Acute, sublethal, antifeedant, and synergistic effects of monoterpenoid essential oil compounds on the tobacco cutworm, Spodoptera litura (Lep., Noctuidae)

Monoterpenoids (terpenes and biogenically related phenols) commonly found i n plant essential oils were tested for acute toxicity via topical applicati on to tobacco cutworms (Spodoptera litura Fab.). The most toxic among 10 su ch compounds was thymol (LD50 = 25.4 mug/larva)from garden thyme, Thymus vu lgaris. 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 aft er administration. Feeding deterrence was determined using a cabbage leaf d isk choice test. The most deterrent compound was thymol, with a DC50 Of 85. 6 mug/cm(2) leaf disk area. Because minor constituents in complex essential oils have been suggested to act as synergists, binary mixtures of the comp ounds were tested for synergy vis a vis acute toxicity and feeding deterren ce, trans-Anethole acted synergistically with thymol, citronellal, and alph a -terpineol, in terms of both acute toxicity and feeding deterrence. On th e basis of these findings, several complex mixtures were developed and test ed as leads for effective control agents. Candidate mixtures demonstrated g ood Synergistic effects. The observed LD50 of mixture 3 was 40.6 mug/larvae compared to an expected value of 74.6 mug/larvae. The result of this resea rch is a proprietary product suitable for commercial production.