I have spent a sizeable part of the day trying to make something of the data that are emerging this year. My feeling is that there are probably quite a few species that have suffered but there will be a few that have benefitted. The same species may have lost out in one part of the country and gained elsewhere. I have started by looking at five examples:
- Episyrphus balteatus, which seems to be having a good year and was far more abundant than expected in the winter.
- Eristalis pertinax, which has appeared to be poorly represented in recent data.
- Eristalis tenax, which appears to have been more dominant in recent data.
- Helophilus pendulus, which has generated regular comments about its apparent scarcity.
- Rhingia campestris, which has been notable by its absence even though it is a highly distinctive species.
To do this, I have compared the proportions of records rather than the absolute numbers of records. This approach is necessary because the numbers of records and recorders vary from year to year; as does the geographic spread of recording. So, context remains an important part of the analysis. Each species is compared against the data for the previous six years (2013 to 2018) extracted from Facebook and other social media.
Episyrphus balteatus
This is an aphidophagous species that occurs throughout the year but in particular dominates the records at certain times. In mid-summer, it vastly outnumbers all other hoverflies but peaks at differing times.
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Figure 1. Episyrphus balteatus
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As the data has grown, it has been clear that
E. balteatus has been far more dominant than in other years. The precise reasons are unclear, but it seems likely that the warm winter allowed dults to be far more active and for the larvae to get started earlier in the year.
Eristalis pertinax
The larvae of
Eristalis (and
Helophilus) are aquatic.
E. pertinax usually has two peaks, one in the spring and the other in late-summer. Numbers should start to rise in July but as yet that has not been detected.Late autumn abundance is very variable, depending upon both temperature and sunshine. It usually starts to disappear in late October but can be abundant into December during warm autumns.
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Figure 2. Eristalis pertinax
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Current evidence suggests that tthe spring generationwas smaller than the long-term average. It is probably too early to be sure that the autumn generation will be smaller than normal, but a recent dip in numbers may be indicative of this species having experienced a poor year.
Eristalis tenax
Adults usually hibernate, but in recent years this has been considerably less clear from the data. Winter 2018-19 was relatively warm but the numbers of
E. tenax recorded seem to have been lower than the longer-term trend.
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Figure 3. Eristalis tenax
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Despite comparatively low numbers of winter records, it seems that
E. tenax is experiencing a good year, with numbers noticeably higher than the longer-term trend.
Helophilus pendulus
Monitoring the Facebook group has pointed to this species having not been as numerous in previous years
. The phenology for 2019 is somewhat odd, with a much shorter spring generation.
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Figure 4. Helophilus pendulus
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I wondered whether there might be a regional reason for this difference so split the data into five regions as shown.
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| Figure 5. Helophilus pendulus phenology in England & Wales |
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Figure 6. Helophilus pendulus in Scotland
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The data present a very confusing picture that is difficult to interpret. If anything, numbers in Scotland appear to be marginally up on the longer-term trend, and South-East England seems t have somewhat elevated numbers but a shorter emergence season. Elsewhere, numbers seem to be lower than recent trends.
Rhingia campestris
This is a highly distinctive species whose larvae develop mainly in cow dung, although its distribution suggests that a wider range of dung is probably utilised. It is a species that has, historically, been considered to respond to hot summers and for some generations to almost completely fail. This was detected in 2018 and the response in 2019 is therefore of interest.
What emerges is a response that seems to be completely different in Scotland. Whilst there seems to have been a uniform crash in England and Wales, numbers in Scotland appear to be robust and possibly higher than in recent years. I suspect that the data for northern England may ultimately show a similar pattern to Scotland once records from various very active recorders are absorbed into the dataset (at the end of the year). Those from more southerly regions seem to suggest that numbers are exceptionally low; which is consistent with the failure of the summer generation in 2018 and thus a failure of breeding success to create the spring generation.
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Figure 7. Rhingia campestris in England and Wales
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Figure 8. Rhingia campestris in Scotland
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I suspect that the elevated numbers in Scotland can be explained by the higher temperatures in 2018, which allowed greater breeding success in a region where higher temperatures would have allowed more rapid population build-up in late summer and consequently more larvae available to create the spring generation.
