DETOXIFICATION OF NATURALLY-OCCURRING CHROMENES IN LARVAE OF THE GENERALIST HERBIVORE SPODOPTERA-LITTORALIS (NOCTUIDAE)

Five naturally occurring chromenes from the Asteraceae including the insecticidal compounds precocene II (1) and encecalin (2) were administered to last instar larvae of Spodoptera littoralis via the food or by topical contact. Metabolites formed and excreted via the frass were analysed by GC-MS and by direct comparison with reference compounds obtained by partial synthesis. In total 28 different metabolites were identified, many of them reported here for the first time. All metabolites detected originated from phase I reactions (most probably catalysed by Cytochrom P-450-dependent monooxygenases) by hydroxylation, demethylation or reduction of the parent chromenes. The resulting metabolites can be regarded as detoxification products based on previous structure-activity studies. The increased polarity of the metabolites will furthermore facilitate their excretion by the larvae compared to the more apolar parent chromenes. The largest number of metabolites (eight for each compound) was detected following oral treatment with precocene II and encecalin respectively. 3-Monool as well as 3,4-trans-diol derivatives predominated in the frass of larvae treated with the latter compounds whereas the 6-hydroxyethyl derivatives were the major metabolites of the other chromenes investigated. The patterns of metabolites originating from precocene II or encecalin were the same following oral application or topical treatment.