Comparison of selection rates of soybean podworm (Lepidoptera: Noctuidae) to key nsecticides using mixture versus rotation strategies

Management strategies for soybean podworm (SPW) are dependent on GMO traits and a handful of foliar insecticides. Consequently, products with the same MOA must often be repeated in the same year to maintain control. To examine differences in tolerance of SPW to various management protocols, we induced and reversed tolerance to insecticides and mixtures in a lab susceptible colony.  A low dose (LC₂₅) of four diet-incorporated treatments were administered every 4 weeks to SPW using diet-incorporated bioassays.  The treatments were: (1) chlorpyrifos (Lorsban®) alone, (2) lambda-cyhalothrin (Warrior II®) alone, (3) rotations of chlorpyrifos, and lambda-cyhalothrin (4) a pre-mix of chlorpyrifos and lambda-cyhalothrin (Cobalt Advanced®).  Concentration-mortality relationships for the bioassays were estimated with PoloPlus (LeOra Software 2005). Treatments were repeated until the mortality caused by the LC₈₅ dose reached ~50%.  At that point, concentration-response bioassays were repeated to assess differences in susceptibility from the baseline dose response curves originally established for the individual insecticides and mixture. Lethal dose ratios (Tolerant LC₅₀/Susceptible LC₅₀) or those differences between tolerant and susceptible individuals were evaluated for each chemical.  Chlorpyrifos+lambda cyhalothrin was first to induce tolerance in 5 generations with a 2.73-fold difference, whereas the rotation of chlorpyrifos+lambda cyhalothrin was last to induce tolerance (7 generations, 10.55-fold difference). Susceptibility was reversed in 4 generations by chlorpyifos+lambda cyhalothrin, 2 generations by chlorpyrifos, 2 generations by lambda cyhalothrin, and 2 generations by the rotation. Follow up research will determine the biochemical mechanisms responsible for these tolerance differences.