What can we do to increase the numbers of hoverflies?

My last post elicited some interesting and challenging comments on the UK Hoverflies Facebook page so it is time to start to think about what we can all do to help improve the numbers of hoverflies. To do this, I think we should start with what what the larval stages need rather than the adults require. Adults are simply the breeding and dispersal stage; whereas the larvae are the critical bit of the cycle. If there is no suitable larval habitat there won't be any hoverflies apart from those that are tourists from suitable sites.

We can break hoverfly biology into a series of guilds:

• Predators (on)
• Aphids
• Other fly larvae
• Lepidoptera larvae
• Coleoptera larvae

• Saprophages (dwelling in/feeding on)
• Rotting plant matter (including dung)
• Decaying sap
• Rotting wood (or more precisely fungal hyphae in rotting wood)
• Fungal fruiting bodies

• Aquatic filter-feeders feeding within
• Shallow parts of large water-bodies
• Small pools
• Water-filled rot holes

• Within plants'
• Leaf and stem miners
• Root and rhizome dwellers

The above is quite a simplification but it will suffice for this analysis. We also need to think about the duration of the life-cycle. Some species have multiple generations each year and their numbers can fluctuate markedly within and between years (often the colourful Syrphus and Eupeodes, the small but very abundant Melanostoma and Platycheirus, and the bee-like Eristalis that make up a big proportion of the numbers we see). Many others have a single generation each year; and a few take several years to go through to maturity (mainly those associated with decaying heartwood such as Callicera rufa and Blera fallax).

And, finally, we need to think about the durability of the food supply: does it fluctuate in volume over short periods of time or is it comparatively constant providing there are no stochastic events to interrupt availability? If food supply is constsistant then the animals probably don't have to move far and, once a new food source has been established, numbers of relevant hoverflies should increase. If the food supply is prone to fluctuation, the numbers of associated hoverflies are also likely to wax and wane. So, when we look at hoverfly abundance we can tell quite a lot about the consistency of food supply.

The more specialised the hoverfly, the greater the likelihood that its numbers will be comparatively low. For example, species that breed in decaying sap under bark will only be able to do so for a short period whilst the sap is available and may occur in high numbers at a small site for a short time; after which they must disperse to find new breeding sites.

Design of measures to enhance numbers

If we want to increase the numbers of 'pollinators' we need to think about which are the main pollinating species and whether we are talking about crop pollinators or pollinators of wild flowers?

For crop pollinators, my impression is that the bulk of these are species that breed up in large numbers over short periods of time – Eristalis, Eupeodes, Melanostoma, some Platycheirus, and Syrphus, and are numerous in the spring. Many more species will play their part in the pollination of wild plants with some being potentially important e.g. Cheilosia bergenstammi gets covered with ragwort and dandelion pollen.

So from a crop pollination perspective, filling in those field boundary ditches probably eliminated your Eristalis and the removal of the hedgerow and associated leaf litter will have eliminated first generation Melanostoma and Platycheirus. The loss of tall herbs such as hogweed and angelica in the hedgerow will have eliminated many Syrphus, Eupeodes and Melangyna. The same holds in our gardens – raking up all the leaves and burning them or composting them will remove over-wintering larvae of the afore-mentioned and, if you have fruit trees, you will be loosing your Epistrophe eligans larvae (which can be very abundant on plums in particular).

A more comprehensive strategy is needed to cover the wider spectrum of species that inhabit unusual or specialised habitats. For example, that single gnarled ash, sycamore, horse chestnut or beech in the hedgerow will provide wet rot holes for specialist filter-feeders and rotting heartwood for specialist saprophages such as Criorhina sp., Mallota cimbiciformis, Myolepta dubia and Xylota segnis. This also applies to our parks and town trees: cutting down the character trees is destructive, but then employing the stump grinder adds insult to injury by eliminating a potential 20-year food supply for the rotting root dwellers.

To many people, a field edge full of creeping thistle might be a major problem, but for some hoverflies they are bliss – Cheilosia proxima for example will be busy mining the stems. Likewise the wet field with marsh thistle may provide breeding grounds for Cheilosia albipila, C. fraterna and C. grossa.

