In the Mediterranean Basin, two species of true seed bugs in the genus Orsillus Dallas, O. maculatus Fieber and O. depressus Mulsant et Rey, share the exploitation of seed cones of Cupressaceae for feeding and reproduction and may carry spores of the pathogenic fungus Seiridium cardinale Sutton & Gibson, responsible for the cypress bark canker disease. We compared the life history of the two species. A total of 89 cone collections carried out in 10 Mediterranean countries confirmed that O. maculatus is most closely associated with Cupressus sempervirens L., although it may also infest some other Cupressus L. species and, more rarely, species of Chamaecyparis Spach. Orsillus depressus appears to be less host-specific, being capable of feeding and ovipositing on most of the native and exotic species of Juniperus L., Cupressus, and Chamaecyparis. On C. sempervirens, the abundance of each Orsillus species follows an inverted longitudinal gradient along the Mediterranean Basin, probably corresponding to the pathway of introduction of this tree species from its native eastern range towards western Europe. The dominance of O. maculatus progressively decreases from east to west, whereas O. depressus becomes dominant in the Iberian Peninsula. By contrast, O. depressus is the dominant seed bug on other Cupressaceae all over Europe and the Mediterranean Basin. In the area in which C. sempervirens has been introducted, both Orsillus species may live on the same tree, but morphological variability of both adults and nymphs often hinders accurate identification of the species. Based on frequency distribution, the relative length of the rostrum compared with that of the body seems to be a diagnostic character for specific identification of nymphs of the last two instars.