BIOACTIVITY OF GRANULAR INSECTICIDES APPLIED AT PLANTING-TIME FOR CONTROL OF MEXICAN CORN-ROOTWORM (COLEOPTERA, CHRYSOMELIDAE) LARVAE

The bioactivity of soil insecticides applied at planting time to prote ct roots of corn from feeding damage by larvae of the Mexican corn roo tworm, Diabrotica virgifera zeae Krysan and Smith, was established by removing soil samples from field plots biweekly from time of applicati on until samples no longer induced larval mortality. This study was co nducted for two consecutive years that were distinctly different. The first year, virtually no rainfall occurred from time of insecticide ap plication through the larval feeding period, and the topsoil remained very dry. During the second year, ample rainfall occurred during this period and topsoil moisture was abundant. Even though the bioactivity of each insecticide during the ''dry year'' was significantly longer, all insecticides had significantly shorter periods of bioactivity in t he alkaline soils from Texas than for the same formulations in slightl y acidic midwestern soils. Results from this study may provide an expl anation for the highly irregular performance of soil applied insectici des and lack of target insect control in south and central Texas.