Small-scale and landscape scale

Within the urban environment we can do a lot by creating small pools, log piles, litter piles or by not over-tidying leaves in the autumn. Our local authorities can stop using stump griders and could place felled trunks in semi-shaded places where they can be felt to gradually rot away (avoid intense sunlight as this dessicates the timber too quickly and creates a hostile environment for saproxylic flies (but some beetles like this sort of timber). Big timber with thick bark is especially valuable as the sap under the bark is protected for a couple of years and this allows the flies time to go through several life cycles.

Within the wider landscape, we need to think about the inter-connectivity of micro-habitats: field margins, ditches, hedgerows and hedgerow trees. Re-wildling is a great idea but is really rather impractical in most areas where we now have arable prairies. Nevertheless, creation of new networks of hedgerows, ditches small ponds and coverts would go a long way to make up the deficit. But, don't forget the specialist features too! For example, seepage lines are immensely important for a wide range of wetland Diptera, including many Platycheirus, Chrysogater and Melanogaster. Set those areas aside as wild places – they are often poor yielding anyway, as are the boggy low-lying areas near water-courses. And in sheep country – reduce grazing pressure on seepage lines so that there is decent vegetation structure rather than a short turf that quickly dessicates on a hot day.

Soil conservation

We hear a lot about 'special sites', but what makes them special? To my mind, the critical issue is that they have not been cultivated for a long time (rarely never) and therefore the soil structure is more suited to maintaining a rich and varied vegetation. We have long-overlooked soil mycology and bacteriology as important conservation features. If the mycology is right you are far more likely to get the hoped-for vegetation. Safeguarding what we have is essential, whereas creation of new habitat on former arable land, that has been sterilised by years of deep-ploughing, fertiliser application and use of pesticides, is always going to be less successful in the short- to medium-term and possibly also way beyond that. 

If, however, all you have is sterilised soil  you must work with what you have and be grateful for what arrives and thrives. But, by thinking about micro-habitat you might just create something that stands a better chance of creating the breeding grounds for hoverflies and other Diptera that are one part of the pollinator assemblage. Measures for solitary and social bees differ somewhat but simply involve the adoption of a multi-layered approach to habitat design.

The moth snowstorm and insect decline

Having just listened to Adam Rutherford's piece on insect decline on Radio 4 tonight I was reminded of my early days as a moth enthusiast. One night in around 1983/4, the late Steve Church (who founded Bioscan UK) and I went to Salcey Forest. We arrived after dark as Steve was never very reliable with his time-keeping! But, perhaps that was fortunate because I well remember the absolute blizzard of moths in the headlights that night! Equally, I am reminded of the wonderful flight of the Ghost Swift moths along the edge of Mitcham Common in July those days. I don't remember when I last saw a Ghost Swift, let alone dozens bobbing around in the gloom.

But, again, I heard a bit of under-play of current levels of recording from Dave Goulson, who commented that we have good data for butterflies and moths, and less reliable data for bees. I do wish that a bit more attention was paid to what actually goes on - we don't lack biological recording but we do lack the tightly regulated monitoring that might be achieved by an expensive long-term study. So, do I detect a bit of propaganda here? I am reminded of a meeting with another academic group that was proposing to seek a research grant to investigate the distribution of Horseflies in the UK who had not even heard of the relevant recording scheme that had been running for way over 30 years!

To be fair to Dave Goulson, he did mention that there were data for bees, but he might also have mentioned that there are around 80 long-term voluntary data collection programmes for Britain's biodiversity and that the UK pretty well leads the World in the use of non-vocational data collection. Despite this, we don't have the data that the German 'Amateur' entomologists have gathered (I hate the term 'Amateur'); but just take a look at the volume of data on GBIF and you will find that for almost all taxa the UK is a bright red mass when compared to the rest of Europe, let alone any other parts of the World. Those data are actually pretty powerful for quite a wide range of invertebrates and have been used in many studies that have identified equally worrying trends in invertebrate abundance.

And, sorry to say Dave, if the data for bees are ok but limited, those for hoverflies are probably rather more robust! As I recall the HRS and BWARS hold roughly similar datasets in terms of numbers of records, but whereas BWARS covers perhaps 550 species, the HRS covers 283 species! We now hold well over a million records (but with some duplication). So lets see some acknowledgement of those other schemes that regularly provide good quality data for academics and Governments to make use of; and we do it all for nothing!

