"When we researchers go into the forest we really need the same kind of nose as these flies to sniff out the sap runs under the bark, because they're often not visible."
Someone like Dr Frank Dziock, whose work focuses on these sap-dependent hover flies, cannot simply lie down in a lush flower meadow and wait to see what comes buzzing along, for these insects live hidden within the woods in very special old trees. They live on micro-organisms which are only found in tree sap in contrast to their more widely known relatives. Although many of the other species of hover flies may be mistaken for wasps because of their black and yellow colouring, they cannot sting. Their larvae are actually very useful, as they consume large numbers of aphids. Around 450 different species of hover fly are found in Germany and they have a wide variety of dietary habits.
A sensation that was almost missed
For the last 12 years Dziock has been investigating the fauna of riparian woodlands. Now, for the first time, the 35-year-old biologist has succeeded in achieving what every entomologist dreams of: the discovery of a new species. Yet the specimen's capture, in the Saalberghau nature reserve by the River Elbe near Dessau, initially seemed quite unspectacular. As part of a research project Frank Dziock had set up an insect trap to examine how hover flies react to flooding. The insects are used here as bio-indicators. At first glance the contents of the trap did not look in any way out of the ordinary. "At first we thought it was a species that was already known. Then we realised that the two tiny spots on the fly's back were clear evidence of an as yet undiscovered species", explains Dziock. On closer examination under the microscope (the fly is only seven millimetres long) it became clear that its body structure was different from all other known species. "So I borrowed the type specimens, known as holotypes and syntypes, of four similar species from the natural history museums in Copenhagen, Stockholm and Oxford and the German Entomological Institute, in order to examine the differences more closely."
Together with hover fly expert, Dieter Doczkal, from Malsch, Frank Dziock wrote a description of the new species. The first description of Brachyopa silviae (Silvia's Brachyopa) was then published in the journal, Volucella. Anyone who discovers a new species can suggest a name for it and Dziock dedicated his discovery to his wife, Silvia, who over the years has had to be very understanding of all the days her husband spends conducting his research in the forests of Central Germany.
A relic from the primeval forest
Frank Dziock found two male specimens in the Elbe biosphere reserve near Dessau (Saxony-Anhalt) and a female in the Hainich National Park (Thuringia). Other insectologists have since found evidence of this hover fly in Hesse and Rhineland Palatinate. "It is no coincidence that this species has been found specifically in impassable areas", says Frank Dziock of the locations where the hover flies have been found in old-growth stands of oak and beech. These relic species are not found in 'newer' forests, which have not been wooded for so long. Because the species are not as adaptable and mobile as other species, they are now found only in insular patches in remnants of old-growth forest. This is because it takes at least 120 years for the structures of old wood, like rot holes, sap runs and rotting heartwood, to form. Most trees in managed woodlands are felled before they reach this age.
Modern forest management threatens species diversity
"Where forests are managed conventionally, this species has no chance of survival. The usual approach is for everything that looks in any way sick to be removed. But it is precisely in these old trees that the sap runs form which these species need." From the biologists' point of view it is therefore essential to leave old trees in the forest and let them die there naturally. Frank Dziock is critical of the Forest Guidelines of the State of Saxony-Anhalt, because they allow too much room for discretion in commercial exploitation and, furthermore, the owner of the forest is not even bound by these guidelines. "It's not enough just to leave some deadwood lying around in the forest", he explains. "After a very short time this wood is so dried out that it's no longer a suitable habitat for the hover fly larvae. Deadwood is no substitute for naturally occurring old wood."
Only around two percent of Germany's forest is over 180 years old. This has serious implications for species diversity. Analyses of the endangered species list show that animal and plant species which have adapted to the typical structures of near-natural forests are disproportionately at risk.