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Don’t jump the gun on pest control

Growers should only use an insecticide when an economic threshold for a pest is reached.

It’s never a good idea to overreact when you see insects on your crops because jumping the gun, and spraying if it’s not really necessary, can do more harm than good.

“Seldom, if ever, do preventative insecticides actually pay us,” says Scott Meers, insect specialist with Alberta Agriculture and Forestry. “When we spray just to try and keep insect pest populations down, we take out the natural enemies too. Natural enemies are really important in the control of the vast majority of our insect pests. It’s really important when we are below the thresholds to let those things do their work because we may never get to the thresholds if we allow them to be successful.”

Predators or parasitoids can have very significant impacts on pest populations in many different crop systems, says Jordan Bannerman, an instructor with the Department of Entomology at the University of Manitoba. Just because a grower sees bertha army worm in a canola field, it doesn’t mean that bertha army worm will necessarily reach economic levels. “Some years you can have 75 per cent of bertha army worms killed by parasitoids in a field and they’re never going to reach a level that’s near where it is cost effective to apply an insecticide,” says Bannerman.

Spraying to control pests that are not at economic levels can also harm pollinators and encourage the development of resistance in pest insect populations, adds Bannerman.

A great example of how natural enemies work is cereal leaf beetles, which are fairly new to the Prairies. “One of the programs through Agriculture and Agri-Food Canada has been releasing parasitoids to the cereal leaf beetle to establish them in newer areas,” says Meers. “Cereal leaf beetles have been in the Prairies for six or seven years, and we have seen minimal spraying, and it’s most likely because this parasitoid is holding populations below thresholds.”

Imprudent pest management can have a long lasting impact — far past the residual period of an insecticide. “A lot of the pest management activities that we are employing, can have a negative, long term because it can take a long time for beneficial insect populations to rebuild,” says Bannerman.

The enemy of my enemy

The first and most important step in a good pest management strategy is to be able to identify both economic pests and their natural enemies. “For insect pests, accurate identification of the pest species, and being able to distinguish them from non-pests and from other insects, and knowledge of pest life cycles is paramount,” says Bannerman. “This facilitates more efficient monitoring strategies and specific targeting of economic pests.”

A good place to start is with provincial government crop protection guides and websites. Visit for regular updates throughout the growing season. This website is funded by the federal and provincial governments and several levy-collecting commodity organizations. They’ve gathered information about most common agricultural pests as well as maps and other valuable information.

Canola growers can find a useful diagnostic tool on the Canola Council of Canada’s website ( The tool can help producers identify canola pest issues and when to spray specific products for specific canola pests.

These resources also provide information about the life cycles and behaviour of different insects to help growers understand when it’s best to scout for them, how to target specific economic pests. When you’re making spraying decisions, these sites are great resources for economic thresholds based on science and economics, as well as the experience of agronomists and entomologists.

If you reach economic thresholds

Growers should only use an insecticide when the economic threshold for an insect pest is reached — the economic threshold is the point at which the cost of control is equal to the loss being caused by the insects activities.

How do you know? It’s all down to careful scouting of the fields with sweep nets to count insect populations. “It’s important to check several spots in a field,” says Meers. “Bertha army worm, for example, we’ll see often see well above threshold in one part of the field, and below threshold in another part, so if you just check one place you may find it really low or really high and that might not be indicative of the whole field. Producers should check several spots in the field before they make their decision if they’re above or below threshold.”

As a general rule, says Bannerman, it’s more productive to scout insects during the heat of the day, when there is no dew on the plants, particularly if you’re using a sweep-net. “Where and how you scout will depend upon the insect or insects you are concerned about,” he says. “There are some cases where scouting should occur at specific times, like wheat midge scouting should occur at dusk. It really comes down to knowledge of the life cycle and behaviour of the pests you are concerned about.”

Some insects will invade the edges of the field first and then spread out through a field, so growers may be able to detect this pattern and get away with spraying just the boundaries of the field. “You can only make that decision with good scouting,” says Meers.

What to spray and how

If insects are at the economic threshold in your field, you’ll have to decide which product to spray. “Refer to the label guide because each product will be different,” says Meers. “Some insecticides are better on certain classes of insects than others, so it’s important to make sure you’re choosing something that will be effective. Producers can consult provincial crop protection guides, or their local agrologist if they need advice about which product to use.”

It’s hugely important to adhere to the pre-harvest intervals for each insecticide product. “Pre-harvest intervals should be adhered to because a failure to adhere to our pre-harvest intervals is going to result in residues in the crop, and could mess up international markets,” says Meers. “You have to get the timing right so you have enough time to allow for the pre-harvest interval.”

Bannerman suggests the following four tips farmers can use to reduce the impact that their pest control activities have on beneficial natural enemies and pollinators:

1. Don’t spray unless the pest is at economic levels.

2. When you spray, use an insecticide class/formulation that is as specific as possible or has reduced impacts on non-targets.

3. If possible, apply the insecticide when natural enemies and pollinators are less active such as in the evening.

4. Use products with low residual times when possible. †

About the author


Angela Lovell

Angela Lovell is a freelance writer based in Manitou, Manitoba. Visit her website at or follow her on Twitter @angelalovell10.



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