Acute, Sublethal, Antifeedant, and Synergistic Effects of Monoterpenoid Essential Oil Compounds on the Tobacco Cutworm, Spodoptera litura (Lep., Noctuidae)

Abstract

Monoterpenoids (terpenes and biogenically related phenols) commonly found in plant essential oils were tested for acute toxicity via topical application to tobacco cutworms (Spodoptera litura Fab.). The most toxic among 10 such compounds was thymol (LD₅₀ = 25.4 μg/larva) from garden thyme, Thymus vulgaris. The compounds were then tested for sublethal effects, specifically inhibition of larval growth after topical application of low doses. Among 6 compounds tested, an LD₁₀ dose reduced growth by 20% on average 3 days after administration. Feeding deterrence was determined using a cabbage leaf disk choice test. The most deterrent compound was thymol, with a DC₅₀ of 85.6 μg/cm² leaf disk area. Because minor constituents in complex essential oils have been suggested to act as synergists, binary mixtures of the compounds were tested for synergy vis à vis acute toxicity and feeding deterrence. trans-Anethole acted synergistically with thymol, citronellal, and α-terpineol, in terms of both acute toxicity and feeding deterrence. On the basis of these findings, several complex mixtures were developed and tested as leads for effective control agents. Candidate mixtures demonstrated good synergistic effects. The observed LD₅₀ of mixture 3 was 40.6 μg/larvae compared to an expected value of 74.6 μg/larvae. The result of this research is a proprietary product suitable for commercial production. Keywords: Monoterpenes; phenols; thymol; natural insecticides; feeding deterrents; synergy; Spodoptera litura