Oviposition of carrot fly (Psila rosae) in response to foliage and leaf surface extracts of host plants

Summary. Foliage of twelve host and two non-host species and surrogate leaves treated with the respective leaf extracts were presented to laboratory populations of the carrot fly (Psila rosae) in oviposition choice assays. The stimulatory activity of dichloromethane surface extracts and the diethyl ether fraction of hot water extracts did not reflect accurately the differences in acceptability observed among intact leaves. A better correlation was found using hexane extracts prepared in a microwave oven. Two out of five fractions of this crude hexane extract obtained by silica gel column chromatography stimulated oviposition. The diethyl ether fraction, which contained the previously identified oviposition stimulants (propenylbenzenes, furanocoumarins, polyacetylenes), could account for only a minor part of the variation in the acceptability of host leaves. The preference hierarchy for intact leaves corresponded better to the ranking of species according to activity of the methanolic fraction, which apparently contains unknown stimulatory compounds.? The water fractions of the hot water extracts were shown to reduce egg-laying underneath surrogate leaves treated with a stimulatory extract. This oviposition-deterring effect was particularly strong with the non-preferred species Pimpinella major, which is also highly resistant in the field. Hence, unidentified inhibitory compounds may also contribute to differential accept ability of host plants. It is concluded that antixenotic (non-preference) resistance of host plants to carrot fly attack depends on complex mixtures of semiochemicals. Received 11 June 1997; accepted 26 November 1997.