2023 Week 12 (Released July 27, 2023)

It sounds like harvest has started in some parts of the prairies in the past week, but crop development and insect pest issues are variable across the prairie region. Insect scouting season continues! Development of many pest insects (and of their host crops) is ahead of schedule this year, thanks to warmer than average weather during this growing season.

Adult grasshoppers are becoming more and more common across the prairies. The stage of grasshopper development and grasshopper population densities can vary between fields, thus scouting individual fields is important to best estimate crop risk. Now that adult grasshoppers are active, egg laying has begun.

Diamondback moth, if present, are into the fourth non-migrant generation in some parts of the prairies now. Keep in mind that diamondback moth develop quickly in warm weather which could lead to rapidly increasing populations over the summer. Scout canola fields for diamondback moth larvae and use the links in the Provincial Insect Updates post to learn about diamondback moth risk in your region. Parasitoids of diamondback moth are highlighted in the Week 12 Insect of the Week!

Watch out for Invasive and Migrating Insects! If you suspect that you have found any of the insects on the Prairie Region Poster, please let us know using the form linked to the QR code on the poster. Note: many of us entomologists on the prairies are members of the Insect Surveillance Community of Practice!

Remember: 1) there are many resources available to help with planning for late-season insecticide applications to ensure Pre-Harvest Interval requirements are met, and 2) insect Monitoring Protocols containing information about in-field scouting as well as information about insect pest biology and identification are available from the Prairie Pest Monitoring Network.

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Questions or problems accessing the contents of this Weekly Update? Please contact Dr. Meghan Vankosky (meghan.vankosky@agr.gc.ca) to get connected to our information. Past Weekly Updates, full of information and helpful links, can be accessed on our Weekly Update page.

Weather Synopsis

During the week of July 17-23, 2023, the prairie average daily temperature was 1°C warmer than the climate normal average daily temperature for the same period. The coolest temperatures were observed across eastern Saskatchewan, western Manitoba, and the Peace River region. The warmest weekly average temperatures occurred across southern Alberta and southwestern Saskatchewan (Fig. 1). 

Figure 1. Seven-day average temperature (°C) observed across the Canadian prairies for the period of July 17-23, 2023. 

Average prairie daily temperatures over the past 30 days (June 24 – July 23, 2023) have been 1°C above normal. Many locations in the Peace River region have reported 30-day average temperatures that were 4°C warmer than average, so it was no surprise that the warmest temperatures in the last 30 days were reported across most of the southern prairies and the Peace River region (Fig. 2).

Figure 2. 30-day average temperature (°C) across the Canadian prairies for the period of June 24 to July 23, 2023. 

Growing season (April 1 – July 23, 2023) temperatures continue to be warmer than normal by 1.7°C. For the growing season so far, the warmest temperatures have occurred across the southern regions of all three prairie provinces (Fig. 3).

Figure 3. Growing season average temperature (°C) observed across the Canadian prairies for the period of April 1 to July 23, 2023. 

Precipitation for the week of July 17-23, 2023 was minimal across most of the prairies; only the Parkland region had rainfall amounts that were greater than 20mm (Fig. 4).

Figure 4. Seven-day cumulative rainfall (mm) observed across the Canadian prairies for the period of July 17-23, 2023. 

In the last 30 days (June 24 to July 23), the average cumulative prairie precipitation was 35 mm, which is only 62% of the precipitation we would normally receive in the same period of the growing season. Cumulative rainfall in the past 30 days was greatest in the Edmonton  and Winnipeg regions and the lowest rainfall totals continue to be those recorded across most of Saskatchewan and southern Alberta (Fig. 5). 

Figure 5. 30-day cumulative rainfall (mm) observed across the Canadian prairies for the period of June 24 – July 23, 2023. 

Since April 1, conditions have generally been dry across the prairies, with some notable exceptions. Most of the prairie region has now received approximately 90% of total rainfall we would expect to receive based on long-term climate normals. Below normal precipitation has occurred across most of Saskatchewan and southern Alberta (Fig. 6). During the current growing season, the warmest and driest area of the prairies continues to be southern Alberta and the western half of Saskatchewan. 

