Toxicity and repellent activity of monoterpene enantiomers to rice weevils (Sitophilus oryzae)

BACKGROUND The rice weevil, Sitophilus oryzae (L., 1763) (Coleoptera: Curculionidae), is a stored grain/seed pest of several crops. Botanicals represent an option to manage this pest, especially those with high toxicity determined by its structure and physicochemical properties and low residue left behind on treated grains/seeds. Enantiomers are bioactive molecules in organic processes. The aim of this study was to evaluate the insecticidal activity of two enantiomeric forms of the monoterpenes citronellal, limonene, linalool, menthone and alpha-pinene against S. oryzae adults through contact, fumigant and repellent assays. RESULTS All results were compared with absolute acetone as the negative control and those from contact assays also with malathion as the positive control. (S)-(-)-linalool was the most toxic by contact to S. oryzae and (S)-(-)-menthone through fumigation, while the two enantiomeric forms of menthone and alpha-pinene were the best repellents. CONCLUSIONS The high toxicity of only one of the two monoterpene enantiomers showed the importance of form and percentage of enantiomer in the commercial product, which affect the success of the product to controlling S. oryzae.

Automated summary

The study evaluated the insecticidal activity of two enantiomeric forms of monoterpenes against Sitophilus oryzae adults. The results showed that (S)-(-)-linalool was the most toxic by contact, (S)-(-)-menthone was the most toxic through fumigation, and the enantiomeric forms of menthone and alpha-pinene were the best repellents. These findings highlight the importance of the form and percentage of enantiomers in commercial products for controlling S. oryzae.