A sensitive bioassay for spider mite (Tetranychus urticae) repellency: A double bond makes a difference

Choice bioassays were used to determine repellency of homologous n-alkanes (C ₈H ₁₈-C ₂₁H ₄₄) to spider mites. When tested at 400 μg/cm ², the C ₁₅-C ₁₉ alkanes were highly repellent; the C ₁₆ n-alkane, n-hexadecane, was most repellent. Subsequently the EC ₅₀ values, the concentration at which 50% of the mites were repelled, were determined for the C ₁₅-C ₁₉ n-alkanes and their analogous 1-n-alkenes (C ₁₅H ₃₀-C ₁₉H ₃₈). The EC ₅₀ value for 1-heptadecene, the C ₁₇ 1-n-alkene, was the lowest observed. Except for the 17-carbon hydrocarbons, the EC ₅₀ values for the n-alkanes were less than those for their analogous 1-n-alkenes. Depending on the compounds evaluated, there was as much as a six-fold difference of repellency between an n-alkane its analogous 1-n-alkene. Thus, the bioassay has sufficient sensitivity to detect behavioral differences associated with the presence or absence of a single double bond. The EC ₅₀ values for the most repellent hydrocarbons were similar to that reported for 2,3-dihydrofarnesoic acid, a naturally occurring repellent isolated from trichome secretions of a wild tomato, Solanum habrochaites, and also were similar to concentrations used to evaluate arthropod repellents. Consequently, this bioassay may be useful for providing a better understanding of the relationships between structures and activities of natural products that are repellent to spider mites.