Patterns of Furanocoumarin Distribution and Insect Herbivory in the Umbelliferae: Plant Chemistry and Community Structure

A chemical survey of 12 species of Umbelliferae in Tompkins County, New York, USA, revealed that species possessing furanocoumarins are most frequently found in open habitats: either swamps or thickets (Sium, Cicuta, Angelica spp.) or disturbed areas such as roadsides or old fields (Conium, Pastinaca, Heracleum spp.). Woodland species (Osmorhiza, Ziza spp.) characteristically lack furanocoumarins, though they may possess dihydrofuranocoumarin glycosides. Restriction of furanocoumarin production to plants in high—light environments is in keeping with the presumed mechanism of furanocoumarin phototoxicity towards insects, that is, ultraviolet—activated cross linkage of insect DNA. Cluster analysis of the insects on these umbellifers demonstrates that insect herbivores distribute themselves according to host furanocoumarin chemistry. By using a weighting factor that takes into consideration the differential discriminatory powers of specialists and generalists, it was possible to show that the insect faunae of plants containing angular furanocoumarins are more similar to each other than they are to the faunae of plants lacking angular furanocoumarins. The same case holds for the faunae of plants containing linear furanocoumarins, and for plants lacking furanocoumarins altogether. The chemical complexity of a plant species is reflected in the degree of dietary specialization of its insect associates; the more complex the furanocoumarin chemistry, as measured by the number and type of structures, the more specialized the feeding habits of the insects on the plant. Thus, plants lacking furanocoumarins have faunae dominated by generalists while plants with both linear and angular furanocoumarins have faunae dominated by extreme specialists. On the community level, then, it appears that furanocoumarin chemistry can account for patterns in distribution and abundance of insect herbivores on Umbelliferae.