By Carter Peru, PAg, Integrated Pest Management Specialist, Crops and Irrigation Branch, Regina
The 2024 growing season posed many challenges for farmers, some of which included high insect pest pressure. Several insect surveys took place in 2024 with populations varying significantly depending on the insect species surveyed. This article provides a brief overview of some of the insect surveys conducted. Although fieldwork for insect surveys is complete, lab work for some surveys is continuing and results have yet to be finalized.
Diamondback moth
Diamondback moth (DBM) is a pest of concern in many brassicaceous crops, including canola and mustard. The DBM survey utilizes pheromone traps that attract male moths. During the monitoring period, traps are examined weekly and the number of DBM caught is reported by cooperators. DBM pheromone traps were set up in late April and monitored until late July. Traps are placed early in the growing season since the main objective of this survey is to detect when populations arrive from the U.S. and Mexico. The population of DBM in Saskatchewan relies on the arrival of adults on wind currents since DBM do not overwinter in significant numbers in Saskatchewan. The early arrival of adults in Saskatchewan favours a more significant population buildup during the growing season. Although we monitor the arrival of adult male moths, it’s the larval stage of this pest that causes crop loss. Individual field scouting for the larval stage of this pest is necessary to determine if an insecticide application is warranted.
For 2024, the DBM monitoring period continued with an extended monitoring period of 12 weeks, adopted in 2022. Traps began to capture male moths at the end of the first week of reporting (early May). Overall, DBM cumulative trap counts were low with some moderate cumulative catches scattered across the province as depicted in the map.
Bertha Armyworm
The bertha armyworm (BAW) survey is another pheromone trap-based survey. Bertha armyworm larvae can feed on many crop species including canola, mustard and alfalfa. BAW overwinter as pupae below the soil surface and emerge as moths in spring. Traps were placed in canola crops during the growing season based on when pupal development is near completion (at least 80 per cent pupal development) to help ensure traps are up when BAW adults (moth stage) emerge. Emergence is highly dependent on temperature. This year, a majority of the pheromone traps were placed in early June throughout the canola-growing regions of the province and were monitored until early August. Counts were submitted weekly by cooperators and used to create risk maps based on cumulative moth catches. These maps are released weekly during the monitoring season and are used to determine regional risk.
The average cumulative trap catches were low in 2024; as depicted in the map, a vast majority of sites had cumulative counts of 300 or fewer BAW moths caught. Monitoring male BAW moths (adults) provides an indication of risk, but adults do not cause crop damage.
It is critical to scout individual fields and apply economic thresholds based on BAW larvae counts. Decisions to spray an insecticide should not be based on regional risk maps.
Pea Leaf Weevil
The pea leaf weevil survey was conducted early in the growing season. The target timing for this survey is when field pea crops are in the second to sixth true leaf stage. It is important to note that although faba beans are not assessed during the survey, faba bean is a primary host in addition to field peas. At each field, the surveyor examines plants for U-shaped foliar notching damage caused by adult pea leaf weevils. The number of notches per plant is averaged per field and used to generate the 2024 Saskatchewan Pea Leaf Weevil Survey Map.
This map provides an indication of regional risk for 2025. Notching damage observed during the 2024 survey was significantly lower compared to 2023. In 2024, the survey was expanded further north. The 2024 map depicts higher notching damage observed in areas in the east and a small area in the northwest near Paynton. Although foliar notching damage assessment is a useful method to monitor this pest, notching damage itself usually does not cause significant yield loss. The larval stage of pea leaf weevil causes the most significant crop damage. Larvae feed on nitrogen-fixing nodules, reducing the amount of nitrogen the crop can fix.
Registered insecticidal seed treatments are the most effective chemical control option for reducing loss due to this pest.
Grasshopper Survey
The grasshopper survey began in July and continued into September. Ditches were assessed for adult grasshopper densities. The targeted timing for this survey is when most grasshoppers are at the adult stage since they are considered the greatest reproductive threat to next year’s population. The survey is conducted by assessing sites once, with a target of surveying four sites per rural municipality.
The 2024 Grasshopper Survey Map illustrates grasshopper densities based on the annual survey. Cool, wet conditions in spring slowed the emergence of grasshoppers (which overwinter as eggs) and development. During the survey, high grasshopper populations were observed in areas in west-central, central, south-central, and southwest Saskatchewan.
The survey results can be viewed in the included 2024 Grasshopper Survey Map. It is important to scout for this pest in all areas of the province. The map is used for regional risk analysis only, not local infestations; high local populations may not be reflected in this map. Many factors, including climatic conditions, will dictate grasshopper populations in 2025.
Wheat Stem Sawfly
In 2024, a wheat stem sawfly (WSS) survey assessed fields for plants cut by sawfly larvae. At least one WSS-infested stub was found in all but five fields. As depicted in the map, high levels of stem cutting caused by WSS were observed in the west-central, southwest, south-central, and Lake Alma (southeast) areas. Spring and Durum wheat are primary hosts of this pest, although several other grassy plant species can support the development of WSS.
The larval stage of this pest is the stage that causes crop damage. The larvae feed on the pith of the host plant which causes significant reductions in both yield and quality. Eventually, the larva will cut a groove completely around the inside of the stem which causes the stem to weaken and become susceptible to lodging. This results in plants lying on the ground which cannot be harvested.
There are no registered pest control products for WSS control. Research has shown that chemical control is not an effective management method for this pest. The most effective management approach is to plant resistant cultivars or crop species.
Cabbage Seedpod Weevil
The cabbage seedpod weevil survey (CSPW) is conducted when canola and brown/oriental mustard crops are flowering. Canola and brown/oriental mustard are the primary host crop species for CSPW. Yellow mustard is resistant to CSPW. During the survey, 25 sweeps are taken per field to catch the adult weevils. The sweep samples are then assessed for CSPW and the total number of CSPW caught per field is determined. The total count for each field is represented on the Cabbage Seedpod Weevil 2024 Survey map. High populations were found at some sites in the southwest near Swift Current and some moderately high populations were found in south-central. Overall populations were low to moderate in the province.
Like all other survey maps, the map should be used for regional risk assessment. Individual fields must be scouted for adult weevils before deciding to apply an insecticide. Both the larval and adult stages of CSPW feed on the host crops, with the larvae feeding being the most damaging. The adults feed on flower buds, young flowers early, and can also feed on pods of late canola crops. Larvae feed on developing seeds which can lead to significant yield loss. Each larva consumes about five seeds during development. The infested pods are also susceptible to fungal infection as the feeding damage serves as an entry point for pathogens.
Additional Survey Information
Although the season for field work has passed, work associated with surveys continues. Soil cores collected during the wheat midge survey are also analyzed well after the growing season. The soil samples collected are analyzed for the presence and abundance of wheat midge larval cocoons and levels of parasitism by the wheat midge’s natural enemy, Macroglenes penetrans. The viable (unparasitized) larval cocoon counts will be used to create the 2024 wheat midge map.