Rolling Out the “Unwelcome” Mat for Soybean Foes

Written by Geoff Geddes

Like taxes and telemarketers, soybean pests and weeds are unwelcome, yet hard to avoid. As with any battle, gaining the upper hand on these crop intruders starts with getting to know the enemy.

Insects: Good guys or unwanted guests?

While soybeans don’t have a lot of regular insect pests to contend with, cutworms and soybean aphids are two that merit attention.

“There are different species of cutworm, but the ones that do early season damage are generally in the larval stage through May and June,” said John Gavloski, an extension entomologist with Manitoba Agriculture. “By late June, they are turning into pupae as they enter the non-feeding stage and are no longer an issue.”

Soybean aphids are another common adversary, though you never know from year-to-year if they’ll pose a problem.

“Because they don’t over winter well and are primarily blown in, soybean aphids don’t always appear at high levels,” said Gavloski. “Last year was a bad one for them in Manitoba, but that doesn’t mean this growing season will be the same. It just depends on what blows in and the conditions at the time. For that reason, scouting your fields is important, as sometimes natural enemies alone can keep soybean aphid populations at less than economic thresholds.”

In many years, crop-feeding insects like cutworms and soybean aphids may not reach damaging levels thanks to natural predators such as lady beetles and parasitic wasps that can help control them. If the level of a crop feeding insect is below a producer’s economic threshold, it’s best to let those natural enemies do their thing. Once above the threshold, farmers should consult the Guide to Field Crop Protection on the Provincial Agriculture website for alternative solutions.

Getting a read on weeds

In general, volunteer RR canola is a major weed competitor for soybeans, usually because of its tolerance to the same herbicides, but it’s by no means the only threat in 2018.

“This year the warm, dry conditions have meant a slower start for soybeans,” said Tammy Jones, Industry Development Specialist – Weeds at Manitoba Agriculture. “As a result, weeds that love the heat, such as barnyard grass, yellow foxtail, redroot pigweed and lamb’s quarters, are having a heyday.”

Herbicide resistant weeds like kochia and wild oats can also be a threat. Manitoba also had instances of ragweed last year that were suspected to be glyphosate resistant.

While she acknowledges that farmers are often too busy or proud to discuss their weed issues, Jones says it’s an important first step to dealing with the problem and helping others in a similar situation.

“Multiple modes of effective action are the key to dealing with soybean weeds. Even in a system where you have a Roundup Ready® or other herbicide tolerant crop, it’s still important to add other modes of action so you’re not continually using the same selection pressure and getting the same results as other areas.”

One option to tackle resistance is herbicide layering a weed management system using two to three herbicides in sequence to tackle difficult weeds and stave off weed resistance. This is especially effective with water hemp, resistant kochia and biennial wormwood.

Regardless of your weed challenge, Jones said one thing remains the same.

“My main advice is to scout, scout and scout again. You should scout before any weed control activity to make sure those weeds are present and at the right growth stage. You must also assess the growing conditions in case you need to adjust water droplet size or water volume.”

A couple of weeks after spraying, producers should scout again to look for escapes and ensure that their spraying was effective. After harvest, a final check reveals what weed growth you have and what to expect next year.

Even the most vigilant farmer can’t rid themselves entirely of every insect and weed. As with telemarketers, and taxes though, there are times to avoid them and turn the other way and there are times when proactive management is required. Finding the right balance is the key!