Reed galls serve as an underestimated but critically important resource for an assemblage of aculeate hymenopterans

Abstract

Common reed (Phragmites australis) beds are frequently considered as aggressive and invasive, being subject to numerous conservation management efforts aimed at their eradication by repeated mowing or more aggressive measures. However, the reed beds are associated with a specific community of reed bed specialists, represented typically by various bird flagship species or by Lipara flies. We show here that the reed beds and particularly the reed galls induced by Lipara flies provide unique habitat serving at least 183 bee and wasp species (amounting to 13.6% of the total bee and wasp species known to occur in the Czech Republic, throughout which the sampling sites were located). The reed galls themselves were found to host 13 species of bees and wasps, five of them red-listed, and some of them considered as reed bed specialists. Pemphredon fabricii and Hylaeus pectoralis were the dominant reed gall aculeate hymenopteran inquilines. Hylaeus moricei, Passaloecus clypealis, Rhopalum gracile and Trypoxylon deceptorium were identified as species tightly bound to the presence of reed galls. Among the other species detected was also one previously considered as regionally extinct (Nysson quadriguttatus), nine were critically endangered, 11 were endangered, and 19 were considered as vulnerable. The species found displayed specific habitat requirements, often requiring not only the presence of reed, but also the presence of loose sandy bedrock below the reed bed. These species, which have nearly disappeared from the surrounding cultural landscape, found their surprising refuge in reed beds occurring newly on the loose bedrock of (post)industrial sites, including gravel-sandpits, ash ponds and tailing ponds. The data obtained challenge the common view of the expanding reed beds as a threat to biodiversity, and highlight the importance of reed beds, particularly those of the oligotrophic nature, for effective conservation of the aculeate hymenopteran reed gall inquilines.