Thomas DEGEN, Hans-Ruedi BUSER and Erich STÄDLER
Swiss Federal Research Station, CH-8820 Wädenswil, Switzerland
Carrot flies Psila rosae lay their eggs in the soil undemeath various umbelliferous plants, and larvae develop on the roots. Defence chemicals in the leaf surface wax of carrot plants were shown to stimulate oviposition (1). They belong to three classes of compounds (see formulae below) which in combination are characteristic of umbellifers. This study attempts to correlate differences in antixenosis resistance to the amounts of oviposition stimulants present in the leaf surface of the respective host plants. Hexane surface extracts of 13 host species were fractionated on silicagel columns using 100 % hexane (f1), 5% Et2O in hexane (f2), twice 100 % Et2O (f3.1 and f3.2) and 100 % MeOH (f4) as eluents. Stimulatory activity was found in f3.1 and f4. The former fraction contained the previously identified oviposition stimulants, whose concentrations were semiquantitatively determined by GC-MS-analysis. The profiles of these stimulants varied widely among the host species. In choice assays, moderate numbers of eggs were deposited below surrogate leaves sprayed with f3.1 that contained high amounts of just one compound class and low amounts of the other two (e.g. polyacetylenes in caraway Carum carvi, furanocoumarins in parsnip Pastinaca saliva). Only f3.1 with medium to high concentrations of at least two compound classes elicited strong ovipositional responses (e.g. phenylpropenes and polyacetylenes in carrot Daucus carota, furanocoumarins and polyacetylenes in hogweed Heracleum sphondylium and hemlock Conium maculatum). High quantities of all three compound classes were not found in any of the plants. Oviposition obviously is stimulated synergistically by the semiochemicals, as has already been suggested earlier, when mixtures of pure compounds were tested (1). Though the known oviposition stimulants seem to be responsible for the different activities of the often highly stimulatory f3.1, they explain only a small part of variation in the acceptability of real host leaves. Still unidentified stimulatory compounds in f4 appear to be more promising in this respect.
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