GAS CHROMATOGRAPHY COMBINED TO MASS SPECTROMETRY ANALYSES OF SOME CITRUS PEELS BIOACTIVE COMPOUNDS AGAINST THE COTTON LEAFWORM, SPODOPTERA LITTORALIS (BOISD.)

Abstract

Hexane (Hex) extracts of fresh navel orange (citrus sinensis) and lemon (citrus limon) peel were prepared to isolate and identify toxic bioactive secondary metabolites for potential use as natural ecofriendly biopesticides against the cotton leafworm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae). Hex extracts were fractionated by column chromatography. The fractions that showed activity against the larvae, were identified and quantified by gas chromatography combined to mass spectrometry(GC/MS), after checking component profile by thin layer chromatography (TLC). The results revealed that F2 and F3 fractions were the toxic active fractions of both lemon and orange peel Hex extracts, but lemon fractions were more toxic than orange ones. F2 and F3 fractions of orange extract causd 60 and 20% larval mortality, respectively. Both lemon extract fractions caused 100% morality. Orange fractions had approximately equal dlimonene content (≈86%) as illustrated by mass spectral fragmentation pattern. However, F2 fraction contained slightly higher amounts of linalool, α-terpinol and citral than F3 fraction. The active fraction of both lemon and orange peel extracts contained d-limonene, α-pinene, linalool, citronllal and citral. Limonene oxide, geranial and terpineol were only detected in orange peel extracts. The results indicated that secondary metabolites quality and quantity differ according to the citrus spp., and the quantity of d-limonene is much lower (≈15%) in lemon than orange extracts in spite of lemon peel was more toxic than the orange. However, d-limonene was a major compound in both extracts. The results suggest that citrus extract contains many toxic compounds, other than limonene, and /or they could potentiate each other to give the observed toxicity. It could be concluded that the quantity of d-limonene is not a determinant factor, but it is probable that other metabolites confer toxicity allover citrus extract depending on the species.