Comparative Susceptibility of Liposcelis bostrychophila Badonnel and Liposcelis decolor (Pearman) (Psocoptera: Liposcelididae) to Spinosad on Wheat

In 2005 the United States Environmental Protection Agency registered spinosad as a grain protectant at 1 μg(AI)/g. The effectiveness of spinosad at 1 μg(AI)/g against many beetle and moth pests of stored grains is well established. In recent years, several psocid species have emerged as economically important pests in various stored-product environments in many countries, including the United States. Information on susceptibility of stored-product psocid species to spinosad is basically unknown, especially in the United States. In laboratory tests, we evaluated adult mortality and adult progeny production of two major stored-product psocid species, Liposcelis bostrychophila Badonnel and Liposcelis decolor (Pearman), exposed to untreated and spinosadtreated wheat. Adults of the two species were exposed to 12 spinosad concentrations ranging from 0 to 16 μg(AI)/g of wheat for 7 d to determine susceptibility and to estimate median lethal concentrations (LC50). Adults of each species were exposed to 0, 0.25, 0.5, and 1 μg(AI)/g for 56 d to determine adult progeny production. In general, at all spinosad concentrations, L. bostrychophila was less susceptible than L. decolor. At 1 μg(AI)/g, L. bostrychophila adult mortality was only 33% whereas that of L. decolor was 88%. The LC50 value for L. bostrychophila was 4.49 μg(AI)/ g and for L. decolor it was 0.22 μg(AI)/g. Spinosad at 1 μg(AI) /g completely suppressed adult progeny production of L. decolor; at this spinosad rate reduction in progeny production of L. bostrychophila was only 91% relative to production on untreated wheat. Our results suggest that spinosad applied to stored wheat at the labeled rate of 1 μg(AI)/g is effective against L. decolor but not L. bostrychophila.