Conserving the Aspen Hoverfly

Ross Poulter is a keeper at Edinburgh Zoo who is involved in insect conservation projects trying to support populations of rare pollinators. Since 2009, Ross has been working to conserve the Aspen Hoverfly in Scotland , by enhancing and protecting old growth aspen woodlands which act as a unique habitat for these fascinating flies.

The Aspen Hoverfly : Ecology and Distribution

The Aspen hoverfly (Hammerschmidtia ferruginea) is a red-listed species which is represented by a UK BAP (Biodiversity Action Plan). This type of hoverfly is defined as a  ‘flagship’ species , which means that by conserving them, other species will also benefit. The Aspen Hoverflys stronghold within the UK is Strathspey in Scotland. Populations of Hammerschmidtia ferruginea can be found along the Spey Valley between Kingussie, Easterness and Grantown; with a stronghold on Insh Marshes RSPB reserve site (opposite side of the Spey River from Kingussie). Their breeding habitat is Aspen (Populus tremula) woodland, including birch or pine woods with an aspen component.

Life Cycle

The Aspen hoverfly gets its name from its preference for decaying aspen wood, where it lays its eggs in the wet rotting cambium (layer of wood beneath the bark), and this wet decaying sap that forms under the bark is where the larvae develop. Dead trees need to be over 25cm in diameter in order for the sub-cambial layer to be deep enough to support the hoverfly larvae. The larger the tree the slower the decay process, providing potential breeding habitat for up to four years before it dries out.

H. ferruginea swing through a cycle of abundance and the reason for this maybe due to the fluctuations in availability of appropriate dead wood, but when there are fewer dead trees H. ferruginea have been seen to breed in sap flows on live P. tremula trees. Stands of P. tremula greater than 4.5 ha are not common but do appear to have continuity of fallen trees and branches sufficient for breeding sites to be continuously present. During a period of fluctuation or in areas where aspen is sparser, one way of ensuring availability of suitable dead wood is by supplementing the area with fallen or cut trees and leaving them to rot.

Conservation and Research

I first became interested in the Aspen Hoverfly in 2008. I knew the site on Insh Marshes where they were breeding and emerging, and that it was close to the Highland Wildlife Park (HWP. Owned and operated by the Royal Zoological Society of Scotland, RZSS). In January 2009 I drove to the wildlife park to survey the aspen trees and measure their suitability as potential breeding habitat. I found three areas at the wildlife park where aspen trees were present. I found three mature trees of suitable size, and several young trees. One area of aspen stood out as an area of interest.  This was a small bank that formed part of the park’s boundary with perimeter fencing. On the other side is the A9, and across the road lays the Insh Marshes.

During this time, I met with an assistant of a PhD student from Stirling University who was studying the ecology and conservation of endangered saproxylic hoverflies in Scotland. Part of the PhD student’s work was to explore dispersal distances of H. ferruginea, and I spent an afternoon watching the emergence of these rare hoverflies being marked with enamel paint before being released. The plan was to test their dispersal range as rotting aspen logs of similar size were distributed at 1 km points extending out from the Insh Marshes RSPB reserve site, up to 4 km away, which is where they were eventually recorded. This was exciting news as I believe the HWP is potentially within its flight range.

Creating Breeding Habitat

I decided that I would make a start on protecting what we had at the HWP with a long-term plan in mind, to create a stand of aspen and perfect breeding habitat for this rare saproxylic hoverfly. I had one of the older dead aspen trees on the HWP perimeter area cut down and left to decay to see if it was enough to entice the hoverflies, and our RZSS gardeners’ rabbit-proofed the other trees within that same area, allowing P. tremula to regenerate. Aspen regenerate by producing root suckers, and new stems can appear up to forty metres from the parent tree, but unfortunately aspen is a very slow growing tree and will take up to seventy years to get to a suitable size. This problem is also felt throughout the current range where the 10–12 main ‘reservoir sites’ sit at considerable distance from each other with large gaps in age structure. The hoverflies can move out of these key areas to colonise suitable fallen trees some distance away at ‘satellite sites’, but the temporary breeding habitat only persists for several years before disappearing again.

Aspen Regeneration

There are now however, things underway at a more strategic level. Aspen is now being treated as a priority species by the Cairngorms National Park, and there have been recent aerial surveys to determine the exact location and extent of aspen across almost all of the park with a view to developing strategic objectives, and there is now a more general move to promote planting and thus regeneration of this biodiversity by both public bodies and NGO’s. Seventy years is not long, but at the same time it does highlight the need to press on quickly to increase breeding habitat and integrate corridors between existing sites, and this is why I feel the aspen trees at the HWP may one day become an extremely important site for this species.

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