<p>Thrips (Thysanoptera: Thripidae) are sucking pests that damage a wide range of crops worldwide. Chemical insecticides are the main strategy for their control, but species can vary in susceptibility or develop resistance. Here, we developed and validated a bioassay method (dip test) using soybean (<i>Glycine max</i> [L.] Merr.) leaf discs to evaluate the susceptibility of thrips to insecticides. We compared the susceptibility of <i>Caliothrips phaseoli</i> (Hood, 1912) to acephate and spinetoram assessed using this method with results of the IRAC Susceptibility Test Method 010, in which bean pod sections are dipped into an insecticide solution. Concentration–mortality data from both methods fit the probit model, with mortality lines showing similar parameters (intercepts and slopes) and mortality increasing with the increase of insecticide concentrations. The lethal concentrations (LCs) of acephate were similar for both methods, whereas spinetoram showed higher LCs in the proposed method. Nonetheless, mortality at field label doses of both insecticides was consistently high (96.5–100%) in both methods. In conclusion, acephate and spinetoram exhibited high lethality against <i>C. phaseoli</i>, and the proposed bioassay method, due to its simplicity, reliability, and statistical robustness, represents a reliable alternative to the IRAC method for assessing susceptibility of thrips species to insecticides.</p>