The chainsaw revved and roared, shattering the peace of a Caledonian forest, as it laboriously sheared its way into the top of a felled Scot’s Pine stump spewing out a flurry of sawdust. Suddenly, a pyramidal wedge of timber pops up revealing a deep cleft as the saw splutters to a stop. After wiping the sweat from his brow, the tree surgeon fills the hole with a pinewood chip and sawdust mixture then replaces the wedge as a cover, before departing for a well-earned lunch.
During winter, the holes progressively fill with rainwater which dilutes the pine resin into an oily soup with sinuous tendrils of sticky goo clinging to floating pine chips. This is the favoured conditions of Blera fallax larvae whom filter feed on bacteria within the pine broth and, come spring, emerges from the rot and decay as a stout fly resplendent with yellow face and black body with a fiery red tipped abdomen.
This humble fly was probably one of the cold adapted insect pioneers that colonised Britain after the retreat of the glaciers at the end of the last Ice Age following the north-westerly march of the Scot’s Pine from their cold climate refugia. It now retains a tenuous foothold in two localities within fragments of pine forest in the central Scottish Highlands where other cold condition loving species still persist. There has been a documented decline in numbers and distribution since the early 20th century and such was the consternation of conservationists that the species is now a UKBAP Priority Species and is included on the Scottish Biodiversity List. It is also considered to be declining and under threat in Europe.
The short breathing tube of Callicera means it has to keep coming to the surface of the water to breathe unlike Blera & Myathropa which use their long breathing tubes, like a snorkel, to breathe whilst remaining at the bottom of the hole.
In the past such drastic intervention on the part of Blera was needless since they naturally occurred in the stumps of large pines which, weakened by fungus attack, snap off during storms. However, with the scarcity of extensive areas of large old pines in Scotland, where this process could take place naturally, the fly relies entirely upon stumps felled during forestry operations that attain the correct degree of decay. A lack of continuity of suitable larval habitat due to inappropriate timing of forestry operations, clear felling, damage to existing stumps and chemical treatment of stumps have further compounded the problems facing Blera fallax. This prompted the creation of artificial breeding sites to mimic natural situations as described earlier.
As part of my apprenticeship, I have two main learning experiences during the first six months; firstly, identification and curation of hoverflies at the National Museums Scotland; secondly, working under the direction of the Project Manager for the SNH Species Action Framework programme on Blera fallax. This provides an excellent insight into the current conservation management of fragile and endangered insect populations.
By virtue of a chainsaw ticket gained during my college years I was put to work, with local RSPB warden Pete Moore, creating artificial breeding sites for this rare insect by cutting into stumps felled during previous forestry operations. These modified tree stumps have become an important tool in maintaining a viable population until their habitat is naturally restored.
As with other conservation management actions, monitoring the effects of such initiatives is important in determining whether or not it is achieving the desired results e.g. increased numbers of Blera! During October I assisted my mentor and the Pine Hoverfly Project Manager in surveying all of the cut stumps and sifting through the decaying pine chips for hoverfly larvae. This called for rapidly learning how to identify Blera larvae from the other hoverfly species that make use of this resource; Callicera rufa, Sphegina clunipes and the ubiquitous wood decay loving Myathropa florea.
Callicera was straight forward as it lacked the long breathing tube, which appears as a tail on the other three species, and much resembles a sausage (I had missed lunch that day). Sphegina tended to be very small, appearing (to my eyes at least) to be shaped like a cartoon speech bubble with its short, tapering ‘tail’ protruding from an elongate, flattened balloon body. The difficulty came in separating Myathropa and Blera as both were of a similar size with very long breathing tubes. The difference is in the coverage of black spots on the larvae’s face (best viewed through a hand lens). With Myathropa, the spots were uniformly spread across the face whereas with Blera the spots were confined to a thin moustache above the mouth.
Blera fallax – if you zoom in you can see the thin black moustache. By the end we had amassed around fifty Blera larvae with the identification characteristics firmly embedded into my memory. This year there was an increase in the number of stumps occupied indicating that the management may be working in improving the fortunes of Blera and thus further modified stumps will be required – I best get sharpening my saw!