A more critical message

I will, however, concede that we and others could have made a lot more noise about the issues of insect declines and that the German data do tell a particularly important story that does not get picked up in UK data. The big point that the German data tell us is the absolute biomass change and that is critical. Sadly, UK recording schemes don't give us this sort of result and it is of massive importance.

Declines in individual species using occupancy models mask the scale of decline that is potentially happening: if they rely on presence/absence, then one record for a 1km or 10km square is equally significant and does not change the model. So, somehow, the next generation of models needs to take account of absolute numbers of records. Add to that, we desperately need to see far more reliable day-to-day recording so that changes in the relative frequency of individual species can be detected. I have previously used the case of the Passenger Pigeon and perhaps there are analogous species in the insect World that are heading in the same direction? Just because it is common does not mean that it is not worthy of recording!

Do we need more funding?

It would be easy to say yes, but I think we have got other work to do that does not involve funding. It requires the recording schemes to make a concerted effort to break the community of natural historians away from a fixation on rarity and upon dots on maps; both of which are a problem if one wants to get a decent dataset that tells us something about overall population changes.

So, as a first step, let us see more recorders making daily counts of the animals that they see. Please try to note full lists for each day rather than just the highlights. For me, this is the great benefit of BirdTrack and as such I want to see the HRS move in the same direction. We really need a network of recorders who record all species on as many days of the year as possible.

Similarly, we also need to generate a much bigger base of people who can identify the full range of species, or who are prepared to retain specimens for technical specialists to analyse. For me, the recent publicity on insect decline highlights the importance of reliable data collection and the importance of whole assemblage data. It shows how the 'take nothing but photographs, leave nothing but footprints' approach leaves conservation science weakened so that we cannot provide the robust messages that the German dataset has provided: full marks to our German counterparts!

Now all we have to do is to move the goalposts and start hitting those targets: that means we need more competent invertebrate taxonomists, which in turn means that there need to be incentives for people to invest the many years that it takes to become taxonomically competent. Perhaps at last we are starting to realise that if you cut soft targets, they start to become a weakness in the system many years later that cannot be rectified just by throwing cash around!

Take photos and retain specimens - best of both worlds?

I have previously written about the way John Bridges has greatly increased his site lists by retaining specimens that he passes on to me for ID. There are a few others who do the same. Yesterday I worked through a batch of specimens collected by Martyn Hnatiuk who is based in South Wales. That was highly instructive and useful because Martyn managed to collect several species that are very rarely recorded and would not have been identified from photographs. Three stand out above the rest:

Cheilosia carbonaria - a species that is regarded as 'Nationally Scarce' and which I have not seen in ten years! This specimen confused me a great deal because the antennae were quite strongly orange but the wings were smoky. At first I wondered whether it was a new species but upon checking Van Veen it ran very easily to C. carbonaria. This is an object lesson in the need for care when using what seem to be reliable characters. In Stubbs & Falk, C. carbonaria sits in a group of species with dark antennae and this specimen refused to go that way. So, we would never have achieved a reliable result from a photo and using Stubbs & Falk. It shows that there are always the exceptions that prove the rule!

Eupeodes nitens -  I have only seen this one a couple of times, several decades ago, and there are very few reliable recent records! Again, it did not wholly follow the key in Stubbs & Falk, but readily dropped out using Van Veen. This one would never have been done from a photo because the critical diagnostic characters are on the sternites of the abdomen.

Trichopsomyia flavitarsis - a species that is closely associated with very acid sites and which we very rarely get records of. At one time I used to see it pretty well whenever I went to the north-west (or at least that is my memory) but I've not encountered it for a very long while so it caused me quite a lot of confusion at first.

So, I wonder what else is out there that more recording might turn up? Martyn got a fair selection of  species that we would not have managed to ID from photographs, thus greatly improving the resolution of his data. John Bridges has done much the same. Perhaps others would like to do something similar?

Do we understand pollinator abundance and population trends?

