Insecticide-retentive carriers. 2. Fenitrothion-impregnated clays

Fenitrothion as a spray or in 1 of 12 claybased carriers was admixed with wheat of 11.5% moisture and held for 142 weeks at 25�C for chemical determination and assays against added adults of Tribolium castaneum (Herbst). The period of protection correlated with the retention of fenitrothion in the carrier; i.e. correlated with the ratio of fenitrothion in carrier to that in wheat plus carrier. Fenitrothion, applied at 6 mg/kg in retentive carriers such as bentonites or halloysites, gave at least 117 weeks of complete protection against T. castaneum and against Sitophilus oryzae (L.) for 142 weeks. Minimum effective dose of fenitrothion to give 3 months of pro- tection against S. oryzae, S. granarius (L.), T. castaneum and Rhyzopertha dominica (F.) was determined as 4 mg/kg in bentonites, 8 mg/kg in halloysites compared with more than 20 mg/kg of fenitrothion applied as a spray. Some insight into the mode ofaction of retentive carriers was obtained by exposing adult T. castaneum to wheat coated with carriers containing either 0 or 1% w/w fenitrothion. In the former case, insects kept themselves free of carrier by preening. In the latter case, however, the insects became coated with fenitrothion-impregnated carriers, with a heavy concentration in and near mouthparts, suggesting lack of coordination in preening.