Economic threshold for lesser clover leaf weevil in red clover
The economic threshold when insecticide application should be initiated was calculated at 6 larvae per 10 shoots at the low market price, 4 larvae at the average price and 3 for the high price. A decision matrix was developed based on a full range of treatment costs and market seed prices to allow growers to use their own treatment and market prices to guide insecticide application.
Red clover seed production is an economically important crop generating in excess of 2.8 million dollars in 2023 to 2024 in Saskatchewan. In seed production, red clover relies on insect pollination, and this means that controlling insect pests such as the lesser clover leaf weevil, (Hypera nigrirostris Fab.) must try to preserve pollinator populations.
However, economic thresholds for lesser clover weevil had not been established in western Canada, resulting in insecticides sometimes being applied unnecessarily with the potential to harm pollinators and other beneficial insects, and speed the development of insecticide resistance.
A research study was conducted in 2023 and 2024 to determine the relationship between lesser clover weevil density and red clover seed yield; calculate the economic injury level, and develop economic threshold levels to guide insecticidal application.
On-farm trials were conducted in 10 red clover commercial fields that were in their second year of growth. Six field sites in northeastern Saskatchewan were selected in 2023, and four were selected in 2024. The fields contained the single-cut variety, “Altaswede” and were managed under typical agronomic practices for red clover seed production. Environmental conditions were near normal in both years.
In 2023, lesser clover weevil treatment levels were 0, 5, 10, 15 larvae per 10 red clover shoots, maintained by insecticide applications, and a “maximum” density. The zero-control treatment maintained larval populations as close to zero as possible. The maximum density was unsprayed and allowed larval populations to increase naturally.
Based on the results of 2023, the treatment levels were refined for the 2024 field trials to 0, 2, 4, 6, and 8 larvae per 10 shoots to allow more accurate analysis of the data.
Decis 100 EC (deltamethrin), a commonly used contact pyrethroid insecticide (IRAC Group 3A), was applied at the maximum recommended rate of 0.25 litre/ha in 200 liter/ha of water.
Yield declined with increasing larval densities
As lesser clover weevil densities increased, red clover seed yield declined linearly in both years. The estimated yield loss was calculated to be 5.83 lbs/ac per larva per 10 shoots (6.55 kg/ha/ per larva per 10 shoots). This indicates that a moderate infestations of four to six larvae per 10 shoots could reduce seed yield by 22.25 lbs/ac to 35.6 lbs/ac (25 to 40 kg/ha).
The economic injury level was defined “as the lowest pest density at which the cost of control equaled the value of crop loss, while the economic threshold was defined as the larval density at which control measures should be initiated to prevent populations from reaching the EIL.”
The cost of control included the insecticide and application costs in 2024/25 , and was calculated to be $21.49/ac ($53.08/ha).
Three market values for red clover seed were derived from Alberta Agriculture Financial Services Corporation averaged over a five year period from 2020 to 2024. Three prices were compared to develop decision tables to incorporate variability market prices. The market prices used in development of the thresholds were $0.20/lb ($0.44/kg; low), $0.31/lb ($0.68/kg; average), and $0.40/lb ($0.89/kg; high).
Economic injury levels were estimated at 6.8 larvae per 10 shoots for the low market price, 4.4 larvae for the average price, and 3.4 larvae for the high market price. The economic threshold when insecticide application should be initiated was calculated at 90 per cent of these thresholds, and were 6 larvae per 10 shoots at the low market price, 4 larvae at the average price and 3 for the high price.
Decision application tables developed
The researchers went further and developed decision tables based on a full range of treatment costs and market seed prices. This allows growers to use their own treatment and market prices to assess whether insecticide application was economically justified, neutral, or not justified.
Decision tables for ET determinations of H. nigrirostris larvae in red clover seed production
Source: Irvine et Prager. 2026
Tables show whether an insecticide treatment is economically justified across a range of realized red clover seed prices ($2.00 to 5.00/kg) and treatment costs ($30 to 75/ha) for observed larval densities of 2 to 7 larvae per 10 shoots. Cells are colour-coded as follows: green (Y), treatment justified; yellow (Y*), treatment equivocal; red (N), treatment not justified. These decision tables help growers coordinate their management choices, considering observed larval densities alongside current market conditions and input costs.
The researchers have developed a Red Clover Economic Threshold calculator that includes some of these tools, with more to come. It is currently hosted at: https://entomologist.ca/tools/red-clover/ as well as our lab website:https://research-groups.usask.ca/entomology/extensionmaterials.php.
The calculator is an interactive decision-support tool designed for red clover seed growers managing the lesser clover leaf weevil, and allows growers to estimate their economic injury levels and economic threshold levels.
This research was supported by funding from the Government of Saskatchewan through the Agriculture Development Fund (ADF 20220261) and the Saskatchewan Forage Seed Development Commission.
Irvine, Jeremy R., and Prager, Sean M. Economic thresholds for Hypera nigrirostris (Coleoptera: Curculionidae) in red clover seed production from on-farm trials. Journal of Economic Entomology, 2026, 119(1), 229–236 DOI: https://doi.org/10.1093/jee/toaf290
Photo courtesy of Jeremy Irvine