Dave Goulson's recent editorial in British Wildlife raises an important point about the issue of pollinator abundance. His analysis does, however, overlook the fact that there are active data collection processes for the most obvious pollinators. Existing datasets compiled by the Hoverfly Recording Scheme and BWARS are regularly used by CEH and various university groups to produce new analyses and in the development of new analytical techniques. So, everybody who posts on the UK Hoverflies Facebook page is contributing to the research. Nevertheless, he is certainly right in saying that there is a lack of data for many families of flies and for some other insect Orders.

In broad terms, we have a very good understanding of what is happening to our hoverfly fauna - somewhere in the order of 40 to 50% is declining and perhaps 15-20% is increasing. We see an expansion in the range and abundance of southern thermophilic species such as Volucella zonaria and V. inanis and Rhingia rostrata. Declines are highly apparent among those species that favour damper conditions. I think we are starting to see the loss of some species from south-east England. For example, the spectacular blue-marked Leucozona glaucia seems to have substantially disappeared from much of south-east England in the past 20 years.

Techniques used in analysis of trends have evolved, and new occupancy models are emerging on a relatively regular basis. They are all dependent upon a continuing stream of data, which makes it imperative that BWARS and the HRS remain active and train new recorders. Perhaps even more critically, we need to develop succession plans to make sure that the process of data assembly and dissemination continues; it is a very labour-intensive  and is starting to become more of a challenge as the numbers of active recorders grow. Nevertheless, this challenge is, in effect, a good news story because it shows that there is positive progress in data assembly.

BUT, are hoverflies really important pollinators?

What are we actually talking about when it comes to pollinators? Politically, the main focus will be on pollinators of commercial crops, many of which flower early in the spring. Thus, a decline in species that fly after critical crop pollination time may be of little commercial interest and therefore equally of little political concern. I am not sure we have really made that distinction yet, but if one was to do so it might help to focus attention on other parts of the insect assemblage such as Anthomyids, Muscids and Calliphorids; or perhaps even Bibionids that occur in vast numbers for very short periods of time?

We do have other proxies for insect abundance. For example, the numbers of insectiverous birds. These too are substantially declining. Why? Well some of the reason lies in general agricultural intensification and loss of wild places in the countryside matrix. Partly it may be associated with the development of vast monocultures and the apparent focus on the same crops in the same fields year after year. A reliance upon pesticides is a likely further factor that diminishes the food supply at critical times of year.

But there are other factors too. For example, there is a developing trend for insects to emerge earlier in the year during a brief warm spell in late March and early April, before a cold snap in late April and May knocks them down. This sort of seasonal change seems to me to be having a profound impact on insect populations. Equally, we quite frequently get short bursts of intense heat in late June and early July, which probably knock out larval stages. I suspect drought and heat stress are a very important factor behind changing insect abundance in south-east England. Warm, damp summers are always looked upon by the public as undesirable, but regular rainfall is probably the single most important factor behind the maintenance of insect populations in Britain. If anything, we are seeing greater stability in insect numbers in northern and western areas - this certainly seems to be the case for hoverflies.

Do we need better monitoring?

Of course the simple answer is yes! But we also need to be clear about the objectives of monitoring and the degree to which we try to link monitoring results to politically sensitive issues rather than to a broad spectrum of ecological factors. Should monitoring focus on species that are potentially important commercially, or should we be looking across a wider spectrum? If so, will the same monitoring systems deliver the results needed for each purpose? Or, can we use proxies - a general pollinator monitoring scheme that assumes that the trends are the same in different families of bees and Diptera?

Looking at the issue of monitoring from a purely practical perspective, I think the current use of datasets compiled by the HRS and BWARS is probably the most viable option in the long-term. Government-funded packages might generate data collected in a more consistent fashion, but they are always going to be vulnerable to cuts. We have already seen this with the long-term data for Atlantic plankton and in the Rothampstead moth trap programme; both of which generate immensely important and instructive messages; unfortunately the messages are not all positive and they don't generate the steady stream of high impact papers that keep research funding flowing. So, voluntary data collection remains the only reliable source of information on trends. That makes it imperative that the core functions of the Biological Records Centre at Walligford are safeguarded.

So, what can you do to help?

There are several aspects to monitoring:

• Range and distribution: what occurs and where does it occur?
• Abundance: in what numbers do individual species occur?
• Variation: how do numbers of species and absolute numbers change regionally and nationally over individual years and in longer-term units?

