A comparison of immediate and delayed efficacy of currently used chemical insecticides with bio-based or mineral alternative active substances for stored-grain-insect control

Abstract: A comparative study was performed in treating 50-kg wheat batches either by currently used contact insecticides or by non-synthetic alternative active substances (a.s.) for control of stored-grain pests (Sitophilusoryzae and Rhyzoperthadominica). The first group of a.s. was: cypermethrin (CYP), chlorpyriphos-methyl (CM), deltamethrin (DM), natural pyrethrins (synergized or not), and pyrimiphos-methyl (PM); the second group of a.s. was: methoprene (with CM), diluted neem oil, spinosad (with or without CM), DE, and silicagel + Sodium bicarbonate. Two series of assays were performed: the first series was destined to the evaluation of immediate efficacy of a.s. on the target insects (curative treatment), and the 2nd series was destined to evaluate the duration of complete protection of treated grain against an external infestation (preventive treatment). The trials were performed in accordance to official method of testing grain insecticides (French AFPP-CEB Method N° 106).With the two organo-phosphates (CM and PM), the efficacy of curative treatments was near 100% on S. oryzae, but less than 65% for R. dominica, 7 d after treatment. For the pyrethroids (CYP and DM), complete mortality of the two insects was obtained up to 8 weeks after treatment. Natural pyrethrins and spinosad (alone) effectively controlled R. dominica. In the case of preventive control assay, either organo-phosphates or pyrethroids appropriately controlled S. oryzae for 6 months. R. dominica adult mortality with organophosphates was ≤ 58% immediately after treatment. However, the survivals were not able to reproduce (observed on the two species). Among alternative non-synthetic a.s., only spinosad (dose 1.0 mg/kg) and synergized pyrethrins exhibited a significant efficacy, the former when associated with CM at half registered maximum dose (1.625 mg/kg), spinosad alone at 1.5 mg/kg being active on R. dominica, only. These results should facilitate the selection of appropriate a.s. for the control of identified grain-insect-pest species.