Alfalfa weevil control is problematic in Kansas for several reasons, but probably first and foremost is the weather. Alfalfa weevils are cool weather insects, thus feeding voraciously in spring immediately after hatching for 2-3 weeks, depending upon temperatures. The larvae actively feed 24/7 when the temperatures are above 45°F. However, the Kansas weather in March/April is often windy with rain/snow/sleet/hail – or a combination of all, which makes foliar applications of insecticides difficult because fields may be too muddy, or it’s too windy, but the alfalfa weevil larvae are still feeding 24/7. However, weather occasionally helps control weevils, as it did in 2017 and 2018 and has apparently so far this year. Temperatures in the mid-to-lower 20s for a couple of hours will actually kill small larvae feeding in the terminals.
Monitoring several fields in north central Kansas on March 16-20 indicated small larval feeding, causing the characteristic pinprick holes in leaves and ragged terminals, which indicated hatching – but no live larvae were found during this time, 16-20 Mar. One live adult was collected (see Figure 1), but no larvae. However, adults are still depositing eggs in stems, and thus, alfalfa weevil monitoring should continue. At least in north central Kansas, the recent cold weather seems to be helping control weevils. For more information about alfalfa weevil management, please refer to the 2024 KSU Alfalfa Insect Management Guide.
Figure 1. Adult alfalfa weevil and leaf feeding damage. Photo by K-State Entomology.
Some factors other than weather that affect alfalfa weevil control include the timing and gallonage (or carrier) used during application. See the results from the 2023 gallonage study shown in Figure 2 using 8 gallons H2O/a vs. 15 gallons H2O/a with other factors the same.
Figure 2. Alfalfa weevil and gallonage trial. (PD = plants defoliated)
Jeff Whitworth, Entomology Extension Specialist
jwhitwor@ksu.edu
Tags: insects alfalfa alfalfa weevils