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Using a lethality index to assess susceptibility of Tribolium confusum and Oryzaephilus surinamensis to insecticides

Paraskevi Agrafioti, Christos G. Athanassiou, Thomas N. Vassilakos, Frank H. Arthur
Plos one 2015 v.10 no.11 pp. e0142044
Oryzaephilus surinamensis, Tribolium confusum, adults, antennae, bioassays, chlorfenapyr, concrete, cypermethrin, equations, fipronil, insecticide resistance, metals, mortality, pirimiphos-methyl
We evaluated the knockdown effect caused by four insecticides: alpha-cypermethrin, chlorfenapyr, pirimiphos-methyl and fipronil against Tribolium confusum and Oryzaephilus surinamensis adults. Furthermore, for the same species and insecticides, we developed a “lethality index”, to assess knockdown patterns. For this purpose, bioassays were conducted on concrete and metal surfaces. Adults of the tested species were exposed on both surfaces treated with the above insecticides at two doses (low and high). Knockdown assessment was done after 15, 30 and 60 min of adult exposure on the treated surfaces. Also, after 1, 3, 5, 7 and 14 d of exposure, a lethality index was calculated with an equation resulting in values from 0 to 100, where 100 indicated complete mortality and 0 complete survival. For the lethality index calculation, each adult exposed on the surface was ranked from 0 to 4, based on a scale as follows; 0: adults moved normally, 1: adults were knocked down, but were able to walk for short intervals, 2: adults were knocked down and unable to walk, but with visible movement of antennae etc., 3: adults were knocked down, with very minimal movement of the tarsi and the antennae and 4: adults were dead (no movement). Knockdown of adults immediately after exposure (15-60 min) was higher for pirimiphos-methyl followed by alpha-cypermethrin, for both dose rates tested and species, but only on the metal surface. The lethality index was high and near 100 for all insecticides after 5d of exposure for O. surinamensis, while for T. confusum adults the lethality index was consistently low for alpha-cypermethrin, suggesting that that recovery occurred. Among the insecticides examined here, chlorfenapyr was the only one that was more effective on concrete than on metal, while the reverse was noted for the other three insecticides.