Friday, 28 September 2012

Volucella zonaria records

This summer, Buglife issued a press release that argued that there had been an exceptional influx of Volucella zonaria from the continent.  Prior to this I had a lengthy exchange with Steve Falk in which I argued that there was very little evidence for a migration. In general the traditional migrants such as Episyrphus balteatus, Eupeodes corollae and Scaeva pyrastri have been very scarce compared to previous years, so it is not a year that is obviously suited to major influxes.

The Buglife case was based on the relative scarcity of males amongst records. This I also think is no reason to assume that the records are of migrants. Males tend to emerge earlier in the season, and also tend to be scarcer in samples of all Volucella species apart from V. pellucens. I think the reasons for the imbalance lie in the behaviours of males and females. Males in some Volucella actively seek females and unless they hold territories (e.g. V. pellucens and V. bombylans) they are less likely to be seen. Females, on the other hand, will actively seek nectar and pollen sources and will therefore be seen more frequently.

Using photographic data for this year, a fascinating picture emerges with the emergence of males and females in each Volucella species differing greatly. What is clear, however, is that males emerge earlier than females and are present for a much shorter season.

 Volucella bombylans 2012

 Volucella inflata 2012

Volucella inanis 2012

 Volucella pellucens 2012

 Volucella zonaria 2012

These histograms raise all sorts of interesting questions about adult recruitment that have yet to be answered. Stuart and I looked at population dynamics in several species a few years ago. There is a lot more to be done. A study of the relative numbers of males and females of V. inanis and V. zonaria would be worth undertaking in a locality where both are abundant (maybe a project for an MSc dissertation). A friend of mine looked at V. zonaria a few years ago but has never written it up, so it is time somebody else got in on this question.

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