A laboratory bioassay of beet armyworm foliar feeding damage was used to evaluate the persistence of Dimilin^® Insect Growth Regulator (diflubenzuron) and Tracer^® Naturalyte Insect Control (spinosad) when applied at respective rates of 0.062 and 0.067 lb ai/acre to foliage of field-grown cotton. In the first of two evaluations, Dimilin was found to be persistent through 31 days exposure in the field. No activity from Tracer was detected at the first leaf harvest which corresponded to 7 days exposure in the field. In the second evaluation Dimilin again demonstrated persistence while Tracer showed no residual effectiveness at the first leaf harvest, following 6 days exposure in the field. In this trial, bioassays of foliage treated with Dimilin or Tracer immediately before field sampling demonstrated good efficacy by both products.