Can we interpret current mass occurrence events?

In the past few days there have been a number of posts of spectacular numbers of hoverflies covering every nectar source or source of fluids and sugar (e.g. a banana skin). These reports seem to be confined to coastal zones in northern England, eastern and northern Scotland. Superficially at least, we might assume that these are mass migrations. An equally valid interpretation, that has been advanced previously, is that these are mass emergence of local provenance. Can we resolve this conundrum from the data?

The dominant species in these events is Eupeodes corollae, with smaller numbers of Episyrphus balteatus and Scaeva pyrastri. All of these species are aphidophagous (larvae are predaceous upon aphids) and have a relatively short development time. In the case of E. corollae there are reports that larvae will feed on fruit in addition to aphids. All three species are also known to be migratory.

Mass occurrences are not uncommon and in most years we see a short-lived peak of Episyrphus balteatus in the third or fourth week of July that often coincides with the start of the harvest of cereal fields. In most years the peak occurs across much of southern England and occasionally there are reports of vast numbers streaming in across the coast. Two reports are perhaps relevant:

• one observation by a friend of mine who witnessed such an event on the Suffolk coast and later told me that numbers were about a thousand per minute per linear metre; 
• the other came from somebody who operated a support vessel for cross-channel swimmers and told me that occasionally the boat and the swimmer would be smothered with hoverflies (mid channel).

Such events do cause local concern and I have previously been contacted by the press to answer questions about a 'swarm of wasps' arriving on the coast.

So, we can be sure that at least on some occasions mass-occurrence are of continental origin, whilst there may be times when the source is local. That would make sense because there must be some parallels between population dynamics on the continent of Europe and in the UK. Local climatic conditions are likely to dictate the degree to which there are differences on any one year. However, whilst we can be sure that there are mass movements from Europe to the UK, we can be less sure about movement the other way.

The mass occurrences in 2018 seem to be different from the norm. Unlike past events, they have occurred from East Yorkshire northwards to the east coast of Scotland, the Moray Firth and Orkney. This complicates matters because the three species in question tend to occur in far lower numbers in northern England and especially in Scotland. They are essentially southern species in the UK and, although there is a resident population of Episyrphus balteatus and Eupeodes corollae, it seems unlikely that there is much of a residual population in Scotland as far north as Orkney. So, the case for mass-occurrences to be of local provenance is less robust.

We must not forget, however, that 2018 has experienced some extremely unusual weather, with a serious heatwave in June and July at least as far as the north of Scotland. I experienced it first-hand on the west and north coasts of Scotland where there was glorious weather in early June. Hoverflies, however, were very scarce and aphidophagous species were extremely sparse (see my post of 4 July Square-bashing in Scotland - hoverfly records). Those of Eupeodes corollae were particularly sparse with just 4 records in about 100 hectads! I guess that this generation might have been responsible for the current generation, given that it has been a very warm summer, but I am sceptical.

I have less evidence for E. corollae numbers on the east coast of Scotland, but the phenology histogram for northern England and Scotland (Figure 1) in June looks to be consistent with those for more southerly regions at the same time. So, we have no real evidence to suggest that there were high numbers of E. corollae in June and therefore the starting point for a mass emergence is not exceptional. Furthermore, going on my own experience in The Trossachs in July, I am doubtful that aphidophagous species have done terribly well in Scotland. That trip was noteworthy for the overall absence of all hoverflies and also of midges. When were midges last absent from such an infested place?!

Figure 1. Phenology of Eupeodes corollae in 2018 represented as southern England, Wales and the Midlands, and northern England and Scotland. All records combined are also shown. This illustrates how numbers have climbed in the north whereas in the south numbers have not maintained their June peak.

Nevertheless, the case has been made that these mass occurences could be of local provenance, with the numbers accentuated by animals remaining in diapause until conditions were favourable. I don't think we have much to go on there but it is certainly a theory that is worthy of further investigation. I think there may be some truth that some species use diapause in response to droughts or extreme cold. As yet, we simply don't know what happens in many species. We do know that spring emergence times are often significantly affected by the severity of the winter. We can also see how numbers of hoverflies drop in periods of extreme heat. This effect is also obvious in the data for Eupeodes corollae in 2018 (Figure 2). Dis-aggregating the effects of migration on populations is more problematic.

Figure 2. Phenology of Eupeodes corollae in 2018 against the five-year average. Unlike previous years, 2018 depicts a sharp decline followed by a new peak as opposed to a gradual build-up of numbers. This decline coincides with the most extreme part of the 'heatwave', whilst the new peak(s) are displaced from the 'average'. This shift in peaks possibly corresponds to a response to the prolonged winter of 2017/18 and is also present in the June peak

So, in summary, I think the coastal locations of most of the reported mass-occurrences strongly suggests that the animals are reaching land and desperately searching for a refueling station before spreading landwards. As the swarm spreads its numbers will diminish but may remain high for some way inland. Elevated numbers are likely to be found far inland, but not in the extremes that are seen on the coast. Nevertheless, we simply don't have the data or and comparative data from Europe to determine where the swarms originated. Maybe it is time to develop a pan-European monitoring network!