LABORATORY ASSESSMENT OF THE RESIDUAL TOXICITY OF COMMERCIAL FORMULATIONS OF INSECTICIDES TO ADULT ORYZAEPHILUS-SURINAMENSIS (COLEOPTERA, SILVANIDAE) EXPOSED FOR SHORT-TIME INTERVALS

Adult Oryzaephilus surinamensis from an insecticide resistant and a susceptible strain were exposed for fixed periods of time (within the range 0.5, 1, 2, 4, 8, 16, 24 and 32 min; 1, 2, 4, 8, 16 and 24 hr) to filter papers treated with commercially formulated insecticides of fenitrothion, chlorpyrifosmethyl, pirimiphos-methyl or etrimfos at a dose of 250 mg active ingredient per m2. The emulsion concentrate (ec) and wettable powder (wp) formulations of fenitrothion produced a delayed toxic response, but were the most toxic insecticide formulations to the laboratory susceptible strain, the ET99 (exposure time) being 0.54 and 0.38 hr respectively at the 24 hr assessment period. Etrimfos 50% ec was the most toxic formulation to the 0213 resistant strain, with an ET99 value of 6.9 hr, whereas the other formulations required much longer exposures, the ET99 values being > 16 hr. The 0213 strain was resistant to all the formulations tested, with a high level of resistance to fenitrothion ec (x 31) and wp (x 162) at the ET99 level. Wettable powder formulations of fenitrothion and chlorpyrifos-methyl required shorter exposures than their respective ec formulations for kill of the susceptible strain. The implications of these results on the survival of O. surinamensis in grain stores are discussed.