Autumn Field Meeting

Two days into the meeing and we are making good progress. It has been hard work though, especially as ever woodland seems to be full of brambles! Last year I described a woodland as a 'bramble patch with a few  trees sticking out of it'. This year the level of bramble infestation is lower but it is still very difficult to avoid shredding one's net!

Impressions so far:

• Fungus gnats numbers are low. Samples are rather homogeneous  and  we have recorded a smaller number of species than at the same time last year.
• Craneflies are in reasonable supply but there are relatively few Tipula species. So far, Limonia nubeculosa has been comparatively scarce.
• There are very few Lauxaniidae - in fact virtually none!
• Drosophilidae are very scarce. 
• Heleomyzidae are in low numbers.
• Hoverflies are scarce but about par for this time of year.

Our group is small but select: Alan Stubbs, Peter Chandler, Andrew Halstead, Malcolm Smart and  John Bratton plus of course yours-truly.

I had a narrow escape today whilst in pursuit of hovers on a patch of rank vegetation, falling  down a manhole whose cover had been removed. I was exceptionally lucky that I escaped with nothing worse than sitting on a few stinging nettles which duly exacted their revenge on me!

We are based in Menai Bridge for this part of the week. It is a place I knew only poorly when based in Bangor  30+ years ago.  Now, it seems positively excellent compared to Bangor; which seems to have few redeeming features. Why on earth did I live there for 3 years?

It is not too late to get out

The weekend approaches and there is still a chance of a hoverfly or two. A few are really dominant, most notably Eristalis tenax. There are plenty of places where this relatively abundant species has yet to be recorded, so a bit of effort may mean a new ten km square record (see map on HRS website for the gaps http://www.hoverfly.org.uk/portal.php?id=10959&scol=HFF0000&bkgrd=1&ecol=H0000FF&page=4). Sericomyia silentis also tends to visit ivy and may be seen occasionally at this time of year. Records are still arriving via ISpot.

October can be an extremely good month of Arctophila superbiens, which is mainly a northern and western species, but it does still occur in North Norfolk and it would be good to see more records from eastern England as well as from more western and northern localities.. This is a species that we think is likely to decline in southern regions as a consequence of climate change, so the more records we get the better its fortunes can be tracked. If it is a nice day, why not go on a quest for this hoverfly in October.

Finally, those in East Anglia should not forget the possibility of Callicera spinolae. It could turn up almost anywhere and has been found in urban parks, formal gardens and in woodlands. A peruse of the ivy patches is a worthy pursuit. But, beware, C. aurata can also fly late and is jolly similar.

As a general point it is worth noting that all records from this time of year are welcome. People tend to hang up the net and the notebook but there are flies to be seen. I noted seven species at a local ivy patch the other day and am hoping for lots of flies when we get to North Wales for Dipterists Forum's Autumn Field Meeting which starts tomorrow.

Ash dieback - a threat to Criorhina and other saproxylic hoverflies

A couple of days ago a friend drew my attention to an article in The Telegraph that highlighted the emerging risk of a new threat to Britain’s trees: Ash dieback caused by the fungus Chalara fraxinea. The story was deeply alarming, suggesting that it had swept across mainland Europe and was killing the majority of Ash trees that it affected.  In Denmark it is reported that 90% of trees are affected! Amazingly, despite the known problems with this pathogen, imports of trees from continental nurseries has yet to be stopped. This is despite known cases of infected trees reaching the UK via the nursery trade. At the moment all known cases involve trees whose health seems to have been monitored, allowing intervention to take place and infected stock to be destroyed. It can surely be just a matter of time before the disease takes a hold unless some urgent action is taken?

Although Ash does not immediately register in the lexicon of important trees for insects, it is actually of considerable interest, especially for the saproxylic diptera that are associated with rot holes and rotting stumps and roots. Woodlands on the borders of Lincolnshire and Northamptonshire are a good example. I never cease to be amazed at the numbers of all four of our Criorhina species in local woods. Two species really stand out: C. asilica and C. ranunculi; neither of which was very abundant in my former stomping ground in Surrey. In northern Northamptonshire they are really quite common and I would expect them to turn up in most long-established Ash woods. I was puzzled by this difference until I discussed it with Alan Stubbs who drew my attention to the key difference in the woodlands.  Those around Stamford support a high density of Ash, whereas in Surrey Oak is much more dominant.

This one small observation makes me very nervous about the future of Britain’s saproxylic hoverfly fauna. Not only would Ash dieback have devastating landscape implications; it would seriously affect one of our most important habitats. The critical issue is that of habitat continuity and in the case of saproxylics that has got to be a key consideration. It is no surprise that ancient woodlands are richer for saproxylics, as they represent the most constant supply of habitat.

What can be done? Probably not a lot, but there is a consultation on the risk assessment by FERA – see http://www.dardni.gov.uk/index/consultations/active-consultations/consultation-by-fera-on-ash-dieback.htm.

This apart, it seems to me that there is an obvious need for urgent measures to survey our Ash woods to determine their real importance for saproxylic invertebrates. This could form a large-scale citizen science project to evaluate Britain's woodlands for the threat Ash dieback poses to one element of our wildlife. 

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.

Hoverfly abundance in 2012 - some early indications

In an earlier post on Volucella zonaria, I highlighted the difference of opinion between my interpretation of Volucella zonaria's  abundance in 2012 and the interpretation in Buglife's press release. I have now done a little bit of analysis which I think sheds light on what has happened this year.

