Secondary metabolites of the leaf surface affected by sulphur fertilisationand perceived by the cabbage root fly

Summary. Surrogate leaves treated with methanolic leafsurface extracts of Brassica napus L. (cv Express) plantsthat received three different sulphur fertilisation treatmentsshowed even more marked differences by the ovipositionchoice of Delia radicum L. than the potted plants. Thisconfirms that the oviposition preference of D. radicum ismediated by chemical compounds on the leaf surface andthat the quality of host-plants in terms of their nutritionstatus can be perceived by the female insect.The oviposition data were positively correlated with thecontent of fractionated surface extracts containing eitherCIF (cabbage identification factor; 1,2-dehydro-3-thia-4,10,10b-triaza-cyclopenta[.a.]fluorine-1-carboxylic acid)or glucosinolates. Electrophysiological recordings from thetarsal chemoreceptor sensilla C₅ and D_3,4 showed that receptorneurons react to glucosinolate- and CIF-fractions. Wefound that the chemosensory activity of specific glucosinolate-and CIF-receptor neurons corresponded with therespective behavioural activity in the oviposition choiceassays. The responses of D. radicum to glucosinolates inthe electrophysiological recordings studies correspondedto the observed oviposition preference on plants or artificialleaves characterised with an higher amount of glucosinolateson leave surfaces. The presented data suggestedthat CIF and glucosinolates are involved in host-plantpreference of D. radicum and are perceived by tarsalchemoreceptors.