Written collaboratively by Adam Varenhorst, Philip Rozeboom, Brad McManus, Stephen Robertson, and Patrick Wagner.
Now that the August 31 deadline for the application of products containing malathion for red sunflower seed weevil management has passed, it is time to determine if the product was effective in 2025. South Dakota State University entomologists have received varying reports on this topic. In some cases, we received reports that the malathion products applied at the labeled rate of 1.6 pints per acre (25.6 oz per acre) were effective. However, we also received reports that the insecticide had little to no effect on some red sunflower seed weevil populations.
In some cases, a product containing malathion was tank mixed with a product containing lambda-cyhalothrin with the malathion rate slightly lower than what is present on the 24c Special Local Need Registration. As a reminder, we do not recommend using products containing a pyrethroid active ingredient for red sunflower seed weevil management in South Dakota. In addition, for the five products that are included on the 24c registration, the only labeled rate is 1.6 pints per acre. For 2026, this is a reminder that you must follow the labeled rate of products, and when a range of rates is not provided you must apply the products at the rate that is provided. Any deviations from that rate are not lawful.
Due to the field failure reports that we received in 2025, we will be conducting a resistance evaluation in 2026. If you experienced a field failure when using a product containing the active ingredient malathion to manage red sunflower seed weevil, please contact Adam Varenhorst. This will help us to determine where to focus our sampling efforts.
