Cutworms bug off when beneficial insects come calling

It’s a bug-eat-bug world when you’ve got beneficial insects in your field

Producers looking to cut spray costs this year may want to think about a different type of pest control.

“(Predatory insects) can be really effective, with cutworms especially,” said Vincent Hervet, PhD candidate at the University of Lethbridge.

Cutworms have “cyclic outbreaks” on the Prairies lasting three to four years, and right now Alberta is at the top of the cycle. That’s great news for predatory insects such as ground beetles, arachnids, and assassin bugs which feast on cutworms, and also for tachinid flies or wasps, which inject their eggs into them (the hatchlings kill their hosts when burrowing out).

These beneficial bugs slowly bring down cutworm populations. Eventually the predatory bugs will do such a good job, their numbers will decline, and the cycle begins anew.

“With cutworms, if you want to avoid having an outbreak situation in following years, you better have a good beneficial population in the field,” said Hervet.

But some common farming practices reduce populations of beneficial bugs.

“If you spray, you kill… the pest and the predators,” he said. “And then you’re back at square one.”

Predaceous insects are attracted to fields by a ready and abundant food source, but practices like spraying and tilling destroy the bugs’ ecosystem, killing pests and predators alike.

“You need to keep a certain number of pests to keep the predators.”

Give them habitat

Maintaining biodiversity in the field will help preserve beneficial insects, said Hervet. In addition to using insecticide only when necessary and avoiding tilling when possible, Hervet recommends keeping flowering plants in the field, including some weeds, as a food source for newly hatched wasps and flies.

“It’s important for the parasitoids to have a food source when they come out of the host,” he said. “You don’t need a 100 per cent clean field. Having some weeds is OK.”

In some cases though, spraying is necessary, but producers should scout their fields carefully before making that call.

“If there is significant damage, maybe your operation is at risk,” he said. “But if the damage is not significant, maybe it’s not.”

It’s also good to know the economic threshold of the pest that’s causing problems, he said.

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“The economic threshold is either based on the number of pests per square metre or per plant.”

For cutworms, that number is one to four larvae for every 12 inches of row, depending on the crop stage.

The life stage of the pest is also a factor before deciding to spray.

“Once a cutworm gets big enough, it’s not going to do any more damage,” said Hervet. “If the cutworm is small, you know there’s still a lot of damage that’s going to occur, and it may need control.”

Just remember, spraying this year could lead to a bigger outbreak down the road.

“By spraying, you know you’re going to kill a good number of good insects,” he said.

“You go back to a situation where there’s nothing in your field… and the pests are going to come first. And you will possibly be again in a situation of outbreak because it will be some time before the beneficials arrive to control the pests.”

About the author

Reporter

Jennifer Blair

Jennifer Blair is a Red Deer-based reporter with a post-secondary education in professional writing and nearly 10 years of experience in corporate communications, policy development, and journalism. She's spent half of her career telling stories about an industry she loves for an audience she admires--the farmers who work every day to build a better agriculture industry in Alberta.

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