Army worms on the march

By Frank Mickan, pasture and fodder conservation specialist, DEPI, Ellinbank Centre

ARMY worms are on the march and you need to stop them in their tracks if they are a problem on your farm.

Reports have been coming in from dairy farmers in West and South Gippsland of army worm

infestations in pasture crops waiting to be cut for hay. One farmer has estimated that his standing hay crop has been reduced by up to 60 per cent.

Farmers in locations around Victoria who have traditionally had trouble from armyworm caterpillars (or larvae) in the distant past should now consider inspecting their pastures to pick up early signs of their presence.

Concentrate on the longer pastures first. The younger instars or stage of growth (there are six instars as caterpillars) chew the leaves and may leave scalloped edges.

Cereal croppers are not immune as army worms can also affect grain crops. Wheat, barley and oat growers should also check their crops.

Heads on the ground are often the first visible sign of army worm caterpillars in cereal crops

although chewed leaves, heads and awns could be apparent. As barley matures and green leaves disappear, part of the stem below the grain head often remains green and palatable. This is the next target for the caterpillar as grubs chew through this section causing the heads to fall to the ground.

Another sign are green, yellow or brown cylindrical-shaped faecal droppings, about one to two millimetres long.

These are found on the ground between the crop rows or caught on the lower leaves.

In some cases when the food supply (pastures/cereals) becomes severely depleted, the

caterpillars start to gather together and they will “march” out of crops and pastures in search of food – hence the name “army worm”.

there are three species of army worm: common, southern and inland army worms found in southern Australia but for all intents and purposes, they look the same as

caterpillars and all do similar damage.

Army worm caterpillars are smooth-bodied and have three white or creamy-coloured stripes running down their back and sides. Caterpillars can grow to about 30–40mm in length.

A similar looking caterpillar is the brown or common cutworm but it has no obvious stripes and are uniformly brown, pink or black. These are not a problem currently.

As adults, armyworms become moths with 30–40mm wingspan which swarm on warm, humid evenings.

It is only as moths that the three species can be identified, which can be a problem as they do their worst damage as caterpillars.

During the day, look under dead leaf litter at the base of the crop or pasture sward and under clods in cereal crops. At dusk or night, a torch should show them up on the leaves and grain heads.

Early detection is essential, particularly when cereals and pasture seed or hay crops are at the late ripening stage.

To get an accurate estimate of caterpillar numbers, considerable effort is required but the potential cost saving is worthwhile. Sample by using a sweep-net or a bucket, or by visually

ground or crop searching for caterpillars.

The sweep-net/bucket method gives a quick and approximate estimate of problem size. Sweep several times across the crop in 180 degree arcs, preferably about 100 times, at different sites within the crop to give an indication of density and spread.

Army worms are most active at night so sweeping at dusk will be the most effective time. If average catch is more than five to 10 per 100 sweeps then do some ground counts to determine approximate densities.

For ground sampling, do at least 10 “spot checks” in the crop and count the number of caterpillars within one square metre.

Young caterpillars (up to 8mm) cause very little damage and are hard to find. This is why many dairy and cropping farmers fail to detect armyworm activity until they are nearly fully grown and damage may be as high as 10-20 per cent by then.

Once they are on the march, the earlier you can detect them, the greater your chance of minimising damage.

Some Gippsland farmers have elected to aerial spray for armyworms, covering both affected

pastures and pastures in their march path.

There are a number of chemicals registered for control of armyworms. For dairy pastures and standing hay crops, consider chemical treatment if damage is obvious.

Also consider the following points:

· timing of harvest;

· green matter available in the crop;

· expected return on the crop; and

· caterpillar development stage (if most are greater than 35-40mm or pupating, it may not be

worth spraying).

If spraying is necessary, it is recommended that this be carried out in late afternoon or early

evening for maximum effect, as armyworms are nocturnal feeders. Familiarise yourself with the chemical’s potency, application and withholding periods.

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Posted by brad on Dec 23 2013. Filed under Rural News. You can follow any responses to this entry through the RSS 2.0. You can leave a response or trackback to this entry