2023 Week 16 (Released August 25, 2023)

Harvest is now well underway and the risk of insect damage to crops is declining as crops mature and are being harvested. This will be the last Weekly Update of 2023. The Prairie Pest Monitoring Network sincerely thanks everyone who has contributed to the 2023 Weekly Updates.

We wish everyone a safe and insect-free harvest season and fall insect scouting season – remember that the annual grasshopper survey is happening now across the prairies and that the wheat midge and wheat stem sawfly surveys will start after harvest is completed.

In 2023, development of many pest insects occurred earlier than normal, thanks to warmer than average weather during this growing season. In particular, we observed adult grasshoppers in mid-June for the first time in more than 30 years (if ever!). Adult grasshoppers will be busy reproducing now, and in looking to next year, insect surveyors are 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 fifth non-migrant generation across the southern prairies now, with the fourth generation occurring in the more northern parts of the prairies. Diamondback moths could pose a threat to crucifer vegetables right now, as well as any canola that is late to mature.

This week, the Prairie Research post highlights studies being done at the University of Saskatchewan to learn more about the ability of ground beetles to contribute to weed seed management.

Use the links in the Provincial Insect Updates post to learn more about what is happening with populations of insect pests (and beneficial insects) in your province. In Manitoba, Dr. John Gavloski notes that grasshoppers remain a concern, that flea beetles are being found in high numbers on the pods in some canola fields and that some bertha armyworm larvae have been found in some canola fields. Dr. Gavloski also noted that there have been a lot of cabbage white and checkered white butterflies active in Manitoba late this summer. Dr. Vankosky observed high numbers of both butterflies in southeast Saskatchewan too. As in Manitoba, Dr. James Tansey reports that high grasshopper densities are a particular problem in central and southern Saskatchewan.


1) There are many resources available to help with planning for late-season insecticide applications to ensure Pre-Harvest Interval requirements are met.

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.

To receive Weekly Updates automatically, please subscribe to the website!

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

This past week (August 14-20, 2023) the southern prairies experienced warmer temperatures and minimal rain. In contrast, cooler, wetter conditions continued to persist across most of the Parkland region.

The seven day average daily temperature was 1.5 °C warmer than average in the last week. The coolest temperatures occurred across the central and western areas of the Parkland region (Fig 1).

Figure 1. Seven-day average temperature (°C) observed across the Canadian prairies for the period of August 14-20, 2023. 

Growing season (April 1, 2023 to August 20, 2023) average temperatures were warmest across Alberta, southern Saskatchewan and southeastern Manitoba (Fig. 2).

Figure 2. Growing season average temperature (°C) observed across the Canadian prairies for the period of April 1 – August 20, 2023. 

In comparing the 2023 growing season temperature to the long-term average temperature for the same period (Fig. 3), growing season temperatures were 2°C warmer than average across Alberta in 2023. In Saskatchewan and Manitoba, the growing season average temperature was 1°C warmer than normal (Fig. 3).

Figure 3. Temperature (°C) anomaly (based on difference of average temperature between observed and climate normals) during the growing season (April 1 – August 20, 2023). 

Precipitation for the period of August 14-20, 2023 was greatest across the Parkland regions of Alberta and Saskatchewan (Fig. 4). Rainfall amounts were negligible for the Peace River region as well as south and central regions of the prairies. A more widespread rainfall event on the prairies occurred between August 21 and 24, bringing precipitation to many parts of the prairies.

Figure 4. Seven-day cumulative rainfall (mm) observed across the Canadian prairies for the period of August 14-20, 2023. 

Growing season cumulative rainfall amounts were greatest in a region that extended from Red Deer to Grande Prairie (Fig. 5).

Figure 5. Growing season cumulative rainfall (mm) observed across the Canadian prairies for the period of April 1 – August 20, 2023. 

Rainfall amounts for central and southern regions of the prairies have been well below average (Fig. 6) in 2023. Areas around and including Lethbridge, Alberta, for example, received 40-70% of the precipitation expected in a ‘normal’ year, based on comparing 2023 to 30-year average weather datasets.

Figure 6. Percent of normal precipitation received during the 2023 growing season (April 1 – August 20, 2023), based on a comparison of cumulative rainfall in 2023 to the climate normal cumulative rainfall (mm).  

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.

The oviposition index provides a relative comparison of grasshopper oviposition rates across the prairies. The oviposition index predicts that oviposition should be occurring across the prairies, and that egg production should be greatest across southern regions of Alberta (Fig. 1), as of August 20, 2023. Egg production should also be high in areas of southern Saskatchewan and southern Manitoba, assuming that grasshoppers are present.

Figure 1. Grasshopper (Melanoplus sanguinipes) oviposition index across the Canadian prairies as of Augst 20, 2023. Higher ovipositional index values indicate greater potential for oviposition (egg-laying). 

Compared to a ‘normal’ year (based on 30-year long-term average weather data), the potential for grasshopper oviposition in 2023 in August has been 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. Long-term average predicted grasshopper (Melanoplus sanguinipes) oviposition index across the Canadian prairies as of August 20 (based on climate normals). 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 20, 2023, indicate that the fourth and fifth generations of non-migrant adults (based on early May arrival dates) are currently occurring across the Canadian prairies (Fig. 1). Fourth generation diamondback moths, if present, are occurring farther north on the prairies compared to fifth generation diamondback moths.

