2023 Week 14 (Released August 10, 2023)

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 now in flight and are expected to be busy reproducing across the prairie region. Scouting individual fields is important to best estimate crop risk. At this time of year, we start to look forward to next season. Insect surveyors are now working to estimate grasshopper populations in ditches/roadsides and may be collecting samples of adult grasshoppers for species identification.

Diamondback moth, if present, are into the fourth non-migrant generation across most 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.

Use the links in the Provincial Insect Updates post to learn more about what is happening with insect pest (and beneficial insect) populations in your province.

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.

Finally, due to the long weekend and my departure for southern Saskatchewan early on Tuesday morning (to help with the aforementioned grasshopper survey), I wasn’t able to post the Insect of the Week until today. This week, we feature Aphidius spp. parasitoids, an important natural enemy of aphids.

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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

Warm, dry conditions continue to persist across most of the prairies. This past week (July 31 – August 6, 2023), the prairie average daily temperature was almost 2°C warmer than climate normals. The coolest temperatures occurred across central and northern Alberta (Fig 1). The warmest weekly average temperatures occurred across Saskatchewan and Manitoba (Fig. 1). 

Figure 1. Seven-day average temperature (°C) observed across the Canadian prairies for the period of July 31 – August 6, 2023. 

Average temperatures over the past 30 days (July 8 – August 6, 2023) have been 1°C above normal; many locations in the Peace River region have reported 30-day average temperatures that were 2°C warmer than average. The warmest temperatures were reported across southern regions of Alberta and Saskatchewan (Fig. 2).

Figure 2. 30-day average temperature (°C) observed across the Canadian prairies for the period of July 8 to August 6, 2023. 

Precipitation for the period of July 31 – August 6, 2023 was minimal across most of the prairies (Fig. 3).

Figure 3. Seven-day cumulative rainfall (mm) observed across the Canadian prairies for the period of July 31 – August 6, 2023. 

In both the last 7 days and in the last 30 days, northern Alberta has had the most rainfall (Fig. 4). The average cumulative precipitation across the prairies from July 8 to August 6, 2023 was 39 mm, which is about 74% of the cumulative precipitation expected for the same period based on long-term average weather data.

Figure 4. 30-day cumulative rainfall (mm) observed across the Canadian prairies for the period of July 8 -August 6, 2023. 

In the 2023 current growing season, the warmest and driest area of the prairies continues to be across southern Alberta and the western half of Saskatchewan. 

Predicted Grasshopper Development

As a result of warmer than normal temperatures, grasshopper development continues to be well ahead of normal in 2023. This year, adults have occurred much earlier than normal (middle of June in some locations!) and simulations also indicate that adult females have begun laying eggs much earlier than normal. As of August 6, 2023, grasshopper models predict that grasshoppers oviposition is now occurring across most of the prairies (Fig. 1), assuming grasshoppers are present. Using an oviposition index, the model indicates the greatest potential for grasshopper oviposition in southern Alberta, southern Saskatchewan, and southern Manitoba.

Figure 1. Grasshopper (Melanoplus sanguinipes) oviposition index across the Canadian prairies as of Augst 6, 2023. Higher ovipositional index values indicate greater potential for oviposition. 

Compared to a ‘normal’ year (based on 30-year long-term average weather data), the potential for grasshopper oviposition in 2023 in early August is very high. In a ‘normal’ year, the grasshopper development model predicts that oviposition would be underway in early August, but only in the southern region of the prairies, and with a lower average oviposition index (Fig. 2) than predicted for 2023 (Fig. 1).

Figure 2. The predicted oviposition index for grasshoppers (Melanoplus sanguinipes) across the Canadian prairies as of August 6, predicted using long-term average (i.e., climate normals) weather data. Higher ovipositional index values indicate greater potential for oviposition. 

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

After arriving in western Canada in the spring, migrant diamondback moths begin to reproduce. In western Canada, there are usually up to four non-migrant generations of diamondback moth produced in ‘local populations’ during the growing season.

Due to warm weather in 2023, development of diamondback moth populations is well ahead of average development. Model simulations to August 6, 2023, indicate that the fourth generation of non-migrant adults (based on early May arrival dates) are currently occurring across the Canadian prairies (Fig. 1).

Figure 1.  Predicted number of non-migrant generations of diamondback moth (Plutella xylostella) expected to have occurred across the Canadian prairies as of August 6, 2023. 

In a ‘normal year’ based on climate normals data (e.g., 30-year averages), we would expect only the third generation of non-migrant diamondback moth to be occurring at this time of the year.

Figure 2. The predicted number of non-migrant generations of diamondback moth (Plutella xylostella) expected to have occurred across the Canadian prairies as of August 6, based on climate normals data.

Some areas of the prairies might be at risk of damage from diamondback moth this summer, based both on pheromone trap results for adult moths this spring and on the presence of high numbers of larvae in canola samples. 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 has found high numbers of diamondback moth larvae in canola samples from Yellowhead county, Parkland county, Lac St. Anne county, the Barrhead area, the Leduc area, and the Bonnyville area (so far).  

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.

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! Issue 4 is now online, as is a crop report up to the end of July, 2023 (available here). 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). The August 9 issue provides an update on Lygus bugs, diamondback moth, flea beetles, grasshoppers and aphids. It also has great pictures of beneficial insects!

Pre-Harvest Intervals

As harvest gets started, 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.

Aphidius Parasitoid Wasps 

Aphidius spp. parasitoid wasps (Hymenoptera: Aphidiidae) are important natural enemies of aphids. Their hosts include over 40 aphid species! Female parasitoids lay individual eggs inside aphid nymphs. After hatching, the parasitoid larva consumes its host, eventually killing it. The parasitoid pupates inside the dead or mummified aphid before a new adult parasitoid emerges. New generation adult parasitoids chew a hole in the mummified aphid to exit and immediately search for new aphid hosts. 

Aphid mummies are the result of parasitism by Aphidius spp. parasitoids. Picture by Jennifer Otani, AAFC-Beaverlodge Research Farm.

Aphid mummies look bloated and discoloured compared to healthy adult aphids. Parasitism rates can be estimated by counting the number of aphid mummies on five host plants at five locations within a field. 

For more information about the predators and parasitoids of aphids, visit 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).   

To learn more about some of the natural enemies fighting insect pests in background visit www.fieldheroes.ca or follow @FieldHeroes on Twitter.