The most compelling evidence for an unusual number of records is that of Helophilus trivittatus which has occurred in exceptionally large numbers. Meanwhile, lots of observers have noted that Episyrphus balteatus has been particularly scarce. Numbers of Volucella zonaria do not appear to me to have been exceptional and a basic analysis of the data from the photographic record seems to confirm this. In order to get a broader picture I also looked at Volucella inanis which has some similar characteristics and is equally noticeable and recordable.

In the past few years I have assembled a dataset of  over 9,800 records from photographs on the web and sent to me as casual inquiries or reports. This dataset is interesting because it largely reflects the species that are seen by the general public and as such is representative of the reports that might have stimulated Buglife's press release.

The data show some interesting trends for 2012.

Aphidophage and aquatics

Figure 1. relative frequencies of Episyrphus balteatus, Eristalis intricarius and Helophilus trivittatus 2004 - 2012

Episyrphus balteatus as a proportion of records has fared very badly with around half the long-term average number of records.

Eristalis intricarius appears to have done relatively well but its numbers do not exceed those of the better previous years

Helophilus trivittatus has had an exceptional year with three times more than the long-term average of records.

Social wasp and bee associates

 Figure 1. Relative frequencies of Volucella species from 2004 to 2012.

Volucella bombylans shows no obvious trend with numbers bouncing around within a consistent level of variation..

Volucella inanis seems not to have changed greatly in its frequency over the past three years and numbers lie below the long-term average which is skewed by data for 2004.

Volucella pellucens appears to have occurred in similar numbers to previous years, but shows a marked drop in frequency in 2011 and a recovery in 2012.

Volucella zonaria numbers are marginally up on the long-term average, but at such a low level that it is difficult to see any reason to assume that anything unusual has happened this year. Part of the reason that numbers are elevated is that additional data arrived via Buglife (about 10% of the sample) and consequently the figures are skewed. Removing these data from the sample would bring the proportion down to 4.89% and this equates to marginally above the long-term average (4.74%) which is not statistically significant.

Analysis

The data presented above are extremely preliminary and will need to be compared against the bigger dataset in the Hoverfly Recording Scheme but they do seem to confirm some of the apparent trends for 2012.

The poor performance of some aphidophagous species is clearly illustrated by Episyrphus balteatus. I will return to this on another occasion as it seems that some greater analysis is needed for this guild.

By comparison, the Eristalines appear to have done exceptionally well.  This must also be a point of further investigation but the data for both Helophilus trivittatus and for Eristalis intricarius, both of which have not been particularly obvious in recent years, suggests a strong gain in numbers.

The data for Volucella zonaria are interesting, however, when you look at the numbers of records in 2011. This shows a quite clear dip in the numbers relative to other species and suggests that V. zonaria was not as frequent in 2011. The reason for this is most likely to be a response to the exceptionally cold winter of 2010/11. There are precedents as Stuart and I showed in our papers exploring the responses of Volucella zonaria and V. inanis over the past 60 years (see http://www.bacoastal.co.uk/Entomology/2004-Volucella-zonaria.pdf and http://www.bacoastal.co.uk/Entomology/2003-Volucella-inanis.pdf). Both species underwent substantial range contraction in response to the cold winter of 1963, but clearly that was considerably more dramatic than that of 2010/11. It is also very noteworthy that a similar dip can be detected in Volucella pellucens which I did not expect. Further analysis is needed for this species too.

Looking at the data, I suspect that any detailed analysis should omit the data for 2004 and 2005 because there were few photographic records exist for those years - see below. However, what is also becoming clear is that web-based data may prove to be a useful means of monitoring certain aspects of the country's biodiversity, even though it has significant limitations because only a proportion of the fauna can be identified from photographs.

Figure 2. Numbers of photographic records from internet sites from 22004 to 2012.

Garden recorders

As I wander around the internet ion search of new hoverfly records, it strikes me that there are lots of potential parataxonomists who take an interest in the insects that visit their gardens. Anybody who regularly keeps a photographic record of what they see can make an important contribution, especially if they do this over a period of several years. There are several people who regularly post photos from their gardens and a few whose garden photographs now form a significant data set in its own right.

Garden recording might just appeal to people whose mobility has declined and yet they want to get involved in biological recording? It might also appeal to a younger generation as a way of getting into insect identification?

It would be great to set up a network of garden recorders but the big question is how to reach them and how to get the message out to people who currently don't use the established mechanisms for posting photographs such as Flickr. I would be grateful for any ideas.

There are some taxa that are better suited to photographic identification than others. For example, shieldbugs, ladybirds and some other beetle families such as longhorns are potentially viable. Hoverflies are not too bad and in my experience about 80% of the species that are regularly encountered in gardens can be identified if the photo captures suitable angles. Some genera are tricky; for example Syrphus, some Eupeodes, Platycheirus, Cheilosia, Eumerus and Pipizines. Even so, progress can be made and I know of one garden where the photographer has recorded well over 40 species of hoverfly. That list would be about 10 species longer if it was possible to ID Eumerus, Pipiza and some Syrphus from photos.

The same approach might be taken with a local wildlife site. We have started to trial the regular monitoring of hovers through our 'Big Hover Watch' - which was largely washed out in 2012! It would be useful to get a feel for levels of interest amongst the non-specialist photographers.

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.