Flower and fruit herbivory in a population of Centaurea scabiosa (Asteraceae): Importance of population size and isolation

Effects of size and isolation of Ceutaurea scabiosa (Asteraceae) populations on herbivory level in flower heads were examined in eight populations of 2,280-44,441 flower heads, each population being separated by 1 to 8 km from the others. Three common flower head herbivores were observed. Two of these (Cerajocera ceratocera [Diptera: Tephritidae] and Isocolus cf. rogenhoferi [Hymenoptera: Cynipidae]) were specialists, and the third (Eucosma cana [Lepidoptera: Tortricidae]) was a generalist on the subfamily Cynaroiideae to which C. scabiosa belongs. All three species increased their herbivory level with size of host plant population. We compared the explanatory power of three measures of population isolation: 1) the traditional measure of distance to nearest-neighbour population, 2) cumulative proximity of all surrounding populations, and 3) a measure termed biological proximity, combining the effects of distance to other populations and their size. Both 2) and 3) explained surprisingly little of the observed herbivory. Thus population size and distance to nearest-neighbour population were the two best predictors of level of herbivory.