With the combination of extreme heat and drier conditions this year, keeping your canola protected from insects is critical. That’s why we sat down with John Gavloski, Entomologist for Manitoba Agriculture, Food and Resource Development, to talk about the insects you should be scouting for in your canola this summer, how to identify them and recommendations for dealing with them.
Keeping Your Canola Protected from Insects is Critical
“The dry conditions are really contributing to pest susceptibility,” Gavloski says. “Lack of moisture is holding the crop back enough that a lot of the fields just aren’t as robust and vigorous as they normally would be.”
Canola growing under favorable moisture and temperature conditions can often compensate well for feeding on leaves, buds and flowers. Conditions that delay crop growth, like drought, can leave your fields more susceptible to insect damage.
Gavloski recommends that you get to know several key insects that can be found in your canola since some pests cause similar damage. With that in mind, Gavloski warns that there are four major insect threats to watch out for right now as canola enters later growth stages:
Species of Grasshoppers in Canola Fields
“Grasshoppers are really keeping me busy this season,” says Gavloski, who thinks that they may be the top concern this year. “A lot of people aren’t really sure whether or not they should be controlling grasshoppers, so I have been providing a lot of guidance.”
Some species of grasshoppers thrive in the hot and dry conditions that have been prevalent across the southern Prairies this year.
While many species of grasshoppers won’t harm your canola, the two-striped and migratory grasshoppers are generalists. They have their preferences but they could feed on almost anything in your field.
Another common grasshopper species this year is the clear-winged grasshopper, which may occasionally be in canola when grassy areas nearby deteriorate or are cut. They prefer grassy plants and won’t damage canola.
Scouting Grasshoppers
When it comes to scouting grasshoppers, the goal is to reasonably estimate the levels in your field using a count per square metre.
“Grasshoppers are tricky to count,” Gavloski notes, “you can’t be too accurate; they’re jumping around within vegetation. Try to estimate approximate levels the best you can.”
Spray Threshold
Spraying may be warranted when populations exceed 7 to 12 grasshoppers per square metre in your field, though some judgement is required in the case of additional crop stresses, like drought.
Sometimes border treatments are enough to manage grasshoppers sufficiently, especially before they become adults and can fly.
When scouting, move deep enough into your field to account for the fact that grasshoppers may move inward as they mature and get their wings, especially if the vegetation where they initially developed becomes less abundant or is cut.
“In canola, there can often be an edge effect,” Gavloski says, “you don’t have to spray the whole field if this is the case.”
Bertha Armyworms
A potential problem for late summer, bertha armyworms are a type of climbing cutworm that spend their day on the soil, often under debris, and prefer to eat the leaves and occasionally pods of plants at night.
Found in a variety of colours including green, black and brown, they can become a big issue in large numbers. But luckily, forecasts suggest growers may be getting a reprieve from these pests this year.
Scouting for Bertha Armyworms
“We monitor the adults to provide a forecast and the good news is that as of today, a little over halfway through monitoring, all 99 trap sites in Manitoba are still in the low-risk category,” Gavloski says. But that doesn’t mean you can afford to let your guard down.
To scout for bertha armyworms, Gavloski recommends:
- Mark out a quarter-metre field section: Define your sampling area before beginning the count.
- Shake the plants: Dislodge hidden larvae by gently shaking.
- Check debris and soil: Search under debris, clumps of soil and cracks for armyworms.
“These insects are easier to count than some,” Gavloski says, “they’ll lie on the surface for you but you do need to look under debris, clumps of soil and cracks in the soil.”
Economic Threshold for Bertha Armyworm Spraying Decisions
Spraying decisions for bertha armyworms are made based on an economic threshold.
Spraying occurs when there are enough larvae that the potential yield loss outweighs the cost of controlling them.
Research shows that you can expect an average yield loss of 0.058 bushels per acre for each larva per square metre.
In drought conditions however, losses may be greater since more dropped leaves mean the larvae are likely to feed on plant pods instead.
Diamondback Moths
Only a threat to crops in the larval stage, newly hatched diamondback moths burrow into leaves, eating them from the inside out. After about a week, they emerge and feed externally.
They can be found feeding on:
- Leaves: Main source of nutrition for larvae.
- Buds: Another feeding site leading to visible damage.
- Flowers: Larvae target this site during blooming stages.
- Seed pods: Pods are sometimes eaten if available.
- Stems: Larvae may damage young stem tissues.
- Seeds: Will eat seeds themselves, but less common.