Figure 6. Growing season cumulative rainfall (mm) observed across the Canadian prairies for the period of April 1 to July 23, 2023. 

Predicted Grasshopper Development

Grasshopper risk may be significant for large areas of Alberta, Saskatchewan, and southern Manitoba. In 2023, grasshoppers developed to the adult stage earlier than normal (by nearly 2 weeks!) and high densities of grasshoppers have been observed in many regions. Grasshoppers are exacerbating crop yield losses in drought-affected areas.

Model simulations were used to estimate the developmental stage of grasshoppers as of July 23, 2023. Simulations indicate that 54% of the prairie population should be in the adult stage (51% last week). Grasshopper development continues to be well ahead of average in 2023. For example, in an average year, based on long-term average weather, we would expect only 10% of the population to be in the adult stage at the end of July.

Following early development to the adult stage, grasshopper reproduction is now expected to be underway. Our grasshopper model indicates that oviposition has begun across most of the prairies (Fig. 1).

Figure 1. Proportion (%) of the migratory grasshopper (Melanoplus sanguinipes) population expected to be in the egg stage across the Canadian prairies as of July 23, 2023. 

Geospatial maps are a tool to help time in-field scouting on a regional scale but grasshopper development and population densities can vary from place to place. Scouting is required to accurately assess the stage of grasshopper development and to estimate grasshopper densities.

Information about grasshoppers and grasshopper monitoring is available from the Prairie Pest Monitoring Network, in the Field Crop and Forage Pests guide, Alberta Agriculture and IrrigationSaskatchewan Ministry of Agriculture, and Manitoba Agriculture

Predicted Diamondback Moth Development

A fourth generation of non-migrant adult diamondback moths could be flying right now in southern Manitoba and in southeastern Alberta (Fig. 1). This prediction is based on model simulations to July 23, 2023 using early May arrival dates for diamondback moth adults that migrated into western Canada in the spring.

Figure 1. Predicted number of non-migrant generations of diamondback moth (Plutella xylostella) expected to have occurred (or be occurring) across the Canadian prairies as of July 23, 2023.  

Compared to past years, warm weather in 2023 has sped up the development of diamondback moth. Using long-term average weather data (also known as climate normal data), model simulations to July 23 indicate that a second or third non-migrant generation of diamondback moth should be occurring right now (Fig. 2). In 2023, we could already be seeing a fourth generation!

Figure 2. The number of non-migrant generations of diamondback moth (Plutella xylostella) expected to have occurred across the Canadian prairies as of July 23, based on climate normal data. 

Some areas of the prairies might be at risk of damage from diamondback moth this summer. Pheromone traps with cumulative counts greater than 25 male moths were located around Cadillac, Rosetown, Makwa, Eatonia, and Swift Current in Saskatchewan and in all regions of Manitoba (see the July 5 and July 19 editions of the Crop Pest Report). In Alberta, Shelley Barkley is finding diamondback moth larvae in canola samples from Yellowhead county, Parkland county, the Leduc area, and the Bonnyville area.  

Because diamondback moth can have multiple generations in a single growing season and because the generation time is shorter when temperatures are warm, their populations can build up quickly. Keep scouting for diamondback moth to avoid unpleasant surprises at the end of this summer.

To scout for diamondback moth, estimate the number of diamondback moth larvae per m2 at several locations in a field. The economic threshold for diamondback moth is NOT based on pheromone traps or sweep net samples, but on the density of larvae per plant. For immature and flowering canola, the economic threshold is 100-150 larvae/m2. In podded canola, the economic threshold is 200-300 larvae/m2. See the Field Crop and Forage Pests guide and monitoring protocol for more information about scouting for diamondback moth.

Watch Out for Invasive Insects

Invasive insects are a threat to agriculture and forestry in Canada. The Insect Surveillance Community of Practice is asking anyone monitoring, photographing, or observing insects to be on the lookout for invasive insects across Canada. They have developed posters for four regions of Canada, including the Prairies, with pictures and information about potentially invasive insects of concern to each region.

Early detection is critical for slowing the spread of invasive insects. Please view the poster for your region using the links below to learn more about insects to watch out for. Use the QR code on the poster to report your detections/observations.

A sample of the first page of the Prairie Region poster of invasive and migratory insects to watch for.

