Identification of host plant attractants for the carrot fly,Psila rosae

Cold-trapped carrot leaf volatiles were analyzed by gas chro-matography with an outlet splitter to a flame ionization detector and to a carrot fly antennogram preparation as the second detector (GC-EAD). Strongest EAD responses were elicited by products whose elution temperatures corresponded to the propenylbenzenes,trans-methylisoeugenol (3,4-dimethoxy-1-propenylbenzene) andtrans-asarone (2,4,5-trimethoxy-1-propenylbenzene) and, to a lesser extent, by-products matching the elution temperatures of the leaf aldehydes hexanal, (E)-2-hexenal, and heptanal, and of the terpenes linalool and caryophyllene. The identity of the propenylbenzenes was confirmed by gas chromatography-mass spectrom-etry. GC-EAD permitted accurate estimation of the olfactory thresholds; it was lowest fortrans-asarone at 500 attogram (5 × 10(-16)g)/ml of air passing over the antenna. Both the leaf aldehydes and propenylbenzenes were attractive when tested individually in the field with yellow sticky traps; fly captures were linearly related to the quantity of propenylbenzenes applied per trap. A combination oftrans-asarone and hexanal was more attractive than either compound singly, suggesting that the fly is adaptively equipped to respond to a mixture of compounds emanating from carrot foliage. In laboratory choice tests, flies were more attracted by vapors from intact carrot foliage than by that from a nonhost; leaf odor alone also mediated oviposition. We conclude that through the selectivity and sensitivity of its response to foliar volatiles, the carrot fly may achieve host-plant orientation and also at close range, in union with its response to less volatile leaf surface components, selection of an oviposition site.