Tarsal chemoreceptors and oviposition behaviour of the cabbage root fly (Delia radicum) sensitive to fractions and new compounds of host-leaf surface extracts

Contact chemoreception plays a decisive role in host selection and oviposition behaviour of the cabbage root fly, Delia radicum L. (Diptera, Anthomyiidae). Glucosinolates (mustard oil glucosides) are known to be perceived by the flies, and when sprayed on paper leaf-models Induce oviposition. Recently it has become clear that other non-volatile types of compounds must also be involved in host selection. A pair of ventro-medial C sensilla on the fifth tarsomere respond strongly to a novel compound called tentatively 'cabbage identification factor' (CIF), but not to sucrose, glucose, fructose and proline, CIF is a new non-glucosinolate oviposition stimulant. A single neurone in each sensillum is activated by this compound and the same is true for glucosinolates. In some flirts a mixture of both types of stimuli evoked an apparent mononeural spike train, whereas in others spikes of two separate cells were activated. The significance of this variability is not yet clear. The new stimulant, CIF, does not evoke responses in glucosinolate receptors in the D sensilla. The involvement of the C-5 sensilla in the detection of host-specific compounds constitutes the first known function for C sensilla in D.radicum. CIF appears to be present in leaf surface extracts from the host-plant Brassica oleracea in quantities as low as 1 ng per gram leaf. In spite of this low level, it stimulates oviposition significantly better than glucobrassicin at higher concentrations, which up till now was known as the mot powerful stimulant for D.radicum.