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The economics of Western bean cutworm

For bean growers, it turns out this is a very different — and difficult — pest

The nature of the crop makes it harder to scout for Western bean cutworm in edible beans than in corn, yet easier to find feeding damage after leaf-drop.

From year to year, edible bean growers face a variety of challenges, including some that are unique to their particular sector. In the past, they have had to deal with bean leaf beetles and potato leaf hoppers, as well as wireworms and seedcorn maggot. As with other field crops, each new growing season seems to introduce another challenge to farmers, adding to the list of pests, diseases and weed species that they’re already managing.

Western bean cutworm (WBC) is the latest challenge to a growing number of edible bean growers. Although not quite “new” by definition, its numbers are on the rise across a wider geographic region than in past. And unlike some pests that arrive with up-and-down populations from one year to the next, the numbers of this cutworm are only increasing.

Native to North America, WBC can overwinter in Eastern Canada, particularly in lighter soils, so they’re flying in to the region and staying. Where other lepidopteran pests have been controlled by Bt technology, cutworm is one of the species that has managed to find a niche.

In past, areas around Bothwell, Ont., were designated as “hot spots” for WBC in edible beans.

Those days are gone.

“We do have a cutworm trapping network. And by looking at the map, Middlesex, Elgin and Chatham-Kent seem to have more,” says Meghan Moran, edible bean and canola specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs. “It’s not just those counties but in that southern area there seems to be more, and every year it seems to be spreading farther north and increasing in numbers.”

Moran adds that there is also movement into Niagara as well as Brant County, and north into Grey and Bruce counties and farther east. She notes too that some Ontario agronomists visiting the Atlantic provinces in 2017 found signs of the pest in various locations there as well.

Hard to fight what’s hard to see

Jim Barclay agrees that Western bean cutworm has definitely increased in its spread across Ontario. As crop retail manager with Hensall District Co-op, he has tried to ramp up an awareness campaign surrounding cutworm, but notes there are several challenges with finding the pest. The larvae are generally tan or pink in colour and have two broad dark brown stripes running the length of its body. Adult moths have a white band along the margin of each wing and a spot or “moon” with a chevron or “boomerang” mark on it.

Evidence of pod feeding indicates the need for an insecticide as well as the potential for higher pick costs. photo: Jim Barclay, Hensall District Co-operative

In spite of such clear and distinct markings, WBC is a tough pest to find. Moran says there are people in the industry who have never seen larvae in a bean field. Barclay, meanwhile, has also worked with OMAFRA entomologist Tracey Baute and some of her interns, garnering more of a confirmation from their combined scouting efforts, further indicating how challenging it is to find them.

“You can set traps to count moths and try to track peak flights, but unlike corn where you go into that next level of looking for egg masses, that doesn’t happen in beans,” Barclay says. Relying on sweep netting can be a challenge when the rows are closed, just because of the nature of how dry beans grow. “It’s not like corn… in dry beans, you’re not looking for egg masses, you’re looking for feeding.”

That is the most readily available and visible sign of a WBC infestation. By the time it’s easier to scout, the leaves have dropped off, which is when feeding becomes evident.

“We do all of our pre-harvest sign-offs on dry beans, so we’re in the fields a lot,” says Barclay. “By that time, at least in the last couple of years, we’re finding more feeding than we used to on the pods, but we’re also looking more too, and we’re seeing more visible signs of Western bean cutworm feeding.”

Asked if there’s an impact that can be expressed in terms of yield or a dollar value, Moran has heard anecdotally that the pick values from 2017 ranged from one per cent to 11 per cent of off-colour or damaged beans that must be cleaned from harvested loads. One per cent in terms of yield loss doesn’t sound like much, but it is when the grower has to pay to have those unwanted beans removed from a truck load.

“It’s not a big yield loss issue, but it is when it comes to quality,” says Moran. “The best way to describe the impact — and it depends on the season — is that cutworms prefer corn, so it’s a timing issue. If a cutworm is looking to lay its eggs and there’s a corn plant in those pre-tassel stages, it’s going to prefer the corn. But if it’s already post-tassel and there’s a nearby bean field, it’ll go to the bean field.”

Timing is the key on spraying

Several factors can challenge management or control measures. Thresholds are hard to establish, especially since the cutworms spend the daylight periods in the soil. It’s only at night when they’ll climb up on to maturing pods, and unlike other pests that target leaves, cutworms only feed on the pods. That’s also why good coverage of the plants is vital, and Moran cites Matador, Coragen and Voliam Xpress (which is a combination of the actives from Matador and Coragen) as products that will provide that coverage.

The most important thing to keep in mind with spraying for Western bean cutworm is to wait for the pods to develop. Although there’s no preference for one market class in particular — small- or large-seed dry beans are targets — cranberry beans are susceptible to oxidation as a result of cutworm damage. Holes in the pod husk allow oxygen into the pod that can discolour and darken the reddish hues on the seed coat, making them unwanted and costly in terms of pick.

From the contractor’s perspective, Barclay says it’s important not to get too convenience-oriented where spray applications are concerned. Although it might be tempting to apply an insecticide with a fungicide in mid-July, it’s best to wait later into August for pod development.

“It’s so different from something like aphids (in soybeans) and we have to holster people from just widespread spraying, yet at the same time, we can’t cost growers money either, so we have to balance the two and take guidance from the experts,” says Barclay. “Practicing the 4Rs — the right product at the right rate at the right time and in the right location — applies to this, as well.”

The learning continues

For the 2018 growing season, Barclay is hoping to learn more about the impact of Western bean cutworm, to intensify in-field scouting and to learn more about pest-crop interactions. The entire bean industry, he adds, can also learn more by monitoring the Western bean cutworm trap network at

Last year was the first year that Hensall District Co-op worked together with OMAFRA and 11 different farms to monitor the pest and begin learning more about it.

Right now, they can’t equate thresholds with yield loss in order to come up with the economics of any damage from WBC. But the hope is that further work with OMAFRA and the University of Guelph’s Ridgetown Campus, together with a widened outreach to farmers will provide more opportunities for learning how best to tackle the pest in future years.

For more information:

For edible bean growers who’d like to obtain green bucket traps for their fields, they’re available from the Great Lakes IPM website.

Make sure to order it in all Green — Item #IPS-G004.

Pheromone Lures are also available from Great Lakes IPM.

Scentry-brand WBC Lures — Item #SC-L206 — are available in cases of 12 lures.

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CG Production Editor

Ralph Pearce

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