Defeating the Armyworm

Source: Saskatchewan Agriculture and Food

To spray or not to spray: that is the question. Whether it is nobler to suffer the slings and arrows of outrageous armyworm infestation, or to risk seeing tight margins eroded by insecticide costs: that is the question facing farmers after an army of insects cut a swath across much of the province this year.

Dale Risula, Integrated Cropping Management Systems Specialist with Saskatchewan Agriculture and Food (SAF), said this year has been a particularly bad year for the Bertha armyworms.

“Historically, if you take a look at Bertha armyworm, it only seems to be a pest, on average, two or three out of 20 years,” he said. “The cycle seems to be dependant on a number of factors – more than just weather, perhaps also the number of acres that are seeded to the host crops where they are attracted to lay their eggs.”

Risula said the timing of the life cycle of the crop and the life cycle of the Bertha armyworm timed out in early August to result in significant damage.

“The worms were in their later stages of development and the crops had lost their leaf material, so the only thing left for them to chew on was the pods. If they start doing damage to the pod, that is a direct impact on the yield,” he said.

“Contrast that to the damage that they would have done to leaf material earlier, that would be less of an impact on the yield than them doing damage to the pod itself,” Risula added.

“If [the armyworms] are there in sufficient numbers – meaning a certain economic threshold – then they will do significant damage, so control measures have to be taken in order to deal with that.”

Aye, there’s the rub.

Given the tight margins in farming, producers have to make a judgement call about whether the cost of spray will outweigh the damage from the insect.

“I think it is important that, if they decide to spray, it be based on good economical threshold information,” said Risula.

According to SAF, if there are under 10 larvae per square metre in a field, it might not be worth spraying.

A chart to help determine the economic threshold for spraying, and more detailed information on the Bertha armyworm, is available at www.agr.gov.sk.ca.

Risula noted there are other options to help control populations for the coming crop year.

“Crop diversification and rotation is probably the most important thing you can do in terms of lessening the potential impact of the moth laying its eggs on suitable host crops the following year,” he said. “Not growing canola every year is a good thing, or, if farmers do grow canola every year, at least space the fields far apart. That, in itself, won’t deal with the problem completely, but it will at least help.”

Risula said another option is to manipulate the timing of seeding.

“You want to avoid anything that flowers at the time when the moth usually emerges, sometime near the end of June,” he noted. “If the plants are flowering, that seems to be a greater attractant to the egg-laying capabilities of the moth. They are attracted to a flowering plant quite readily.”

Another option is crop selection, since cereal and pulse crops are less vulnerable to Bertha armyworms than canola.

“None of these options are 100-per-cent foolproof,” Risula stated. “It’s difficult to get around the outbreak of Bertha. Sometimes the only means left for producers to deal with them is through chemical action.”

More information on Bertha armyworm control can be obtained by calling the Agriculture Knowledge Centre at 1-866-457-2377.

For further information, contact:

Dale Risula
Integrated Cropping Management Systems Specialist
Saskatchewan Agriculture and Food
Agriculture Knowledge Centre
Phone: (306) 694-3714