Odours attracting Medetera (Diptera) predators to spruce bark beetles in a multitrophic system

Abstract

Tree-killing bark beetles cause great economic and ecological damage worldwide. Several long-legged fly species of the genus Medetera are natural enemies of tree-killing bark beetles and are of interest as potential biological control agents. However, flies within the Medetera genus have been poorly studied to date, partly due to very difficult species identification that often requires studying the fine structures of male genital morphology. In this thesis, morphological analysis and barcoding were applied in species level identification of different Medetera specimens collected from the bark of Norway spruce trees in Southern Sweden infested with the Eurasian spruce bark beetle, Ips typographus. Adult Medetera flies use olfaction to locate bark beetle-infested trees. Olfactory cues (odours) are detected by various types of hair-like structures called sensilla distributed on the fly antennae and maxillary palps. This thesis shows that the number and subtypes of sensilla differs between M. signaticornis and M. infumata, the two most common species found at the study sites. Therefore, it is possible that these two species respond to different odours, or process the same odours in different ways. Further analysis showed that male and female M. signaticornis were able to detect more than 20 compounds emitted from Norway spruce trees infested with I. typographus. These compounds included metabolites produced by the trees and also compounds produced by bark beetles and associated microorganisms. Such detection of odours from different trophic levels by Medetera flies may facilitate the location of infested trees throughout a bark beetle attack. Multiple comparisons of synthetic blends containing different combinations of these compounds demonstrated that some compounds (e.g. (– )-cis-verbenol, ipsdienol, myrtenol, α- and γ-terpinene; limonene, camphor, terpinen-4- ol and borneol) were more important than others in the attraction of Medetera species. Overall, the findings presented in this thesis can facilitate identification and monitoring of Medetera flies and might help to improve management of I. typographus in the future.