season are the common armyworm (Mythimna convecta), southern armyworm (Persectania ewingii) and inland armyworm (Persectania dyscrita).
At first glance one caterpillar can look much the same as another, however armyworms have several morphological features that can aid in a correct identification.
They have four sets of abdominal and one set of anal 'prolegs' - the fleshy appendages that stick out from the middle and at the end of the body respectively that allow caterpillars to crawl.
Some caterpillars only have one set of abdominal prolegs and one set of anal prolegs, and in this case it would be referred to as a looper.
Taking a good look just behind the head of the common, southern and inland armyworms will reveal three light coloured, parallel stripes on an apparent collar, which often run down the length of their back.
Perhaps a good way to remember these stripes is to think of armyworms as being 'in the military'.
While some caterpillars are noticeably hairy, armyworm are quite 'naked', with a smooth appearance and little in the way of bristles that could be viewed by eye.
It is not critical to distinguish between common, southern and inland armyworms as they have similar crop feeding preferences and management tactics.
Wheat, oats and barley are all crop hosts of these armyworm species, with barley being particularly susceptible to damage if mature caterpillars are present in spring, due to a relatively thin stem and the potential for lopping of flower heads.
As host crops approach the ripening stage detecting an armyworm infestation is important in order to make a decision about whether active control is necessary or not.
However, just the presence of armyworm in the lead up to ripening is not enough to merit chemical control - taking note of the size (larval life stage) of caterpillars is important in order to judge risk to crops.
Armyworm caterpillars will start out as very small larvae, only 2-3 mm in length, while mature caterpillars will reach approximately 40 mm, with patterning features such as the longitudinal stripes becoming clearer as the caterpillars age.
The final larval stages are most ravenous and tend to result in the most vegetative tissue loss, although crops can compensate for a certain amount of defoliation before grain yield loss is a risk.
At this time of year, the presence of medium to large armyworm is unlikely to result in head lopping in crops such as barley as these caterpillars will pupate before the crop enters the high-risk ripening period.
If very small caterpillars (larger than 3 mm) or egg batches are found in the crop or near the crop (they would be very hard to see) it will be important to correctly identify them as they mature and monitor numbers as plants approach senescence.
As barley begins to dry out, reductions in green leaf tissue can lead to overcrowding of armyworm caterpillars, and it is at this point in the plant growth cycle that larger caterpillars may begin targeting stems, which can then lead to loss of heads.
For more information on armyworm, and for threshold advice, visit PestNotes Southern: http://www.cesaraustralia.com/sustainable-agriculture/pestnotes/insect/armyworm
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