Patterns of oviposition stimulants for carrot fly in leaves of various host plants

Undamaged leaves of 12 host-plant species differing widely in acceptability to ovipositing carrot flies were extracted with a microwave-assisted method with hexane as solvent. The highly stimulatory diethyl ether fraction obtained by separation on a silica gel column was semiquantitatively analyzed by CC-MS for previously identified oviposition stimulants of the carrot fly (phenylpropenes, fluranocoumarins, polyacetylenes). Various plant species exhibited widely differing profiles of these compounds. In choice assays, moderate numbers of eggs were deposited underneath surrogate leaves sprayed with fractions that contained high amounts of just one type of compound and low amounts of the other two types. Only fractions with medium to high levels of at least two compound classes elicited strong ovipositional responses (e.g., phenylpropenes and polyacetylenes in Daucus carota, furanocoumarins and polyacetylenes in Heracleum sphondylium and Conium maculatum). None of the examined plants contained high quantities of all three compound classes. The contents of the stimulants seemed to account in a synergistic manner for the variation in activity of the diethyl ether fraction. However, they could not explain adequately the observed preference hierarchy of the carrot fly for the host-plant species.