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Dealing with soybean aphids and cutworms

Hot, dry summer weather can bring soybean aphids into your fields

Soybean aphids tend to come along with hot, dry weather, says Cassandra Tkachuk, production specialist for Manitoba Pulse and Soybean Growers (MPSG). “Moist conditions mean fungal pathogens can help take down populations of soybean aphid. Hot, dry conditions are conducive to them reproducing rapidly.” While this year’s harvest weather wasn’t hot and dry in most of the soybean-growing areas, the sunny summer weather may have brought aphids.

John Gavloski, Manitoba’s provincial entomologist, has been tracking soybean aphid levels since 2001, and the province saw infestations at economic threshold levels only in four years — and never back-to-back.

“A year with economic problems hasn’t been followed by a year that reached those levels. It’s not that it can’t happen. It just hasn’t happened yet,” says Gavloski.

So far, there’s no evidence that soybean aphid overwinters in Manitoba because of our extremely cold winters. Most soybean aphids in the province are likely to have blown in on south winds.

The best time to scout for soybean aphid is around the end of June or early July, says Tkachuk. Ideally, weekly.

Manitoba Agriculture’s John Gavloski recommends scouting fields twice at minimum before applying an insecticide. An average of 250 aphids per plant is the economic threshold at which producers should start thinking about spraying, but only if populations are still increasing.
photo: John Gavloski, Manitoba Agriculture

Gavloski recommends scouting fields twice at minimum before applying an insecticide. An average of 250 aphids per plant is the threshold at which producers should start thinking about spraying, but only if populations are still increasing.

In other words, if aphid numbers seem high, scout the field, says Gavloski. If the count is average 250 aphids per plant one day, scout again before spraying. If counts remain at roughly 250 aphids per plant the population may have stabilized; if they’re up to 400, the population is increasing and may be moving toward economic injury levels.

Once seeds are at the R6 stage it’s too late to spray, says Gavloski. But most years, he says, spraying isn’t necessary.

Additionally, because fungal pathogens can help keep soybean aphid populations low, producers should be cautious about over-use of fungicides.

“Scout your fields, use pesticides and fungicides when needed, but overuse of any [treatment] causes more harm than good,” says Gavloski.

Cutworm and wireworm

Cutworm is the insect pest both Gavloski and Tkachuk would place second on the list in terms of economic importance for soybean producers.

A good time to start scouting for cutworm is just as the crop is coming up, says Gavloski. “If emergence doesn’t seem good or there’s evidence of clipped plants or bare patches, dig around in those areas,” he says. “It’s a matter of being on top of it early. If there are high levels, control them before they do a lot of damage.”

That being said, cutworm can be really patchy across fields, he says, so producers should scout entire fields and treat only the affected areas. In a couple hundred acres, only 10 to 20 might be affected.

Thresholds are not well researched for cutworm, but producers can take this as a general guideline, says Gavloski: one or more larvae per three feet of row, with the larvae less than two centimetres in length; or 20 per cent of plants cut.

Wireworm is another pest whose impacts on soybean are still poorly understood. “There’s a lot we don’t know about wireworm,” says Tkachuk.

In 2018, MPSG began working with Brandon University professor Bryan Cassone on a project testing wireworm bait trapping methods. Grainews will follow up on this study later in the winter.

In terms of control options for wireworm, producers should be cautious about using neonicotinoid insecticides, she says — unless there’s a history of wireworm infestations in a field.

MPSG has conducted on-farm strip trials comparing treated versus untreated seed and yield response.

“This seed treatment study has been going on for quite a few years, started in 2015,” says Tkachuk. “We’ve got 28 site years. Only in four of those site years did we see a significant positive response to the seed treatment — only 14 per cent of the time did we see a positive response in yield.”

She says that as neonicotinoids are under review by Health Canada’s Pest Management Regulatory Agency, it makes sense to limit use of neonics to where they’re really needed.

“We’re funded by farmers and we want to help farmers,” says Tkachuk. “If they can save money on seed treatment, why not? We’re in a honeymoon phase with soybean. Pest pressure will continue, so this doesn’t mean there won’t be a need for seed treatment moving forward.”

MPSG’s website houses many great resources for producers, including a scouting calendar and pest identification guide.

Manitoba Agriculture’s John Gavloski recommends scouting fields twice at minimum before applying an insecticide. An average of 250 aphids per plant is the economic threshold at which producers should start thinking about spraying, but only if populations are still increasing.
photo: John Gavloski, Manitoba Agriculture

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Julienne Isaacs is a Winnipeg-based freelance writer and editor. Contact her at [email protected]

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