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Maturing cereals threatened by armyworm

Maturing barley is most susceptible to head lopping as cereal crops finish. 

 

Sightings of armyworm in maturing barley, wheat, and oat crops in various cropping regions in Victoria and NSW have been reported. The population sizes are not consistent between paddocks. In some crops, populations are light and have not warranted action, while some paddocks are hosting up to 30 armyworm per m2. Common armyworm (Leucania convecta) is the likely suspect as this species is more commonly found in spring and summer. Other species that are common in south-eastern Australia, including the southern armyworm (Persectania ewingii) and inland armyworm (Persectania dyscrita), are more prevalent in autumn and winter, although their activity can sometimes extend into spring. 

Armyworm larvae grow up to 40 mm in length, and are identifiable by the three parallel body stripes, mostly visible on the collar behind the head and usually continuing down the length of the body. The three species of armyworm commonly found in southern Australia all have these distinguishing stripes. This feature can be used to distinguish armyworm from other caterpillar species in cereals such as herringbone caterpillars (Proteuxoa spp.), cutworms (Agrotis spp.), and corn earworm (Helicoverpa armigera).

Armyworm have three visible stripes on the collar behind the head (source: cesar)

 

Understanding risk to crops

Maturing cereal crops are at risk of armyworm dama­­ge until the last green matter has dried out. Barley plants are most at risk because of their relatively slender stem; as crops finish caterpillars can chew through the last part of palatable green material remaining just below the head, severing it right off. While there are no signs of head lopping yet this season, crops are being monitored closely as damage can occur rapidly with one large caterpillar capable of lopping several heads in a day (or night). Wheat, triticale and oats are less susceptible to losing their heads because of the nature of their seed heads or panicle, however all cereals are at risk of chewing damage to grain and foliage. Rice, grass pastures and grass pasture seed crops are also susceptible to armyworm damage. The rate of feeding damage will increase substantially as larvae progress towards maturity.

Our advice

When cereals and pasture seed crops are approaching ripening this spring, early detection of armyworm is important. The most critical time is 3-4 weeks prior to harvest. Assessing the number of armyworm can be difficult because their movements vary with weather conditions and feeding preferences. Larvae are most active at night, seeking shelter under debris and vegetation on the ground during the day. Armyworm larvae can sometimes be seen during the day feeding on leaves and stems of crops, but research has shown that larger, more mature larvae are mostly nocturnal. This means that monitoring with a sweep net or beat sheet during the day is not the best way to assess the number and size of larvae within a crop. Instead, we recommend incorporating a direct ground search, along with sweep net (preferably in the evening) or beat sheet monitoring. Watch a quick demo on ground searching with QDAF entomologist Dr Melina Miles in the video below.

Demonstration of a ground search for armyworm (source: The Beatsheet)

 

Current thresholds to guide decision making are, 2–3 large larvae/m2 for barley, and 10 larvae/m2 for wheat and oats. There are several insecticides registered to control armyworm including various synthetic pyrethroids. Be sure to check the withholding periods if spraying is warranted in crops that are close to harvest. There are ‘softer’ options such as Bacillus thuringiensis (Bt), although these are only suited for quite small grubs. As armyworm tend to shelter at ground level during the day, good spray penetration in the crop canopy is necessary for effective control.

There are a number beneficial natural enemies that attack armyworm caterpillars, and will assist in supressing populations in spring. These include parasitoid wasps, spiders, carabid beetles, and predatory shield-bugs. 

 

Armyworm captured by a spider in a cereal crop (source: Rohan Brill)

 

 Field reports of armyworm

Rohan Brill – NSW DPI (Central West Slopes & Plains, NSW)

David Crowley – Delta Agribusiness (South West Slopes, NSW)

Bronwyn Hunt – Merriwa Pastoral Company (Mallee, VIC)

Emma Robinson – Landmark (Riverina, NSW)

Bob Ronald – Landmark (Riverina, NSW/North East, VIC)

Vanessa Warren – Delta Agribusiness (Riverina, NSW)

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