Time matters: delayed toxicity of pirimiphos-methyl on Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) and its effects on efficacy estimation of residual treatments

Three time-related toxicity parameters of pirimiphos-methyl were compared using a common pest of stored cereal and grain products, Tribolium castaneum (Herbst). The effective exposure time sufficient to knock-down half of the beetles was much shorter than the median knock-down time from a continuous exposure test (8.6 min and 80 min, respectively, at the same insecticide concentration). The difference between these two measures can be attributed to delayed toxicity of pirimiphos-methyl. Pirimiphosmethyl was effective against T. castaneum at a low application rate of 0.0047 g a.i. m(-2), when beetles were exposed on the treated surface for short time intervals. The median post-exposure knock-down time for animals placed for 20 min on insecticide-treated surface and then kept on an untreated surface was 180 min. Our results suggest that actual effective exposure time might be a more realistic efficacy measure of pirimiphos-methyl at a given dose than LT50 (median lethal time for continuous exposure), and that the timing of the toxic effects should be included more often in residual toxicity testing. (C) 2010 Elsevier Ltd. All rights reserved.