All at sea!

We know that some hoverflies cover considerable distances over land and sea. Moreover, there is some evidence of northward movement through mountain passes in the spring and southward movement in the autumn. In the UK, we occasionally get vast influxes of hoverflies but in most years there is probably more of a gradual trickle. It is an area of research that offers all sorts of questions and perhaps opportunities.

One of the regular contributors to the UK Hoverflies Facebook group works on the Floating Production Storage and Offtake (FPSO) vessel Curlew, which lies 130 miles East of Montrose. From time-to-time, he reports sightings at this offshore location. The hoverflies involved must be part of a larger 'migration' but the big question is 'where are they coming from?' Many will doubtless be continental in origin, but perhaps some also come from the UK and are heading north-eastwards?

These conundrums got me thinking: how big a proportion of the records gathered for hoverflies in July and August are 'home-grown' and how many are continental visitors? I doubt we can say with any certainty, but perhaps we can do some basic investigation by packaging together the records for the main migratory species (Episyrphus balteatus, Eupeodes corollae, Helophilus trivittatus and Scaeva pyrastri) and see how much of a contribution they make to the data (Figures 1 & 2).

Figure 1. Total records for 2017 extracted from UK Hoverflies Facebook page broken down to include the contributions of four known migratory species. Note that Syrphus ribesii, Syrphus vitripennis and Sphaerophoria scripta are also regular migrants but there are also a substantial resident populations.

Figure 2. Records for 2017 data in Figure 1 with migratory species excluded.

I think Figure 2 demonstrates how a substantial dip in records occurred in 2017 if migratory species are excluded. Then, there was nothing like the heatwave that we have seen in 2018, but even so there seems to have been a pronounced drop in numbers. It will be interesting to see the differences for 2018 at the end of the season.