The efficacy of linalool, a major component of freshly-milled Ocimum canum Sims (Lamiaceae), for protection against postharvest damage by certain stored product Coleoptera

Abstract

Linalool was present at 8.6 ± 0.9 mg/g in the dried leaves of Ocimum canum Sims, an annual mint used in Rwanda to protect against postharvest insect damage. Direct exposure of adults of Zabrotes subfasciatus (Bohem.) to milled, dried O. canum leaves resulted in 100% mortality of males and 50% mortality of females after 48 hr. Dose-response curves for linalool were completed with adult Z. subfasciatus, Acanthoscelides obtectus (Say), Rhyzopertha dominica (F.), and Sitophilus oryzae (L.) using a filter paper bioassay. The LC 50 values were: 428 μg/cm 2 for Z. subfasciatus; 405 μg/cm 2 for A. obtectus; 428 μg/cm 2 for R. dominica; 427 μg/cm 2 for S. oryzae. Knockdown was occasionally followed by recovery at doses less than the LC 50 for all species. There are significant differences in the LC 50 and LT 50 values for male and female Z. subfasciatus. At the lower dosages hyperactivity rarely preceded moribundity and mortality where these occurred, while at higher dosages hyperactivity occurred soon after initial exposure and preceded imminent death. A concentration increase from 250 to 750 μg/cm 2, representing a tripling of dosage, spanned th 10–100% response mortality for all species at 24 hr. Air-exposure of linalool-treated papers (500 μg/cm 2) for up to 24 hr significantly decreased toxicity to both sexes of Z. subfasciatus. Quantitative analysis showed the only significant decrease in the amount of linalool to occur after 0.25 hr, and this did not fully correlate with the resulting decrease in efficacy against both sexes of Z. subfasciatus. The results are discussed in terms of the efficacy of using O. canum for the protection against loss due to insects in the traditional food storage systems of Rwanda.