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Summary A preference hierarchy of the oligophagous carrot fly for 30 umbelliferous host-plant species or varieties and six non-hosts was assessed. Foliage of the test plants was presented to a laboratory population of the fly in oviposition choice experiments together with leaves of a standard plant, the susceptible carrot cultivar “Danvers”. Only two species (Anthriscus cerefolium, Carum carvi) were more acceptable than the standard plant, while about half the species received significantly fewer eggs. Some umbellifers (Anethum graveolens, Pimpinella major) had a low acceptability similar to that of non-hosts (non-umbelliferous plants). The results obtained with dual and multiple choice assays were in agreement. A multiple choice assay with potted plants yielded a similar ranking of the species as assays using detached leaves, indicating that foliage is representative for whole plants. Variability in the exploratory runs performed by the females on the leaves prior to egg-laying is described and quantified for hosts and non-hosts. Differences in post-alighting pre-ovipositional behaviour of the female flies on the leaves accounted for a major part of the variation recorded in egg-laying.

Summary Several apiaceous and two asteraceous species were tested for their suitability to support larval development of the carrot fly. Plants grown in pots or transplanted from seed beds into pots, were inoculated with a specific number of eggs. Pupae and non-pupated larvae were collected 6–7 wk after inoculation. Both the number and weights of pupae produced varied widely among the species. Cultivated carrots Daucus carota sativus often gave rise to only moderate numbers of pupae, but these invariably attained the highest weights. Pimpinella major was the only apiaceous plant tested that did not yield any carrot flies. The two asteraceous plants Cichorium intybus and Tanacetum vulgare failed to support larval development. Total carrot fly biomass produced per plant was influenced by both the host species and the root weight. Emergence rates of adult flies were positively correlated with pupal weights. Small individuals tended to have a longer total developmental time from egg to adult fly.

Foliage of twelve host and two non-host species and surrogate leaves treated with the respective leaf extracts were presented to laboratory populations of the carrot fly (Psila rosae) in oviposition choice assays. The stimulatory activity of dichloromethane surface extracts and the diethyl ether fraction of hot water extracts did not reflect accurately the differences in acceptability observed among intact leaves. A better correlation was found using hexane extracts prepared in a microwave oven. Two out of five fractions of this crude hexane extract obtained by silica gel column chromatography stimulated oviposition. The diethyl ether fraction, which contained the previously identified oviposition stimulants (propenylbenzenes, furanocoumarins, polyacetylenes), could account for only a minor part of the variation in the acceptability of host leaves. The preference hierarchy for intact leaves corresponded better to the ranking of species according to activity of the methanolic fraction, which apparently contains unknown stimulatory compounds.¶ The water fractions of the hot water extracts were shown to reduce egg-laying underneath surrogate leaves treated with a stimulatory extract. This oviposition-deterring effect was particularly strong with the non-preferred species Pimpinella major, which is also highly resistant in the field. Hence, unidentified inhibitory compounds may also contribute to differential accept ability of host plants. It is concluded that antixenotic (non-preference) resistance of host plants to carrot fly attack depends on complex mixtures of semiochemicals.

Seven species of plants from the families Umbelliferae, Rutaceae, and Leguminosae known or suspected to cause photophytodermatitis were quantitatively analyzed for psoralen, 8-methoxypsoralen (8-MOP, xanthotoxin), and 5-methoxypsoralen (5-MOP, bergapten). The leaves of all species contained these coumarins, which were also found on the leaf surface, but both the concentrations and the proportions varied greatly. In the interior of the leaves the highest total was in Angelica archangelica, followed by Psoralea bituminosa, both of which are species with severe toxicity, and the lowest in Levisticum officinale (lovage). On the leaf surface, in contrast, the highest total was in P. bituminosa, followed by Pimpinella major. The sum of the surface concentrations, while low in absolute terms, was in some cases a substantial percentage (as high as 24%) of the total in the plant, although there was great variation among species. For individual coumarins this value was sometimes much higher, for example, 60...

Seeds of 10 members of the Umbelliferae were sown separately into plots of unmanaged turf in October 1984. Emergence was monitored at frequent intervals and took place mostly between February and May 1985. Nine species emerged in relatively large numbers but Conium maculatum did not. Survival was recorded until 1987, by which time only four species were still present. Three of these species flowered (Anthriscus sylvestris, Heracleum sphondylium and Pimpinella major), but the fourth (Conopodium majus) did not. Most flowering took place for the first time in 1988. 3. Several attributes of the 10 species are considered as possible determinants of establishment success (...)