Most Unwanted List: Pests

The Top 5 undesirable insects that are coming for your crops this year.

by Becky Dumais

Every year producers have to deal with pests creeping onto the farm – those unwanted guests that infiltrate the field, eat their way through their crops of choice and leave a path of destruction and damage in their wake.

While it’s safe to say that some species of insects will inevitably be coming, they’re not the same invaders from year to year. Five invertebrate pests we expect to come at us this season are: two-spotted spider mites, corn rootworm, fall armyworm, wireworm, and seedcorn maggot.

With this list of potential suspects in mind, how can producers prepare for, and deal with these crop-crippling creatures?

Two-spotted spider mite

Appearing from Chatham-Kent all the way to Bruce and Grey Counties, spider mites didn’t experience a banner year in 2021, but they’re still a pest of concern, according to Tracey Baute, a field crop entomologist at OMAFRA. “It was a wet season, and for me to have people call 10 fields that sprayed for mites and saw them not get knocked down, that concerned me because that (begs the question), what’s going to happen in a good year? It’s going to be concerning when (there are) ideal conditions and sprays aren’t effective, because we don’t have an alternate product to turn to.”

In Deb Campbell’s experience, it tends to be a one-in-three-year pest. Campbell is a certified crop advisor in Dundalk. “All it takes is an extended period of hot, dry weather and, bam, we have spider mites.

“We haven’t had that hot dry spell in the last few years. We had reasonable (moisture) so it wasn’t a big issue. This year, because it’s been long enough, I’m concerned. The odds are we’re on the third year and expecting to see (them). It causes significant yield loss in those seasons where it flourishes.”

Of course, regular scouting is vital; monitor for any patches along the field edges where the plants turn a bit yellow or bronzed, Campbell says.

Two-spotted spider mite
    Two-spotted spider mite. - Syngenta Canada photo

“Catch it very early – as soon as it’s arriving in the field and you’re seeing those symptoms; it’s your best time to be treating it, before it (infiltrates) the field and before it’s laying eggs and getting into multiple generations. Then it’s more difficult to kill and it’s a matter of spraying a foliar insecticide.”

Baute agrees regarding spraying, though producers may run into situations where they walk away from the field “because the dimethoate products aren’t knocking the population back now. They’re able to continue and build up.” Last year was, she reiterates, a year that wasn’t even ideal for them.

An issue, she says, is that soybeans are a major crop so any new insecticides can’t be a minor use registration. “Even an emergency use registration would be difficult to secure,” she says. “It would be a full registration of a new product which will take a few years to happen.

“It’s going to be challenging to find one that’s going to be economical to use. This year, if it’s warm/hot and dry then we’re going to struggle and have a challenging year.”

Two-spotted spider mites overwinter in Ontario and when the wheat starts to get harvested, they’ll progress into the soybean fields. “Windy days help (them) and we are seeing more windy days. Ironically, the windier the days, more often the harder it is to get in and spray.”

Baute advises producers to contact her if they find sites of mites even before a spray so that the mites can be tested.

Bt-resistant corn rootworm

Although there wasn’t an increased number of reports last year, a population of rootworm was detected to be resistant in Middlesex County. “It’s not just a Huron-Perth area now. It’s important to take this seriously because we’re starting to align with what the U.S. has been experiencing.

“They’re tolerant to most of the Bt proteins, even in pyramid hybrids, so if we at all want to save (the) Bt tools, we have to use them more wisely,” Baute says.

She also cautions that once the rootworm is resistant to one protein there’s often cross-resistance, “so it’s getting more troubling. They can’t just turn to another hybrid or trait. They’re really needing to stop relying solely on Bt hybrids.”

Bt-resistant corn rootworm
    Bt-resistant corn rootworm - Tracey Baute photo

Other management tools to keep rootworm at a lower population level can be done via crop rotation, what she refers to as a reset. “We essentially need to do a reboot by crop rotation out of corn for a year to bring those beetle populations to a lower level so then the resistance isn’t as prevalent as it is now.”

Corn Rootworm
    Corn Rootworm - Syngenta Canada photo

Another control method Baute suggests is using bio-control nematodes which persist in the soil. “It’s not meant to be the only (tool) but by putting them in problem fields, it keeps that rootworm population at a lower level.

“This is our reality now,” she says. “The last few years we’re seeing continuous high rootworm populations that are challenging our Bt hybrids. This isn’t going to go away.”

Fall armyworm

This pest originates in the southern U.S. and migrates in mid to late summer. “We’re starting to see them come earlier and they’re also getting a more ideal environment when they get here,” Baute explains, noting that they were catching some in June up until September 2021.

Last year was an outbreak year in the Great Lakes areas and Quebec, according to Baute. “We were naive in thinking those late August/September flights weren’t going to have an impact,” she adds, noting that a typical cool fall would knock out the population. They were still doing damage even at that point in the year.