Figure 1.  Predicted number of non-migrant generations of diamondback moth (Plutella xylostella) expected to have occurred across the Canadian prairies as of August 20, 2023.  

In a ‘normal year’ based on climate normals data (e.g., 30-year averages), we would expect the third generation of non-migrant diamondback moth to be occurring across the northern prairie region at this time of year, with the fourth generation occurring across the southern regions of the prairies (Fig. 2).

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 20, based on climate normals data.

Considering advanced canola development, risk across the southern and central regions of the prairies associated with the development of a fifth generation of diamondback moth should be minimal. In regions where broccoli, rutabaga, and other brassica vegetables are grown and where crops are late to mature, these crops could still be at risk of damage from diamondback moth. Keep scouting for diamondback moth where crops are still green and could provide a desirable food source for developing diamondback moth larvae.

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 5 is available now and though it focuses mostly on plant pathogens and weed management, there is an updated Crop Report for the period of August 15-21. The crop report notes that grasshoppers and gophers are causing some late season damage to crops, as well as winds, localized flooding, and drought. Aster yellows has been observed in crops. 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 23 issue provides an update on grasshoppers, flea beetles, and bertha armyworm. It also has features insect pests that are often found in storage bins that could affect grain quality and a quiz for identifying butterflies!

Prairie Research: Carabid Beetles that help Manage Weeds

*Text for this post prepared by Daniella Canon-Rubio and Christian Willenborg, University of Saskatchewan.

At the University of Saskatchewan, Christian Willenborg, Khaldoun Ali, and Daniella Canon-Rubio are studying the role of carabid beetles in the biological control of weeds. These remarkable beetles have gained acclaim for their vital ecological role across diverse agroecosystems, including as predators of pest insects and for contributing to weed management by actively reducing the population of weed seeds in agricultural fields.

Stereoscope image of an adult of Pterostichus melanarius. Picture provided by Daniella Canon-Rubio, University of Saskatchewan.

The seed selection process in carabid beetles is a multifaceted phenomenon, subject to the influence of various ecological factors. The objective of Daniella’s study is to examine the effects of imbibition on the preferential tendencies exhibited by Pterostichus melanarius and Amara littoralis carabid beetles towards the seeds of Bassia scoparia (kochia) and Thlaspi arvense (stinkweed).

Stereoscope image of Thlaspi arvense seed eaten by P. melanarius next to an intact seed. Picture provided by Daniella Canon-Rubio, University of Saskatchewan.

Their research will involve conducting field experiments to trap live adult insects using pitfall traps. In the laboratory at the University of Saskatchewan, studies in controlled feeding environments will be conducted with varying imbibition length times to allow for the evaluation of seed preferability and consumption. Seed imbibition occurs when dry seeds take up water. Concurrently, we will utilize an olfactometer to evaluate how the odor emitted by Bassia scoparia and Thlaspi arvense seeds, treated with various imbibition times, influences the seed selection and favorability of Pterostichus melanarius and Amara littoralis. To integrate and analyze the behavioral data obtained from this experiment, we will employ Ethovision, a sophisticated software platform, to track the subjects through video analysis, perform movement analysis, and accurately identify specific behaviors exhibited by the beetles.

Through the implementation of this research on biological control in conservation, our objective is to significantly expand and enrich the knowledge base concerning carabid beetles and their feeding behavior in relation to diverse weed seed characteristics. By fostering a comprehensive understanding, we seek to promote awareness among farmers and fellow researchers about the valuable role carabid beetles play as beneficial organisms in North America. Furthermore, we aim to encourage the utilization of carabid beetles as an alternative approach to weed management.

Pre-Harvest Intervals

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.

Parasitoids of Wheat Stem Sawfly

Bracon cephi and Bracon lissogaster are the primary parasitoids that attack wheat stem sawfly and help to regulate wheat stem sawfly populations in North America. These closely related parasitoid species are described as idiobiont ectoparasitoids. The parasitoid larva, after hatching from an egg laid on the surface of the wheat stem sawfly larva, consume the entire host except for the host’s head capsule and exoskeleton. Both Bracon species complete their development and pupate inside the wheat stem. Their pupae are generally found inside the exoskeleton or beside the remnants of their consumed wheat stem sawfly host. There are two generations of B. cephi and B. lissogaster per year.  The first generation completes its lifecycle then exits the wheat stem to locate a new host to parasitize.  The second generation overwinters within the wheat stem. 

Adult Bracon cephi parasitoid, pictured inside a vial (hence the artistic effect!) by Dylan Sjolie, AAFC-Saskatoon.

Bracon cephi and B. lissogaster are similar in appearance. The wasps are typically 2-15 mm long and brown in colour. They have a narrow waist connecting the abdomen to the thorax and the combined length of head plus thorax is equal to the length of the abdomen.  These parasitoid wasps have long antennae and two pairs of transparent wings. Females have a noticeable ovipositor protruding from the end of the abdomen. 

Parasitoid population dynamics and efficacy are influenced by crop management practices. Parasitoids can be conserved by increasing the height of stubble when harvesting and reducing insecticide applications in grass ditches where natural enemies of the wheat stem sawfly are abundant. 

For more pictures and information about the natural enemies of the wheat stem sawfly, check out our past Insect of the Week post about Bracon cephi!