I've tried a variety of ways of expressing the data in graph form and think the two following versions do help, but they are tricky to interpret!
The first thing that strikes me is that the height of the female peaks in 2015 and 2018 (Figure 1) are of similar height. There are two possible reasons: comparatively fewer males were recorded in these years; and the season was shorter in these years. In the case of 2018 I suspect both factors are at work.
Rather more significantly, I think Figure 2 indicates that numbers in 2018 were substantially lower than in previous seasons and hence the peak for females is markedly down on preceding years. The male peak seems to be broadly analagous to 2015 and 2016 but is substantially smaller than 2017. Some inter-annual variation is to be expected, I don't think we can make too much of this. What is possibly more significant is the substantial gap between the male and female peaks in 2018 that coincides with the heatwave between weeks 25 & 28 (i.e. 20 June to 10 July).
At this stage, it may be conjecture, but there is the possibility that the heatwave had a significant effect on the lifespan of the male generation and possibly also that of the females. If so, why might this be? It seems unlikely that the heat itself would have been a problem - the flies could simply have sought out shady places, especially as they tend to be woodland denizens.I think the more probable explanation is that neither males nor females could access sufficient nectar because normal sources had failed due to the heat and water stress. Is this a species that has suffered as a result of the heatwave?
Will the impact be expressed in numbers present in 2019? Only time will tell!
|Figure 1. Volucella pellucens records extracted from social media covering the years 2015 to 2018 and represented as proportions of the records for this species in each year.|
|Figure 2. Volucella pellucens records extracted from social media covering the years 2015 to 2018 and represented as proportions of the records of all species in each week.|