Wednesday, 3 October 2012

Episyrphus balteatus in 2012

In my post yesterday I touched on the low numbers of Episyrphus balteaus recorded this year. This is made much clearer by comparing the data for the years 2010, 2011 and 2012 (graph below). The graph was compiled solely from photographs posted on websites and as such it is incomplete. The messages that it conveys are, however, a useful indication of the pwer of ad-hoc recording that is based on what non-specialists actually see.

Figure 1. Relative frequency of Episyrphus balteatus in 2010, 2011 and 2012 against week number (sequential from 1 January).

There is one very clear similarity between the three years: they each show a distinct peak in occurrence towards the end of July, followed by a rapid decline in numbers towards September. The differences are more striking, however:

In 2010 there seemed to be little or no occurrence prior to the spike at the end of July and the period of peak occurrence lasted for no more than five weeks.

In 2011, despite the very cold winter of 2010/2011 there were regular records throughout the winter. Small peaks prior to July suggest that numbers were gradually building prior to the main peak which started in mid-July and ended in early September.

In 2012 the spring was very early and it appears as though one or two small emergences took place prior to the main peak which commenced in mid-July and rapidly fizzled out. The drop in numbers at the end of August remains a constant feature.

What do these data tell us about Episyrphus balteatus populations?

Each graph tells a different story. My initial interpretation of the spike in 2010 is that this largely represents an influx based largely on continental migrants. A similar situation obtains for 2011 although this appears to involve two spikes, possibly reflecting native bred and continental populations? The picture for 2012 suggests that neither the native or the continental spikes really happened.

These data are only a proportion of the data available and a clearer picture should emerge when we analyse the Recording Scheme dataset. That will be some time in the future as there will be several months before the bulk of this year's data arrive and are processed. However, it does seem that the phenology of E. balteatus offers some interesting trends that might be linked to climate variation and which in turn might help to inform predictions about the ways in which critical predaceous species may respond to differing scenarios of climate change.

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