Should you spray for armyworms?

Dilemma: should I spray for armyworms?

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Armyworms are on the march again, threatening to ravage pastures. But when should you spray and what with?

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MARAUDER: Armyworms on the march are identified by three stripes along their bodies.

MARAUDER: Armyworms on the march are identified by three stripes along their bodies.

Armyworms earn their name when they arrive seemingly overnight and march across paddocks, leaving devastation in their wake.

Outbreaks can affect both pastures and broadacre crops and Saputo agronomist Scott Travers has warned farmers to make daily paddock checks as infestations have begun to appear near Yarram.

In November 2017, armyworms wreaked havoc across Gippsland.

"They were climbing up fence posts, crossing laneways making it look like the ground was moving, they were so thick," he said.

"It was a disaster for a lot of people who assumed they had standing feed, only to discover all the leaf was gone - it was just stalks waving in the breeze."

"We can't afford to lose a lot of hectares again but there's a lot of standing grass around now and people might not realise just how many caterpillars are there until hay is taken off.

"Check daily and get any infestations sprayed straight away."

Cesar research, adoption and extension scientist Julia Severi said three types of armyworms affect pastures and crops in Australia's south but all could be managed the same way.

The grey-brown moths can travel long distances - perhaps hundreds of kilometres - to lay eggs, which hatch into larvae that grow 35-40 millimetres long before pupating to fly away again.

The relatively hairless caterpillars can be green, brown or yellow but all have three parallel white stripes running from the 'collar' behind the head to the tail.

Finding the caterpillars early can be difficult as they crawl around at night and mostly stay curled up at the base of the plant under clods of soil or in the plant crown during the day.

"They stay hidden during the day - that's why we recommend doing a ground search rather than doing sweeps with a net or trying to look at them on the foliage during the day," Ms Severi said.

Most damage is caused by the final larval stages and the rule of thumb for a problematic infestation varied.

"In crops like wheat and oats, we say eight to 10 per square metre because there's a low risk they will lop off the head but with something like barley, it's one to two grubs per metre squared because they can saw the head of barley straight off," she said.

"With armyworm in pastures, we have a rough rule of thumb that's similar to the 8-10/m2 for wheat.

"That's not an economic threshold, just a very basic kind of guideline."

Armyworms grow faster in warmer conditions but can be affected by fungal attack in humid weather.

They are normally controlled by birds, shield bugs, parasitoid wasps, spiders and other predators.

"There are a number of insecticides registered, which are the main tool for armyworms but it's also about maintaining your beneficial populations as much as possible," Ms Severi said.

"Try to avoid using broad-spectrum insecticides frequently that can impact them."

The options for control are Bacillus thuringiensis (Bt) and synthetic pyrethroids (SP).

Sumitomo Chemical Australia regional sales manager Barry Kerr said if farmers could see armyworms marching towards unaffected pastures, a "barrier spray" about 30 metres wide could be very cost-effective.

Bt products, like DiPel, also had the advantage of being gentle on beneficial insects, suitable for organic properties and had no withholding or stock exclusion period.

"Their Achilles heel is that (Bt products) hate sunlight and rain will wash them off," Mr Kerr said.

"To get the best bang for your buck, we always recommend applying it in the evening, so you get eight or nine hours of darkness.

"If there's a little bit of rain coming and you do it the next night after a shower, you always get a better result, too, because they feed harder on the sweeter grass.

"They won't die instantly.

"They will stop feeding immediately but they'll be alive for a couple of days.

"Ideally get them in one of the early instars - the bigger they are, the higher the rate you may need to use.

"Using the suspension concentrate formulation, you can go as low as a litre for small ones but if you've got big grubs, you might need four or five."

Bt products remained active for five or six days and while use on pasture was off-label, it was legal in Victoria.

Similar to the chemicals used in fly spray but much more concentrated, SPs were cheaper than Bt products and lasted longer.

Because SPs had longer withholding periods of up to 28 days, it was often useful to spray some paddocks with Bt products and others with SPs.

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