You may recognise evidence of diamondback larvae by the small, irregular holes that they leave behind. If there are enough of them, they may eat entire leaves, leaving only the veins.
So far this year, it has been just a few fields with economic levels but it’s still important to scout for them in case they become a problem in your field.
Scouting for Diamondback Moths
When scouting for diamondback moths, Gavloski recommends:
- Shake plants over a tray: Dislodge larvae for an accurate count.
- Remove plants in a square foot: Use a consistent area for sampling.
- Beat and count larvae: Beat removed plants over a surface and count larvae present.
“If you’re removing plants, remove in a square foot area. Beat them over a surface and count the number of larvae.”
The adult moths, which won’t damage your crop but will show up before larvae are present, are monitored by provincial specialists and agronomists in May and June using pheromone-baited traps.
Spray Threshold for Diamondback Moth Larvae
It’s recommended that action be taken to control diamondback larvae when the larvae exceed:
- 10 to 15 larvae per square foot (applies to immature to flowering plants)
- 20 to 30 larvae per square foot (applies to plants with flowers or pods)
Lygus Bugs
The last pest that Gavloski recommends watching out for is the lygus bug, which prefers to feed on the most nitrogen-rich points of the plants, such as buds, flowers and young seeds.
While many plants can compensate for feeding on buds and flowers, they require adequate soil moisture to do so, which could be an issue in such a dry year.
While the other insects we’ve mentioned have mandibles and chew on the front tissues of plants, lygus bugs have a beak which they use to suck out sap.
Sweeps with a Sweep Net
“Get out in your field in later stages of flowering,” recommends Gavloski, “do sweeps with a sweep net and see what your levels are like.”
Gavloski recommends performing sweeps at a minimum of five sampling areas in your field and during the late morning or early afternoon when lygus bugs are active in the canopy.
Spray Threshold for Lygus Bugs
Spraying for lygus bugs is based on economic threshold tables, similar to bertha armyworms.
Typically based on the amount of lygus bugs sampled per 10 sweeps with a 38cm diameter sweep net, the thresholds are calculated based on a loss of 0.12 bushels per acre for each lygus bug per 10 sweeps when flowering is complete and 0.09 bushels per acre for each lygus bug per 10 sweeps at pod ripening.
Current research in Alberta is re-evaluating these thresholds.
Keep Up to Date on Pest Concerns
To keep up to date on pest concerns, subscribe to the Manitoba Crop Pest Update, co-edited by Gavloski, a weekly report compiling agronomists’ observations and summarizing the greatest threats.
Contact your BrettYoung Regional Account Manager to take a look at your canola field with you and learn more about what to watch out for this summer.
FAQ
What are the four main insect pests to watch for in canola this summer?
The four main insect pests to watch for in canola this summer are grasshoppers, bertha armyworms, diamondback moth larvae and lygus bugs.
How can grasshoppers be scouted and when should spraying be considered?
Grasshoppers can be scouted by estimating their numbers per square metre in the field. Spraying may be warranted when populations exceed 7 to 12 grasshoppers per square metre although some judgement is needed if additional crop stresses exist, such as drought.
What is the recommended method for scouting bertha armyworms and what determines if spraying is necessary?
Bertha armyworms are scouted by marking out a quarter-metre area, shaking the plants and searching under debris to count the larvae. Spraying decisions are made when larvae numbers exceed the economic threshold where potential yield loss outweighs the cost of control.
How do diamondback moth larvae affect canola and when should growers take action?
Diamondback moth larvae burrow into and feed on leaves, buds, flowers, seed pods and stems, often leaving irregular holes or consuming entire leaves. Control is recommended when larvae exceed 10 to 15 per square foot in immature to flowering plants or 20 to 30 per square foot in plants with flowers or pods.
How are lygus bugs identified and when is spraying recommended?
Lygus bugs are identified using a sweep net during late stages of flowering by making 10 sweeps at multiple sites within a field. Spraying is guided by economic threshold tables which relate bug numbers to potential yield loss per 10 sweeps.
Why are canola fields more susceptible to insect damage this year?
Canola fields are more susceptible to insect damage this year due to extreme heat and drier conditions which result in less robust and less vigorous crops that cannot compensate as well for insect feeding.
Where can growers find more information on pest concerns for canola?
Growers can subscribe to the Manitoba Crop Pest Update, a weekly report summarizing pest concerns or contact a BrettYoung Regional Account Manager for field inspections and additional guidance.
