Comparison of Bioassays Used to Determine Onion Thrips (Thysanoptera: Thripidae) Susceptibility to Spinetoram.

Onion thrips (Thrips tabaci Lindeman) is one of onion's most damaging insect pests and has a history of developing resistance across insecticide classes. The susceptibility of T. tabaci populations to insecticides can be determined using laboratory bioassays. Three types of bioassays have been documented in the literature specifically for use with T. tabaci: vial assay (contact only), feeding assay (ingestion only), and leaf-dip assay (contact + ingestion). The objectives of this study were to 1) compare insecticide susceptibility levels of a T. tabaci population using these three assays and 2) determine which bioassay's results were most similar to those generated from exposing thrips to whole plants treated with insecticide. All experiments were conducted using a colony of T. tabaci known to be susceptible to insecticides and all were evaluated for their susceptibility to spinetoram (Radiant SC). Results indicated that 1) each bioassay generated a unique concentration-mortality relationship and LC50 value (0.01, 0.03 and 1.6 ppm for leaf-dip, vial, and feeding assays, respectively), and 2) all bioassays overestimated the susceptibility of the population relative to the whole-plant assay (LC50 = 5.3 ppm). Attributes of these bioassays are discussed relative to their future use in insecticide resistance monitoring programs for T. tabaci.