Following gas chromatographic analysis of citruspeel oils, 30 components of these oils were tested against Callosobruchus rnaculatus (F.). Several compounds (22), including the major component of all citruspeel oils--( +)-limonene-were found to be bioactive, having a strong vapour insecticidal activity. These components and some of their isomers were shown to have varying levels of activity (a-terpineol, 24-h LC,, 4.05 pl litre-' to n-tridecane, LC,, 13.3 p1 litre-I), but no single component could account for the activity measured for a typical citruspeel oil (lime) on a proportional basis. Joint action studies established that in artificial mixtures several pure components of citruspeel oil potentiated their individual fumigant action in a manner consistent with an additive model against C. maculatus.

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A B S T R A C TStudies on the mode of action of fixed vegetable oils on Callosobruchus maculatus eggs revealed two possible mechanisms of action which may be complementary: ( 1 ) that the oils exert lethal action slowly by drastically reducing respiratory activity and elimination of toxic metabolites as a result of the oil 'barrier effect ', and (2) direct toxic efects of oil or oil constituents that possibly penetrate the eggs.The slow lethal action of oil was demonstrated by the continuous low level respiratory activity (measured by carbon dioxide emission) of dying eggs over a six-day monitoring period, and the fact that a significantly higher proportion of the dead eggs found on groundnut or traditional coconut oiltreated grains developed up to the late embryonic stage (first instar larvae) when compared with the control. Furthermore, in an experiment carried out by trapping C. maculatus eggs on oil-treated and untreated artificial membrane (Sellotape@), it was found that oiling did not affect egg development up to the late embryonic stage but prevented hatching of significantly higher numbers of developed first instar larvae.Eggs that successfully hatch on oily grain surfaces may find it more difficult to penetrate, since egg attachments on such surfaces were found to be significantly less secure than on the control.The proposed modes oj-action show that the complete spread of an oil film over the surfaces of a commodity is vital for the successful utilisation of fixed oils as stored-product protectants.

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