Fall armyworm
    Fall armyworm - Tracey Baute photo

“In mid-October, they took out our fall crops like winter wheat and winter canola and cover crops,” she says. “This speaks to the change in our thinking of these, what we would have called late-season pests.” Discouragingly, there’s not enough research currently to help determine how to manage them in the fall – what products and what thresholds would be applicable. “None of that is known for September/October growing crops.

“Usually, frost will take them out sooner.” That occurs when we have cooler fall temperatures, yet Baute also says last fall was warmer. She says there’s a need to have improved monitoring or predictive tools to really understand if and when they’re going to be a concern.

Climate modelling over the next 10 to 20 years indicates they’ll be better able to overwinter closer to us in areas such as in Indiana or closer to the Great Lakes. “That will be a game-changer in how we think of those pests and when they’ll come here and start to be a problem.”


Campbell has very big concerns surrounding wireworm pressure on corn and grasses, and the inability to control it with the current slate of seed treatments. “We’re seeing significant stand loss and we’ve seen this problem develop over the last three years. In 2021 it was quite significant and it’s a multi-year pest, so I do anticipate it to continue.”

Although it may not have been an issue going back as far as five years, Campbell says it’s becoming a problem again.

“I have no data to verify but I’m concerned that our cover crops have created a bridge and an extra window of survival and feeding that wouldn’t otherwise be there for this pest because a lot of our cover crop mixes have grass species or cereal species in them. It’s ready food for them.”

Last year wireworm populations were quite common in Mitchell to east of Lake Simcoe, she says. “Based on what I saw last spring in the fields where it was a problem you couldn’t seed the spring grain back into it because it would have caused the same issue. We ended up having to rotate out to a broadleaf crop like soybeans or canola in order to avoid the pest.”

    Wireworm - BASF photo

Once the soil temperature warms up, it becomes active, feeding for about five weeks. It will move below the soil and resurface again in the fall, primarily eating wheat or other available crops, though it’s considered not as damaging a pest during this part of the season. “We were having to terminate large wheat fields (last) spring from large patches being killed out from wireworm.”

Unless you’re actively digging and looking to see what’s lurking in your soil, wireworm can be overlooked. “You could miss it and blame it on frost, seed quality, or several other problems that have similar symptoms,” says Campbell. “I think it’s necessary to have on the radar (and) that folks should be digging up some of these thinned-out patches in the fields to be diagnosing if it is, in fact, wireworm.”

Winter conditions so far haven’t created an environment to cause the pest to die off or be less active in the spring, so she anticipates an “even bigger population causing (feeding damage) again this spring.”

For management of this pest, current seed treatments don’t kill wireworm, but instead, cause an avoidance-like reaction “which has been successful to date. For example, the neonics: they were effective because the wireworm wouldn’t feed on them, even though it didn’t kill them.”

Protection is helping at around a 50 per cent rate, she says, adding that while there are some new seed treatments coming on the market, they’re not commercially available in Ontario.

Producers should scout and identify the pest and have the best seed treatment option available and also have a diverse crop rotation.

Seedcorn maggot

A problem presents itself with seed-corn maggot because the industry has moved to planting soybeans earlier for their higher yield potential. “But ... often those soybeans that we plant end of April/early May, the ground is cold, they sit in the ground for three weeks, they’re slow to emerge.

“The longer it takes those soybeans to emerge, the more opportunity there is for the fly to lay those eggs and for those maggots to start feeding on those soybean fields,” Campbell explains.

She feels an increased seedcorn maggot population is a side-effect from planting early.

Seedcorn maggot
    Seedcorn maggot - Syngenta Canada photo

“We do have seed treatments however they have to feed on those treatments to ingest it.” Unfortunately, with the populations that were happening in some fields, by the time a collection of perhaps five maggots on a seed/seedling consumed enough of the treatment to die, they terminated the plant at the same time. “We were dealing with super-high populations in some of these fields that even the seed treatment isn’t enough to keep them in check.”

Populations of this pest can also be compounded by other factors. Campbell says they tend to be high on livestock farms and where manure is being applied. “The odour attracts the fly,” she says. “If someone’s a livestock producer, we try to get their manure on in the fall, get their cover crop terminated in the fall so that we have less attraction (for them) in the spring.”

Its future is unpredictable, she says. “The main factor that’s changed the whole dynamic is early planting. We could avoid it by planting later and getting the beans out of the ground quicker, but then we’re not capitalizing on the full season.”

Without a finite method to measure just how much – or how little – of a particular pest population Ontario’s crops may get this season, always be scouting and monitoring.

As Baute says, “it’s getting more challenging. Things are becoming more unpredictable and not a lot of research is able to back up our current knowledge on situations that are really unprecedented.”BF

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