Control of alfalfa weevil infesting alfalfa, 2022

Alfalfa weevil is a major pest of alfalfa in Delaware. In 2022, chlorpyrifos tolerances were canceled reducing available insecticides for alfalfa weevil management. The purpose of this study was to evaluate the efficacy of select insecticide treatments in the control of alfalfa weevil. Treatments were applied to an existing stand of alfalfa established in 2016 at a cooperator farm located in Houston, Delaware (Kent County). Plot size was 9 ft by 25 ft. Treatments were replicated 4 times in an RCB design. Insecticides were applied with a CO₂-pressurized backpack sprayer with a 9 ft boom equipped with 6, XR11003 nozzles calibrated to deliver 18.7 GPA at 26 PSI on 8 April 2022. The organosilicone adjuvant Kinetic (0.5% v/v) was included in all broadcast treatments. Treatment efficacy was assessed by sampling from each plot 15 random alfalfa stems severed at the crown and counting the number of alfalfa weevil present. Samples were collected at −4, 5, 12, and 28 days after treatment (DAT). All data were analyzed using ANOVA and Tukey–Kramer honestly significant difference (HSD) means separation using SAS JMP. The Plinazolin and Endigo ZCX plots were destroyed after sampling.

All treatments significantly reduced the number of alfalfa weevils 5 DAT compared to the untreated check (Table 1). All treatments except Sivanto Prime reduced the number of alfalfa 12 DAT compared to the untreated check. Plots treated with Plinazolin had fewer weevil larvae than those treated with Warrior II. At 20 DAT, all treatments except Sivanto Prime reduced the number of alfalfa compared to the untreated check plots, with the greatest reductions in the Plinazolin and Steward-treated plots.¹

Footnotes