At the moment, we have a relatively small number of people who regularly record from their garden or wildlife area. There are several very committed members of the UK Hoverflies Facebook group who record on a daily basis. More of this sort of recording is likely to be helpful because it may be possible to take sub-sections of data for analysis if we have more such datasets. Occasional ad-hoc records are also useful because they help to fill in gaps in distribution data. Occupancy models depend upon good general coverage, so even a few common species recorded from a location can make a difference; this is particularly important in places where there are limited numbers of active recorders or that people don't regularly visit.

Dipterists Forum Autumn Field Meeting - Farnborough and district

In addition to the week-long trip we had at Loch Lomond in September, There was a 'spur of the moment' trip to the Surrey/Hampshire borders by a small group of die-hards who find it difficult to hang up the net and pooter for the year. In the past there has been a four or five-day meeting around the third week in October and a few of us wanted a bit more field work before winter sets in. Thus, I booked three rooms in the Travelodge in Farnborough for Peter Chandler, Alan Stubbs and myself; Andrew Halstead, Tony Davis and Mark Mitchell joined us for all or part of the time, travelling from home.

Most of the sites we looked at were in north Hampshire (many thanks to Tony Davis for organising access permission), although Alan, Mark and I did take a look at the area around Shere and the Winterfold Forest in Surrey on the Saturday.

Mark Mitchell demonstrates the newest technique for extracting flies from a pooter - centrefugal force to stun them and send them to the end of the pooter with Peter Chandler looking on in amazement!

The trip was relatively uneventful and we have still to get the results from the samples of fungus gnats that were gathered. My impression was that there were plenty of gnats, but that range and variation was limited. I saw precious few Boletophila or Macrocera and it seemed to me that the majority of specimens were from within the Mycetophilidae. Maybe the results will differ, as there were at least four of our party collecting gnats and there should be a very big selection to choose from. If we are lucky, we might have managed to find somewhere around 130  species but I think I might be a bit over-optimistic in that estimate.

Craneflies were incredibly sparse in both numbers and species diversity. I would be amazed if we managed to find 30 species over the four-day trip! On the plus side, I cannot recall ever seeing so many Heleomyzids but even this assemblage was odd: the bulk of my samples were Suillia with just the occasional Tephrochlamys. Drosophilids were fairly abundant (especially D. suzukii) but Platypezids were also noteworthy by their absence.

Alan Stubbs, Tony Davis and Andrew Halstead deliberating over the choice of next site

Although perhaps not the most rewarding meeting from a recording perspective, we did manage to visit nearly 20 localities and covered an area that has not previously been investigated by an Autumn field meeting.

In addition we experienced the most odd weather at Greywell Moor where light intensity eerily dropped to dusk-like levels around 2pm as the edge of Hurricane Ophelia drifted past on Monday the 16th. One could almost imagine that this was the start of the Martian Invasion and would have made an amazing backdrop for a rendition of Jeff Wayne's 'War of the Worlds' (the Richard Burton version) ---- and it was not that far away from Horsell Common either!

Mitcham Common survey - A reflection

From August 1983 to September 1984 I led a team of six recent graduates employed to conduct an 'Ecological Survey' of Mitcham Common. It was funded under the Manpower Services Commission's 'Community Programme'. The timing was far from ideal because most of the time was Autumn and Winter, whilst by July we were stuck into writing our report. Nevertheless, we managed a great deal and produced a useful report. Looking back, I certainly learned a great deal from the project, and I know that at least one other team member benefitted from the experience and went on to a career as an ecologist.

But, I hear you ask, 'were the results reliable and trustworthy?' Well, they were as good as we might hope to achieve. Where possible, we sought expert advice to validate our diagnoses, and I am pretty sure that most of the results were reliable; even if there were misidentifications in places. At the time, I felt the biggest problem area was the macro-fungi; this remains my biggest concern in terms of diagnostics. Today, I have greater misgivings about some of the recommendations that were made. In hindsight I (as team leader and editor) lacked the experience to make some of the judgments I would have made today. Hindsight is a wonderful thing!

