In his book “Guide to fishing Lake Vyrnwy” G.V. Westropp quotes a passage from Edward Durands 1938 book “Wanderings with a fly-rod” who attributes the decline of the chafer beetle presence in late June and early July and the success of the coch-y-bondhu fly at Lake Vyrnwy/Llyn Efyrnwy on the proliferation of coniferous forests that now almost encircle the lake since its construction. This got me wondering if this causal link was accurate as my recollection of the 1887 OS 1:2500 scale county maps, which I referenced for a previous blog post, was of there being a number of broadleaf woodlands present at the time of its construction.
There are two major chafer beetle species that the Coch-y-Bondhu fly is designed to mimic, the garden chafer (Phyllopertha horticola) and the Welsh chafer (Hoplia philanthus), the latter especially prevalent in sandy soil areas which is not the case near Lake Vyrnwy. The adult chafer beetles emerge from their pupal stage in late May to July and dig their way to the surface. They may fly short distances and congregate looking for a mate, but once having mated the females dig back into the soil to lay their eggs. The chafer grubs hatch within 2 or 3 weeks and start to feed on the roots of grasses (Poaceae). The grubs will be fully grown by mid-October to mid-November at which time they burrow deeper into the soil to over-winter before pupating in April the following year.
The 1900 Ordnance Survey 6 Inch map indicates that much of the Lake was surrounded by woodland right from the outset. However, the density of these woodlands is hard to judge from the topographic map legend symbols used at the time. Analysis of historical photos from that period suggests that many of these areas were already quite densely planted. Despite this, there were large tracts of the shoreline that were directly connected with the surrounding moorland where chafer would be expected to be present.
Analysis of the 1949 OS 6 inch topographic map suggests that there was an intensification of forestry right around the lake during the intervening 50 odd years particularly on the northern shore between Cedig and Rhiwargor. In the following 65 odd years to 2015 this intensification continued albeit at a slower rate and particularly on the southern shore. The change in tree species composition is also apparent with the broadleaf and mixed woodlands present at the time of the construction of the lake being replaced by coniferous forest. The result is quite literally a ring of coniferous forest around the lake cutting off the connectivity with the surrounding moorland.
Westropp notes that as late as the 1970s some success with the coch y bondhu fly could be had around the straining tower. This is unsurprising as the amount of woodland in the vicinity of the straining tower and along the shore to the Hotel is much lower, is more highly fragmented and has a good mix of coniferous, broadleaf and mixed species woodland. As such the connectivity with the surrounding grassland and moorland is better and it would be unsurprising to find chafer in this part of the lake. Something to explore next July.
Tight Lines
Greg