Fate of Conifer Terpenes in a Polyphagous Folivore: Evidence for Metabolism by Gypsy Moth (Lepidoptera: Lymantriidae)

Abstract

Conifers comprise a substantial portion of the host range of the gypsy moth, Lymantria dispar L. In this study, the effects and fates of the predominant conifer phytochemicals, monoterpenes and diterpene acids, were determined. Gypsy moth larvae were fed physiological levels of three monoterpenes (bornyl acetate, limonene, and myrcene) and two diterpene acids (isopimaric acid and neoabietic acid) for the duration of their second, third, and fourth stadia. Frass was collected daily, and larvae and exuviae were collected after the completion of the stadium. These samples were chemically analyzed for parent terpenes or their metabolites. Overall, gypsy moth larvae were highly tolerant of all terpenes administered, despite some treatment effects. Limonene, myrcene, isopimaric acid, and neoabietic acid were not recovered suggesting that, once ingested, these terpenes were converted into other products. Bornyl acetate was metabolized in part to borneol and also was recovered in small quantities from frass. Its metabolite borneol was mostly excreted, with a small amount remaining in body tissues. Additional experiments were performed, determining that monoterpenes were effectively ingested, and that monoterpenes were not lost substantially through volatilization from diet or larvae. There was no evidence that terpene metabolism was due to P450 enzymes or esterases, although these results are inconclusive. Gut bacteria do not appear to be primarily responsible for terpene metabolism.