Identify pests in the field

Friday, September 17, 2021

Before they become a problem.

By Michelle Jones

Insects are a pest that crop farmers deal with every year. And while pest varieties can change from year to year depending on a number of factors, the Prairies have some pests that farmers are almost always certain to deal with. Better Farming connects with some industry experts about the most common pests on the Prairies, how they affect crops and how they can be controlled.

Flea beetles

Flea beetles are an early-season canola pest that all farmers should be watching for this growing season. There are two species of flea beetles on the Prairies: striped and crucifer. The beetles will overwinter as hungry adults and emerge in the spring when it begins to warm up and will begin feeding on the canola crop early on.

All the canola that farmers purchase has a seed treatment that looks promising for both types of flea beetles. This treatment is important since early planting is no longer effective with the early-emerging striped flea beetle now being the dominant pest.

"Some of the work that was done back when the crucifer beetles were the dominant pest indicated that you could seed canola earlier to try to escape the flea beetle infestation. But now the striped flea beetle has moved down from the north and is dominant in a lot of areas, so that doesn't hold true anymore. Early-seeded canola can still get attacked by flea beetles," Dr. Tyler Wist tells Better Farming.

Wist is a field crop entomologist at Agriculture and Agri-Food Canada (AAFC) in Saskatchewan.

To help mitigate flea beetles, Wist suggests planting into stubble is the best option. Using no-till methods and leaving higher stubble may reduce flea beetles, since they "love running across the black earth," says Wist. This approach is currently being studied.

Flea beetles are the biggest issue early in the season for canola, according to Anne McRae, a technical service specialist at BASF in Ontario. The beetles are above-ground feeders and feed on leaves, which causes defoliation in the plant.

"They will feed on the cotyledon and young leaves, so they cause that shot-hole look as feeding damage. They like it when it's calm and hot, and come out and can devastate a crop early on," she tells Better Farming.

When scouting, look for defoliation on the plant, as well as the presence of beetles. McRae suggests scouting for them every two to three days.

According to Rory Cranston, a market development manager for cereals and pulses at Bayer CropScience in Alberta, "Scouting is always going to be number 1 for knowing when, where and how to look for them. You can see flea beetles on the leaves. If it's windy, look under the leaves."

Cranston recommends scouting for flea beetles during the first few weeks of growth. Scouting should take place from emergence to the four-leaf stage. After that, the plant can outgrow the pest. Once the crop is a few weeks old, the beetles can't really do much damage, he tells Better Farming.

Rob Garland agrees. He is a farmer in the Moose Jaw, Sask. area. This year, he's growing red wheat and canola on about 900 acres, and he rents out pasture. He is also an independent sales representative for Pioneer Hi-Bred, Top Krop Fertilizers and Agtiv.

"Look for holes in the canola leaves. If it's 25 per cent or more defoliated, which is the economic threshold, you probably want to spray at that point," he tells Better Farming.

"Ideally, to reduce flea beetles, we would have a one-in-eight rotation. Economically, would that actually work, though? I'm not sure," says Cranston. "So, it's going to be using tools at hand: seed treatments, planting and effective plant-stem population."

Cutworms

The cutworm does just what its name says: it cuts. Some stay in the ground, cut the plant underground and pull it down, which will cause a bare spot. Some will come out of the ground and cut the plant above ground and eat it while it's laying in the field, says Wist.

"Later on, we get climbing cutworms, and they have a slightly different life strategy. They come out later in the season when the plant is taller. The larvae bide their time in the ground until the crop shows up, and like the name suggests, they'll be climbing on it. That's when they do their cutting and feeding damage," he says. The climbing cutworms are best known as Bertha armyworms, and are a very common Prairie pest.

Cutworms are a multiple-host crop pest that can attack canola and cereals, and can quickly devastate crops. Cranston says they'll clip the stem right at ground level, which completely kills the plant.

Garland recommends looking for large gaps in the fields when scouting.

"If there are missing plants, that is a giveaway that cutworms are present. They march up the row eating the plants," he says.

Garland also says that cutworms are more prevalent on darker soils like lentil stubble, so it is important to closely inspect those fields because they warm up and become active sooner. He recommends seeding wheat into lentil stubble to help with this.

McRae says cutworms are hard to see because they're on the soil surface and are dark in colour. This can make them difficult to detect.

"They will chew on the stem and cut it off. Sometimes you don't always know what it is that hurt your crop because they come out and feed at night. So, you'll just see a lot of plants that have been cut off," she says.

If you're scouting for cutworms, Cranston recommends doing it at night.

"They're nocturnal, so if you're scouting, you have to be doing that later at night when it's starting to get dark."

