Toxicity of Avermectin B1 Against a Susceptible Laboratory Strain and an Insecticide-Resistant Strain of Plutella xylostella (Lepidoptera: Plutellidae)

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Abstract
Avermectin B, (AVMB,) is a relatively slow-acting (96 h) pesticide whose activity appears to be independent of temperature (15-25°C). By topical application, AVMB, was more toxic than cypermethrin, malathion, or DDT against a standard susceptible laboratory strain of Plutella xylostella L. and a field (Thailand) strain resistant to a number of insecticides. When applied in ethyl methyl ketone (technical grade) or as a diluted emulsified concentrate (113MI2, 1.8% EC), AVMB, was 26-fold more active against the laboratory strain. However, when safflower or Sunspray 6E oil was added to the 113M12 formulation, or when technical AVMB, was applied in an aqueous-acetone mixture with Triton X-100, the difference between the two strains was less (17-, 6-, and 8-fold respectively). Piperonyl butoxide increased the toxicity of technical AVMB, to the field strain by 2-fold. In foliar contact-ingestion and ovicidal tests, the differences in susceptibility to AVMB, (113M12) between the two strains were also less (13- and 3-fold, respectively). These differences in sensitivity to AVMB, may have been due to vigour tolerance of the field strain in comparison with the laboratory strain and to a very low level of cross-resistance in the former. With the laboratory strain, AVMB, was considerably more active than cypermethrin on foliage against fourth instars (100-fold), whereas cypermethrin was 14-fold more toxic than AVMB, against eggs. The ovicidal activity of AVMB1 (113M12) against both strains was enhanced by the addition of oils.