Recognising armyworm in crops
The first visible sign of armyworm caterpillars is often their green to straw-coloured droppings, about the size of a match head, found on the ground between the cereal rows.
Damage to weeds, especially their preference for ryegrass, is also a sign of their presence.
In barley crops
In barley crops, chewed leaves, heads and awns can be apparent, although the first sign is usually barley heads on the ground. As barley matures, part of the stem often remains green and appetising after other parts have dried. The caterpillars chew through this part causing the heads to fall to the ground.
In oat crops
In oat crops, the caterpillars bite off pieces of the panicle, causing grain to fall.
In wheat crops
Wheat crops are less frequently attacked and usually minor damage is caused as compared to damage in barley crops.
Where to look for armyworm
Caterpillars of various sizes up to 4cm long may be seen in the crop. They may be on the plant or under leaf litter on the ground.
Description
The caterpillars hatch from batches of eggs laid in crevices, such as under the sheathing at the base of leaves. Caterpillars undergo a series of moults before reaching their full size of about 40mm long. They are fat and smooth and may be distinguished by the three parallel white stripes on the collar just behind the head.
When mature, the armyworm caterpillars burrow in soil to form pupae.
Adult moths emerge from the pupae.
Four species of armyworm occur in barley growing areas. They are distinguishable in the moth stage; all are stout-bodied, light brown to grey moths with a length of about 30mm. Along the south coast, the most abundant species is the common armyworm (Leucania convecta).
Other similar species armyworm can be confused with
Cutworm
Cutworms are also smooth caterpillars but are usually only pests of cereals at the seedling stage and appear at a different time of the year. They do not climb and sever heads from the stems.
Helicoverpa puntifera
Another caterpillar, Helicoverpa puntifera, which is a closely related species to the native budworm, feeds only on individual grains and is not usually as damaging in barley as the armyworm.
Helicoverpa puntifera may be recognised by its rough skin sparsely covered with bumps and bristles.
Lifecycle of armyworm
The speed of development depends on temperature. In winter it may take several months for eggs to develop into moths. A generation may be completed in six weeks in warmer weather when crops are maturing.
Occasionally, armyworm moths move in large flights in search of food, but usually a damaging population will have bred within the paddock or local area. Caterpillars which have bred in high densities may be almost black, compared with the usual lighter shades.
Monitoring
Assessing the numbers of armyworm in a cereal crop can be difficult, as their movements will vary with weather conditions and feeding preference. Sometimes they are found sheltering on the ground and under leaf litter, whilst on other days they will be high up on the plants or on the heads and easily picked up using sweepnets.
If ryegrass is present in the crop, they often prefer to feed on that, until it runs out.
Armyworm caterpillars may be confined to only small portions of a crop. Several different locations within the crop should be checked for caterpillar numbers before deciding on control measures.
A suggested monitoring procedure is:
- Look for signs of caterpillar droppings and damaged ryegrass heads (if present).
- Look for damage to the foliage of the crop.
- Look for caterpillars on the plants and on the ground after shaking the plants and searching the leaf litter between rows.
- If caterpillars are present, check frequently for the first signs of head-lopping in barley.
If you can't find caterpillars in spring, it can be two or three weeks before a population of damaging-sized caterpillars could develop, so check again in at least two weeks time.
Thresholds
In barley
Late maturing barley crops need to have three large caterpillars or more per square metre, before insecticide spraying is considered.
If the barley crop is commencing to hay-off, continue checking daily until fully mature, as caterpiller head-lopping can be unpredictable. Seven large heads or 14 small heads lopped per square metre justify the cost of treatment. This assumes an on-farm return of $150 per tonne and a control cost of $11 per hectare.
In oats and wheat
The threshold for wheat or oats is much higher as only grains are consumed and heads are very rarely dropped. The threshold of oats will be the same as for wheat depending on the price of grain.
Laboratory trials have shown that the yield loss in wheat per hectare per grub per square metre equals 5.4 kilograms per hectare. Given a wheat price of $180 per tonne and the cost of insecticide and its application of $10 per hectare, the threshold number of grubs, above which spraying is warranted on economic grounds, is approximately 10 per square metre.
Control
On the south coast spraying is unnecessary in most years, as natural events can control the pest or allow the crop to mature without damage. The presence of large larvae in spring should not prompt treatment automatically.
The most serious situation is the presence of many large caterpillars coinciding with the maturing of the crop. Usually, little damage occurs in the leafy stages, but it is advisable to check crops regularly after the flag leaf appears.
Weather is the most important factor determining the size and stage of the pest population. Outbreaks appear in spring, following successful preceding generations. The weather is also important when the crop is maturing, as an extended ripening allows the pest more time to develop and damage the crop.
Biological control agents can be important in some years. These include parasitic flies and wasps, predatory beetles and diseases.
Chemical control may finally be necessary as the crop begins to ripen. A number of effective synthetic pyrethroid insecticides are registered for the control of armyworm. However their effectiveness is often dependent on good penetration into the crop. This can sometimes be difficult to achieve in high-yielding thick canopy crops, especially when caterpillars are resting under leaf litter at the base of plants.
Monitor after spraying
Crops should be check after spraying to ensure that the application is effective. Consideration of insecticide withholding periods is important in late sprayed crops.
Acknowledgments
Yield loss data and supporting information was provided by past department entomologists Kevin Walden and Mike Grimm.