The acaricidal activity of certain natural plant extracts and some of their main terpenoids was evaluated
against the female adults of the two-spotted spider mite, Tetranychus urticae (Koch) under laboratory conditions using the slide dip and surface residue techniques. The relationship between chemical structure of terpenes and acaricidal efficacy was evaluated. The results revealed that peppermint, thyme, caraway and clove extracts were the most effective against the spider mite, followed by chenopodium and visnaga extracts and then eucalyptus and sweet basil which showed relatively the least miticidal activity. Of the terpenoids tested, the monoterpene hydrocarbons limonene and ocimene were completely ineffective on the mite at all concentrations tested, whereas, the oxygenated terpene alcohols menthol and linalool exhibited promising acaricidal activity. The phenolic compound thymol was the most active among tested terpenoids followed by carvone, a monocyclic ketone and the main constituent of caraway oil and proved to be biologically more active against mites than camphor (the bicyclic monoterpene ketone).Visnagin, a furanochromone with one methoxy group, was significantly more toxic against adult spider mite than khellin which has two methoxy groups. The results obtained suggest that certain terpenoids of plant origin can be used as potential parent structures for new acaricides and as alternative and safe acaricidal agents.