Phenology of Episyrphus balteatus in 2014 and 2013

In a few moments of inactivity I took a look at the phenology of a few species seen this year. One that I thought worth looking at was Episyrphus balteatus as it is reported in very large numbers. The attached histograms are strictly for photographic records - I will look in more detail once the garden records have arrived.

Data were plotted in four blocks:

•  All records for 2014 and 2013 plotted separately
• South of a line created by the northern limits of ST/SU/TQ plotted as separate years 2014 and 2013.
• Midlands - up from the northern limits of ST/SU/TQ to the southern limits of SE/SD plotted as separate years 2014 and 2013.
• North - north of the limits of the southern limits of SE-SD plotted as separate years 2014 and 2013.

The results were a bit of a surprise:

1. It looks as though there might be as many as four generations in southern England - I expected 3 but there is a bit of a March generation too.
2. In 2014 I found no migration peak at the end of July but there is quite a spike by late June - and a pronounced spike in the Midlands.
3. The number of generations in the midlands is a bit messy - probably 3, but I cannot be sure. It begs the question - can one come up with a series of thermoclines to come up with differences in phenology?
   
4. The northern population seems to have a distinct single generation.

What I think it starts to tell us is that every year is very different, and that this species probably has slightly different emergence strategies even between southern England and the Midlands. In 2014, in which we had an exceptionally mild spring and autumn it looks as though there were probably at least 4 generations in southern England, whereas the further north one gets it seems to be broadly univoltine.What I find particularly puzzling is that the late July migration spike was not picked up this year. I have a feeling that this may only be detected by absolute counts.

Figure 1. Total 2014 records of Episyrphus balteatus against week number.

Figure 2. 2014 records of Episyrphus balteatus from southern England.

Figure 3. 2014 records of Episyrphus balteatus from the English midlands and Wales.

Figure 4. 2014 records of Episyphus balteatus from northern England and Scotland.

Figure 5. Total 2013 records of Episyrphus balteatus against week number

Figure 6. 2013 Episyrphus balteatus records from southern England

Figure 7. 2013 Episyrphus balteatus records from the English Midlands and Wales.

Figure 8. 2013 Episyrphus balteatus records from northern England and Scotland