Grasshopper management in canola: Thresholds, identification, and control strategies

Grasshoppers can be a major pest of canola. If grasshoppers are present at numbers and crop damage is getting worse, spray.

A lot of grasshopper species exist on the Prairies. Four species – two-striped, clear-winged, migratory and Packard’s – can pose a significant threat to canola crops.

Identify the pest

The primary grasshopper pest species on the Prairies, the two-striped grasshopper, prefers broadleaf plants and can cause significant damage to canola. The clear-winged grasshopper prefers grasses and cereal crops, but will feed on canola. The other two major grasshopper pest species, Packard’s and migratory, will consume a wide array of crops.

Damage usually starts around field margins as grasshoppers hatch and move in from egg-laying sites in ditches, treelines, pastures and preferred crops. Start scouting in field-margin areas.

Weather

Grasshoppers, like many other insects, tend to eat more and grow faster in hot, dry weather. This weather also limits fungal infection.

Rain promotes rapid plant growth and will reduce the economic risk from grasshoppers. That’s because robust plants can suffer more damage without a yield loss.

Rain has the opposite effect on grasshoppers: Wet weather in May and June slow grasshopper growth, and can actually kill the smallest nymphs. Wet conditions in July slow maturity of the larger grasshoppers, which delays egg laying. Wet conditions in August interfere with mating and egg production. August wet, warm weather can increase infection by the grasshopper fungus Entomophaga grylli.

Stages

The major pest grasshoppers have one generation per year on the Canadian Prairies. Grasshoppers lay eggs in late summer. Eggs overwinter and then hatch between mid-May and late June, depending on the accumulation of heat. Cool conditions delay the hatch.

Grasshoppers grow through five nymphal stages, or “instars,” before reaching the adult stage. Growth from hatch to adulthood takes just over a month in warm temperatures and close to two months in cool temperatures.

Look for accumulating damage

The simple approach to scouting and thresholds: If grasshoppers are present at numbers and crop damage is accumulating, spray.

Crop damage is key. If a site has numerous grasshoppers, but the canola field shows no damage, the site may contain a species that does not typically feed on canola.

The current threshold for grasshoppers in canola is 8-12 adults per square metre. This is based on counts in mature crop. This threshold is limited in that it does not apply to nymphs eating small plants early in the season.

New thresholds

The Prairie Pest Monitoring Network has new broader considerations:

• 12-14 fifth instar nymphs and adults per square metre of crop is suitable for average or above average growing conditions.
• 30-45 nymphs per square metre has been proposed to remove the threat early when grasshoppers are small and easy to target. This also allows for a larger suite of insecticides. Group 3A (synthetic pyrethroids) are not recommended for adult grasshoppers.
• Ditch thresholds of 24-50 fifth instars and adults and 50-75 nymphs per square metre are in discussion. With a ditch threshold, farmers could consider an early targeted spray before pest grasshoppers move into fields.

Plant condition affects thresholds. A lower threshold of 5-7 fifth instars and adults per square metre may be a better target for hot and dry conditions where grasshoppers thrive and crop growth suffers.

Count them (or give it the college try)

Prairie Pest Monitoring Network recommends this grasshopper monitoring protocol:

• Mark off a distance of 50 metres with flags on the road or field margin.
• Walk along that 50 metre distance counting grasshoppers within a one metre square in front of you. A preferred option is to take separate counts for each one by 10 metre segment, providing five counts per 50 metre stretch. Make some disturbance with your feet to encourage any grasshoppers to jump. The grasshoppers per square metre will be the average of all your counts.
• Do this in the ditch and again inside the crop. Repeat a little farther down the road. And possibly again in the middle of the field to see if grasshoppers are confined to field margins.
• To confirm grasshopper growth stages and species, use a sweep net to capture a few so you can look at them. Contact a provincial or local extension expert if you want help with identification. Most grasshopper species are not crop pests, so confirm crop feeding before making a spray decision. Grasshopper nymphs also look like leafhoppers.

Apply insecticide if necessary

It is generally easier to control earlier instars before they grow into adults.

A Manitoba Agriculture factsheet provides useful control tips, including this tip on insecticide timing: “Insecticide applications are most effective and the costs are lower when they are applied after the eggs hatch and while the nymphs are still concentrated in their breeding areas.”

However, waiting until the majority are in the third and fourth instar stages is a more accurate indicator of a problem. A lot can happen to early instars (ex. disease outbreaks, predation, weather) before they reach the third instar stage and start to cause damage.

Spraying only when grasshopper feeding is accumulating also protects non-pest grasshoppers and other off-target species.

Farmers have several insecticide control options for grasshoppers in canola.

Avoid spraying on hot, hot days. At temperatures above 25°C, grasshoppers can metabolize insecticide more quickly, and it can be less effective. Warm, not hot, days result in slower metabolization and better control. Always consult the insecticide label for details prior to applying. (For more on this, read the section on insecticide performance in hot temperatures at the bottom of this article.

One option, EcoBran, is particularly effective on younger grasshoppers within field margins.

More…

• Canola Encyclopedia chapter on grasshoppers
• AAFC factsheet on grasshoppers, including non-pest species
• Prairie Pest Monitoring Network risk maps and monitoring protocols on grasshoppers
• Prairie Pest Monitoring Network grasshopper lifecycle and thresholds factsheet