Savage unsung hero is a gardener’s best friend

TWICE a year, in the spring and autumn, the National Biodiversity Data Centre produces an online magazine called Biodiversity Ireland.

It’s something to look forward to because the articles are about fascinating aspects of natural history and are always beautifully illustrated and written in very simple language.

The spring issue is just out and the article that caught my attention was about hoverflies. I see them about the place but I don’t know much about them, apart from a vague notion that they are important pollinators in my garden. But the distinguished zoologist Martin Speight convinced me in his article they are fascinating insects which I want to find out more about.

Some 850 species of hoverfly have been identified in Europe and around 180 have been recorded in Ireland. None of these, as far as I know, has a common English name, so the amateur naturalist trying to educate him or her self about them has to contend with the binomial scientific names, which are usually impossible to pronounce.

Episyrphus balteatus (inset) is a common and rather pretty Irish species in a livery of orange and black. It’s a gardener’s friend. Its larva, a multi-coloured maggot, is a fierce predator of greenfly. It stabs them and sucks their juices out. It can account for over 600 greenfly in a fortnight. When it hatches into a winged fly it becomes much more docile, lapping up nectar and pollinating many plants, including orchard fruits.

This species hovers in a horizontal position, like a helicopter, its wings beating so fast they can’t be seen — which makes the whole thing look rather miraculous. Other species hover vertically, with their tails down, which looks even stranger.

About a third of the species found in Ireland have predatory larvae. About another third live on dead organic matter and the remaining third eat living plant tissue. These can attack bulbs, tubers and corms.

Many hoverfly species protect themselves by imitating venomous insects like bees and wasps. But spider orchids of the genus Ophyrs have turned this trick around by producing flowers that look like female hoverflies from the genus Microdon. Sex-crazed male Microdons attempt to mate with the flowers and the orchids get pollinated.

This must be a very ancient evolutionary trick. Today both the fly and the flower are very rare, spider orchids are not found at all in Ireland, and they seldom occur in the same place, so the orchids must have developed a new trick to get themselves pollinated.

Some hoverflies spend their larval stage under water, or more often in organically enriched mud. These larvae are called ‘rat-tailed maggots’ because of their very long, thin tails. The tail is in fact a kind of snorkel which the larva uses to get oxygen by sticking it above the surface of the water or the mud and sucking in air. Eristalis tenax starts life as a rat-tailed maggot and develops into a hoverfly that mimics a bee.

Syritta pipiens is another common garden species. It’s larva is a saprophage, meaning it belongs to the group that feeds on dead and decaying organic matter. They often live in compost heaps and bins, helping to turn your weeds, grass cuttings and kitchen waste into valuable fertiliser.

Rhingia campestris is particularly common in Ireland because of our generally mild and humid climate and our habit of leaving cattle out in the fields all year round, something that’s not common in other countries. This leads to a lot of cow dung being deposited in the fields and the larva of this species specialises in living in cow pats.

I’m delighted that Martin Speight has introduced me to such a varied and fascinating group of insects.

* dick.warner@examiner.ie

See also: www.biodiversityireland.ie


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