The four horsemen of the crop-pocalypse

Three of the big four crop pests could be poised for a comeback in the coming season

Flea beetles.

Four pests caused the most problems last season, and given the right spring conditions, three are poised to return.

That’s according to John Gavoloski, provincial entomologist, who says farmers should head into spring watching the weather and with their eyes open.

“If I had to predict which three pests farmers could be at a higher risk for large levels it would be flea beetles, cutworms and grasshoppers,” he said.

“Weather is one factor and for others it might just be the lack of natural enemies. It’s usually a combination of these things that regulate their cycles.”

The weather set the stage last year. There was a late frost that held some crops back but overall the temperatures were close to historical averages. It was drier than usual with higher-than-normal winds, predominantly from the south. These conditions favoured the big four — grasshopper, cutworms, flea beetle and army worm — and led to their high numbers.

“Grasshoppers like hot and dry summers,” Gavloski said. “Heavy rain can make a dent in the population if it occurs shortly after eggs hatch so when the majority of grasshoppers have just hatched, some heavy rain can decimate a population.”

Two-striped grasshopper. photo: John Gavloski

That’s because they’re susceptible to a fungal pathogen and, if it’s wet the fungus does real damage to the hatchling grasshoppers. That’s why they like it warm and dry, the fungus isn’t as effective.

“And the other critical time is egg-laying time, August through the first weeks of September,” he added. “If they get good, hot, sunny weather they will lay more eggs.”

A late frost last spring was another factor and it set the canola crop back. Surviving seedlings took longer to mature and this made them even more vulnerable to flea beetle.

“With the seed treatments you’ve got roughly three weeks of effectiveness from the time you seed,” he explained. “So anything that holds the canola in that seedling stage for more than three weeks is going to increase your risk of flea beetles.”

The late frost, in combination with drier conditions and a powerful south wind caused multiple stresses on the canola and provided the beetles with a good feed. They did a fair amount of damage before the plants could get to the four-leaf stage where they’re less vulnerable.

Cutworms were also a concern and farmers in many parts of the province sprayed them in corn and the already stressed canola, most of it in early to mid-June.

Dingy cutworm (right) and redbacked cutworm (left). photo: John Gavloski

Army worm, the last of the big four, is a migratory pest brought here on those strong southerly winds. As far as we know they don’t overwinter in Manitoba but arrive when the wind is right.

“Last year they arrived in some pretty big numbers and they were quite widespread,” Gavloski said. “Some years when we have army worm they can be localized but last year there were issues anywhere people were growing small grains and forage grasses. Certainly not all fields were affected but they have their preferences where they lay their eggs and there are reports of army worm damage from all agricultural regions.”

As for next year, army worms are dependent on the wind but grasshoppers, flea beetles and cutworms will be around and may be abundant. No season is ever the same as the last and it will depend on a few factors. For instance, we are seeing an increase in the number of certain natural enemies so next year’s pests may meet some opposition. It’s also dependent on next year’s weather and this winter’s snow cover.

“People tend to speculate that, while we’ve had this big stretch of minus 40 for a couple of weeks, won’t that kill the grasshopper eggs?” Gavloski said. “The short answer to that is, don’t count on it. It needs to get down to about minus 15 where the eggs are, and that’s usually a few centimetres into the soil. If we ever get that you’re going to be losing all your winter wheat and a lot of other things.”

Armyworm. photo: Manitoba Agriculture

Most winters, even with a little bit of snowfall just don’t give you those soil temperatures. Additionally, grasshoppers tend to lay eggs in areas with lush, green vegetation, along field edges and in ditches and these are places where you get snow accumulation.

“Potentially, you can get some cutworm kill,” Gavloski said. “There have been studies with some species showing that if you’ve got very little snow cover and bitter cold you can get some mortality but, again, you have to have almost bare soil. A little bit of snow can actually insulate quite well.”

Flea beetles like to overwinter in sheltered areas where there is some debris that they can get under.

“These areas often accumulate some snow so the flea beetles tend to overwinter well,” he said. “We need more research on flea beetle overwintering but we make the assumption not to count on high winterkill even when we get the cold temperatures.”

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