Prairie Region (French version here)

British Columbia (French version here)

Ontario & Quebec (French version here)

Atlantic Canada (French version here)

This initiative is a collaborative project developed by the Insect Surveillance Community of Practice of the Canadian Plant Health Council, a multi-partner body that coordinates action for the protection of plant health in Canada. 

Pre-harvest Intervals (PHI)

As harvest is nearing (if not already begun), it is necessary to consider PHI before applying pesticides for late-season pests. The PHI refers to the minimum number of days between a pesticide application and swathing or straight combining of a crop and reflects the time required for pesticides to break down after being applied. PHI values are both crop- and pesticide-specific.  Adhering to the PHI is important for a number of health-related reasons and to ensure that crops being sold for export meet pesticide residue limit requirements.

Helpful resources include:
• The Keep It Clean website, with information about PHI and Maximum Residue Limits (MRL)
• The Pest Management Regulatory Agency fact sheet, “Understanding Preharvest Intervals for Pesticides”, with a free copy available to download
• Keep It Clean’s “Pre-Harvest Interval Calculator” that will help to accurately estimate PHI for a variety of crops
• The Pre-Harvest Glyphosate Stage Guide
• The provincial crop protection guides include the PHI for every pesticide by crop combination. The 2023 Crop Protection Guides are available as FREE downloadable PDFs for AlbertaSaskatchewan, and Manitoba.

Provincial Insect Updates

Visit the Alberta Insect Pest Monitoring Network and Crop Insects pages for information about insects and monitoring in Alberta, including links for live maps from the 2023 monitoring season for diamondback moth, bertha armyworm, cutworms, and cabbage seedpod weevil.

Saskatchewan Crop Production News issues are now online! Use this link to read Issue #3. Issue #3 for 2023 includes information about grasshoppers and Lauxanid flies, as well as about plant diseases and some notes about plant development across Saskatchewan. There are links on the Crop Production News page so that interested readers can subscribe to the newsletter or read issues from past years.

Weekly Manitoba Crop Pest Updates for 2023 are available online with timely updates about insect pests, weeds, and plant pathogens. Watch their website for new Crop Pest Updates (usually published on Wednesdays this year).


This week, our insects of the week are the natural enemies of diamondback moth found on the Prairies! Four important parasitoids attack this pest: Diadegma insulare, Diadromus subtilicornis, Microplitis plutellae, and Trichogramma praetiosum.

A pupa of the parasitoid Diadegma insulare inside its cocoon. Picture credit: Andrea Brauner, AAFC.

Some of these species (like Diadegma insulare) follow diamondback moth on its yearly migration from the southern United States and some (like Micropletis plutellae) overwinter in Canada and can help with early-season control. These small, dark colored wasps occasionally completely control diamondback moth outbreaks in Canada! 

Parasitoids of diamondback moth. On the left: Diadegma insulare. On the right: Microplitis plutellae. Both pictures taken by Amanda Jorgensen, AAFC-Beaverlodge Research Farm.

The four parasitoid species attack during different stages of the diamondback moth lifecycle. Diadegma and Micropletis parasitoids attack larval diamondback moth. Trichogramma and Diadromus species attack the prepupal and pupal stages.

A female Diadromus parasitoid preparing to parasitize a diamondback moth pupa. Picture credit: Andrea Brauner, AAFC.

There is a long list of other wasp species that have been found to parasitize diamondback moth larvae to a lesser extent. Hoverfly larvae, yellowjacket wasps, lacewings, plant bugs, pirate bugs, beetles, spiders and birds also prey on diamondback moth larvae. 

Biological and monitoring information related to diamondback moths in field crops can be found on our Monitoring page as well as on provincial Agriculture Ministry pages (Manitoba, Saskatchewan and Alberta). For more information, visit the diamondback moth page in the Field Crop and Forage Pests and their Natural Enemies in Western Canada: Identification and Management field guide. (en français : Guide d’identification des ravageurs des grandes cultures et des cultures fourragères et de leurs ennemis naturels et mesures de lutte applicables à l’Ouest canadien). You can find more information about some of the parasitoids of diamondback moth on the field heroes website or learn about Braconid wasp life cycles here.