The timing of cutworm scouting is similar to that of flea beetles, Cranston says. Scouting for cutworms should be done in the first few weeks up to the four-leaf stage when the plant is able to outgrow it.

Wheat midge

Wheat midge are a problem later in the season. They are extremely small; about half the size of a mosquito. The females come out in the evening to lay eggs on the wheat heads and the larvae then damage the kernels, says McRae.

Like most of the other crop pests, populations are contingent on the weather. A dry spring is not conducive to wheat midge development, according to Wist.

"It's something that we create risk maps for by sampling fields in the fall and predicting what can happen the next year," he says. "There were a lot of viable wheat midge in fall 2020, so the risk map doesn't look very good for 2021."

Some work from AAFC a few years ago says we need about 25 millimetres of rain to get wheat midge developing in the ground, he says.

Although most farmers have moved to no- or low-till agriculture, Wist suggests that tilling helps to reduce the wheat midge population. The tilling brings them to the surface where ground beetles can eat them, he says.

To combat wheat midge, Cranston recommends starting with a wheat-midge-tolerant variety of wheat. When you're scouting, it should be done when the sun is setting, since that's when they're most active.

"You'll be looking on the head of the wheat. You only need to be looking when the head emerges from the boot and a few weeks afterward, because that's the time they can do damage," he says.

Wist agrees. "For wheat midge, you can plan ahead and plant midge-tolerant wheat that has a resistance gene in it that kills the larvae."

But even if you're using a midge-tolerant variety, McRae suggests scouting daily during peak times.

"You have to be out there every evening to see what the pressures are, because you want to make sure you hit that threshold," she says.

Diamondback moths

AAFC monitors the early winds for diamondback moths (also called cabbage moths), a migratory species that comes in on winds from the south.

"Earlier in the year, we had suspect winds from areas that could be carrying diamondback moths into Saskatchewan," says Wist. "There are pheromone traps for those across the Prairies. The moth comes in early, lays eggs on cruciferous weeds and has a small generation.

"Those moths then come out when there is canola in the fields. And they can just keep on going, so they don't have fixed generations. It's as many as they can get in, so the population keeps building up," Wist says.

"The thing about diamondback moths is they can't overwinter usually with our cold weather, so they come in from the south," says McRae. "It depends on the weather in the south, then you need wind and warm weather."

Pheromone traps are put out early in the spring to assess the risk to see if it will be a bad year for the diamondback moth.

The larvae chew on canola plants, reduce leaf area and sometimes chew on pods, which releases the seeds.

According to the Canola Council of Canada website, "The diamondback moth larva is easily identified by its peculiar reaction to being disturbed. It will wriggle backward violently and may drop from the plant, suspended by a silken thread. After several seconds, the larva will climb back onto the leaf and continue feeding."

Wist says scouting for diamondback moths is the best way to combat them.

If things are getting bad, he recommends that a spray would be needed.

Also watch for moths in your fields when the crops are disturbed.

"If you see the moths, then you know to mark that down and check for larvae later on," says Garland.

Scouting is key

In general, all pests do the most damage in the larvae stage when they are feeding on the plant.

"Pests will eat the foliage, which will damage the plant by taking away the green mirror. The underground pests like cutworms will eradicate the plants, so your plant population goes down, which negatively affects yield," says Garland.

Typically with diseases or weeds, you know you have them every year, but with insects, you won't always have that action threshold. So, you still have to monitor, even if you've used a seed treatment, says McRae.

"Seeing moths and flies is a good way to know that larvae may be present in the field. That's why scouting is so important," she says.

"Probably one of the biggest overlying things for all these pests is boots-on-the-ground scouting, and proper timing, because you just don't know until you look."

"You need to know your enemy," says Cranston. "Scouting is always going to be number 1 for knowing when, where and how to look for them.

"There are also foliar defences, but that is a last resort, because it puts your beneficial insects at rest, too. So, you want to use good rotations, seed treatments, and if there are varieties that have tolerance, use those. Then when scouting, know when you have an action threshold and when it's time to spray," he says.

Garland agrees.

"If (pests) are in high-enough numbers, you have to use an insecticide. For most farmers, this is a last resort," he says. "If the numbers are borderline or threshold, we typically won't spray. Targeting one of those pests will also target beneficial insects. Ladybugs will eat aphids, so killing aphids will also kill them."

Talking to your agronomist, reps and tech service specialists is an important part of pest management.

"Some people hire agronomists and there is a schedule to come out and scout once a week. They will watch for outbreaks and a lot of farmers are doing this now. At least once a week, your boots should be in the field. You need to be diligent in your crop checking, since different areas can be affected," says Garland.

"If you see anything, it's important to talk to people to find out what's going on in the fields. And there are scouting apps that can be used to help identify some of those action thresholds," says McRae. BF