Abstract :
Variation in resistance to thistle-head insects was studied in plants of Centaurea solstitialis L. (yellow starthistle) derived from Nearctic and Palearctic populations and grown in common gardens near Rome, Italy. Significant variation was found in the percentage of insect-damaged flowerheads among plant populations, with aggregate rates of herbivory higher on plants of the local (Italian) population. Two tephritid flies, Acanthiophilus helianthi (Rossi) and Chaetorellia succinia (Costa), accounted for most of the damage to all populations. In addition, more species of insects exploited the capitula of Italian plants (eight vs three to six species for the other populations). That only Italian plants were suitable host plants for Urophora jaculata Rondani suggests a host-plant resistance factor was responsible for the failure of this tephritid fly to become established on yellow starthistle in California. These results indicate that all populations of yellow starthistle are not equally susceptible to herbivory by thistle-head insects. On the other hand, adults of two potential biocontrol agents, U. quadrifasciata quadrifasciata (Meigen) and Terellia uncinata White, emerged from American plants; thus, there is reason to believe these tephritid flies would establish on Nearctic populations. Variation in resistance among plant populations was not related to differences in plant size. The overall results are consistent with the findings of other ecologists that noncrop plant species vary significantly in their resistance to individual herbivore species and to groups of insect herbivores. The results also point to the usefulness of the open field approach in host-specificity determination of insects for biological control of weeds.
