Chemical communication and conservation ecology of two rare saproxylic beetles: osmodema eremita (sensu lato) and elater ferrugineus

Abstract

Osmoderma eremita (Coleoptera, Scarabaeidae) and Elater ferrugineus (Coleoptera, Elateridae) are threatened saproxylic beetles associated with old hollow broad-leaved trees in mature forest ecosystems of Europe. The larvae of O. eremita feed on decaying wood, while those of E. ferrugineus are predators of the immature stages of large Scarabaeidae, including O. eremita. The females of E. ferrugineus are attracted to the O. eremita male-emitted sex pheromone, (R)-(+)-γ-decalactone, and exploit this compound as a kairomone in order to locate suitable tree cavities in which to lay eggs. In addition, the males of E. ferrugineus are strongly attracted to the sex pheromone emitted by conspecific females (7-methyloctyl (Z)-4-decenoate). The overall goal of the present project is to increase the knowledge about O. eremita and E. ferrugineus ecological relationships and their chemical communication system. The antennal sensilla of both species were studied using scanning electron microscopy. O. eremita antennae did not show any sexual dimorphism concerning the distribution of sensilla placodea on the antennal club, the ones responsible for pheromone reception. In fact, both the sexes are attracted by the same pheromone. In contrast, E. ferrugineus showed a strong sexual dimorphism, with one type of thricoid sensillum occurring only on the male antenna. Probably these sensilla are responsible for the reception of female-emitted sex pheromone, to which only males are attracted. A mark-recapture study was performed using traps baited with the two pheromones. On the whole, 13 O. eremita and 1,247 E. ferrugineus were trapped. For E. ferrugineus, 7-methyloctyl (Z)-4-decenoate was a much more efficient lure than the kairomone, and 1% of the individuals dispersed farther than 1,600 m from their natal site. In contrast to some studies on these beetles conducted in northern Europe, the activity pattern was not influenced by variation in temperature during the season. Scent collection and analysis on O. cristinae, endemic to Sicily, showed that this species uses exactly the same sex pheromone compound as used by O. eremita, demonstrating a strong conservation of this sexual trait within the genus. Data on mtDNA cytochrome C oxidase I gene (COI) and morphology of male genitalia supported the divergence of the two species and suggested a species status for O. cristinae.