Abstract

RUSSO, Romina; GAMUNDI, Juan C.  and  ALZOGARAY, Raúl A.. Evaluation of pyrethroids toxicity in a laboratory strain and a fi eld population of Rachiplusia nu (Lepidoptera: Noctuidae) using two bioassay techniques. Rev. Soc. Entomol. Argent. [online]. 2012, vol.71, n.1-2, pp.115-123. ISSN 0373-5680.

Soybean is the most important crop in Argentina and Rachiplusia nu (Gueneé) (Lepidoptera: Noctuidae) is one of its main pests. In this study, the toxicity of five pyrethroids applied topically and by exposure to films on filter paper was evaluated on third instar larvae from a laboratory strain and a field population of R. nu. Four cyanopyrethroids and one non-cyanopyrethroid (permethrin) were tested. All cyanopyrethroids showed the same order of increasing toxicity, regardless of the form of application and origin of the larvae: cypermethrin < ?-cyhalothrin < deltamethrin < ß-cyfluthrin. Knock down Dose 50% and Knock down Time 50% values increased as a function of the solubility of cyanopyrethroids in water. Permethrin showed a different behavior: it was the most toxic insecticide for the laboratory strain when applied topically, but the least toxic when larvae were exposed to filter papers. In general, the pyrethroids were more toxic for laboratory larvae than for the field ones. After calculating Resistance Factor (RF) values, low-moderate resistance to permethrin, cypermethrin and ?-cyhalothrin was observed in the experiments with topical application. However, exposure to films on filter papers failed to detect resistance. There was not correlation between the RF values obtained by both methods. These results suggest that the population of R. nu studied here has low-moderate resistance to some pyrethroids, and that topical application is a more appropriate method for quantifying resistance than exposure to insecticide films on filter paper.

Keywords : Soybean looper; Soybean pests; Insecticide resistance.

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