Friday, 27 October 2017

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.

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