There were about 60 copies of our report; most of which went to the Board of Conservators and to the London Borough of Merton. Two went to the then Nature Conservancy Council and one to the London Ecology Unit but I cannot recall any others going anywhere useful. How many survive? Very few, I suspect, and of those most will be in inaccessible personal libraries or filing systems. I expect many will have been pulped!

Fortunately, I do have a couple of copies and last year I decided that it was a lamentable waste of public resources to have the report but not to have it in an accessible form. I therefore OCRd the whole report (nearly 300 pages of tables, diagrams, maps and text). The OCR process and scanned illustrations required a lot of work to turn them into a decent machine-readable version, so in the end I re-typed many of the tables and re-drew the graphs. Sadly I did not have the original raw data so some of the graphs are best estimates based on the originals. Nevertheless, the whole package is now available in machine-readable form. I have sent it to the Warden so he has a copy, and hopefully it will be made available to a wider audience; not that it is thrilling reading, but it is a valuable baseline.

Future thinking

In my view, Mitcham Common is one of the most important wildlife sites in south London and therefore it is important to make sure that there is a permanent archive of relevant science. There is a very old paper on the birds by my father (sadly his diaries were lost when Mum cleared his effects) and of course there are papers by Louseley and Parsons on the botany. Since 1984 I have published accounts of the aculeate Hymenoptera and some aspects of the Diptera, but there is a lot that has yet to be studied or reported. I have a good many more Diptera records now, so maybe I will write another account. That leaves an awful lot more to do.

The 1984 team barely scraped the surface of the Coleoptera and Arachnida, whilst I dare say a lot more could be made of the Lepidoptera (I know David Lees did a lot on micro-leps in the early 1990s but I guess his report has been lost).

Looking at the 1984 report, I lament the lack of foresight on my part. I should have created a photographic library of the site; it has changed immensely and having detail would help to put some context into the changes in both the animal and plant components. I also wish that I had been more diligent in collecting Diptera from 1984 onwards – I am sure the fauna has changed markedly as the site has dried out very substantially.

So, the big question is 'can anything be done to create a new baseline?' I don't feel equipped to take on the full panoply of taxa and my botany is too rusty to re-do that part of the study. I do wonder, however, whether there is scope to gather together a group of local specialists to re-survey the site and to try to fill in some of the gaps using modern techniques? If there was interest amongst local Coleopterists, Arachnologists, Mycologists etc. then perhaps a new project could be developed, perhaps providing training to a new generation of aspiring specialists. What is needed is a vision to provide focus, some leadership and willing volunteers: any takers? At the moment I am able to offer time and enthusiasm, but I would not do so unless there is a group that would also be willing to participate.

An opportunity to improve our knowledge of Diptera

The fungus season is with us, and yet again I find myself thinking that I really must start to try to breed flies out of fungi. Each year I fail to do so; mainly because I am not confident that I can reliably identify the fungi concerned. This year is no different! There are lots of Russula and other species to choose. So perhaps I will have another go!

What do you need to do?

If you find a fungus with larvae in it, retain it in an open container - do not place in a plastic bag as CO2 will build up and kill the larvae.  When you get it home, place it on top of some sterilised coir (heated and dried in the oven to kill existing larvae) or similar material that will soak up the decayed fungus and provide a pupation site for the larvae. Cover the container with a fine gauze and place in a position where you are likely to regularly inspect it.

In due course, fungus gnats and other fungus-feeding flies (e.g. Platypezidae, Helomyzidae and Drosophilidae) should emerge. These can be removed from the container and stored for later identification. If you don't have access to ethyl acetate, which is the normal killing agent used by entomologists, then the freezer works just as well.

Keep a record of where your fungus was found, what species it was (if not to species then to genus) and when the flies emerged from it. Later in the winter these flies can be passed on to the fungus gnat recording scheme and will help to fill in gaps on the maps but, more importantly, they might add useful ecological information on the host fungi used by different species.

Alternatively

Try watching fungi for the flies that visit them, and then catch the flies up for passing on to the recording scheme (care needed not to inhale fungal spores if you use a pooter). Armillaria is especially good and can yield a lot of species if one takes a diligent approach to watching and collecting.

The majority of fungus gnats require dissection and cannot be identified from photography, but there is an exception in the Platypezidae, some of which can be identified from photographs.