Influence of essential oils on toxicity and pharmacokinetics of the plant toxin thymol in the larvae of Trichoplusia ni

The present study was undertaken to determine the influence of some major constituents of plant essential oils on the toxicity and fate of thymol, a phenolic monoterpene from garden thyme (Thymus vulgaris L.) (Lamiaceae), following topical administration to the cabbage looper, Trichoplusia ni (Hubner) (Lepidoptera: Noctuidae). Tested individually, trans-anethole (LD sub(50) = 71.2 mu g/larva) and methyl salicylate (93.6 mu g/larva) were the most toxic, whereas phenylethyl propionate (555.8 mu g/larva) and mineral oil (2279.3 mu g/larva) were the least toxic. However, when coadministered, mineral oil was the most synergistic (synergy ratio, SR = 25). A proprietary blend of essential oils (A4 Blend) was strongly synergistic (SR = 9), as was trans-anethole (SR = 5), while the majority of the other compounds tested yielded SRs of 2-3. Based on experiments using tritiated thymol as a radiotracer, the fate of this plant toxin within 60 min of topical administration varied widely with the essential oil carrier. Some carriers facilitated rapid disappearance of thymol from the insects' integument and rapid appearance internally, whereas others prolonged the tenure of thymol on the integument and delayed entry into the insect. The highest internal levels of radioactivity were observed when phenylethyl propionate was the carrier (47% of the topical dose), the lowest when rosemary oil was the carrier (9%). The highest proportion of the dose was excreted when eugenol was the carrier (20% of the topical dose), the lowest when mineral oil was the carrier (4%). Overall, there are no apparent trends linking the fate of thymol in the different carrier oils to the synergies observed or to the relative polarities of the carriers