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See a sample issue of The Irish Garden!

June 2012

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Hovering about!

Dick Warner admires the abilities of hover flies

Hoverflies are common garden insects and many species mimic bees, wasps or bumble bees. They are very good at this, despite the fact that they are true flies of the order Diptera and therefore have only two wings, while the venomous insects they are trying to imitate all have four wings. The likeness is certainly good enough to make many gardeners nervous when they encounter one, but no species of hoverfly can sting, and indeed most species are extremely beneficial.

There are about 180 species identified in Ireland, though not all of them are likely to turn up in gardens. The majority of them have no common English name. One exception to this is Episyrphus balteatus, sometimes called the marmalade fly. It's a handsome orange and black insect and its larva, a multi-coloured maggot, is a fierce predator of greenfly and similar sap-sucking pests. It skewers the aphids on a sort of hollow needle sticking out of its face and sucks out the internal juices. A single larva, which looks a bit like a jelly bean, will account for about a hundred greenfly a week, which makes them useful things to have around. In fact they are being experimented with to see if they can be used as biological pest control in tunnels and greenhouses.

When the larva hatches into an adult marmalade fly its character changes completely. Instead of being a ravenous killer, it flits from flower to flower sipping nectar. This makes it an important pollinator of fruit trees and some vegetables. The decline in the number of bees in most gardens increases the importance of hoverflies. Hoverfly population dynamics can be used to evaluate trends in local environmental quality. Already this tool has been used in Ireland, France, Spain, Italy, Holland and Switzerland.

Most hoverflies hover and this is the best way of telling them apart from the insects they are mimicking. They remain stationary in mid-air and while they're doing this their wings beat so fast they become invisible, which makes the whole thing look rather miraculous, as if the insect had developed some sort of science-fiction anti-gravity device. Typically they hover for a few seconds and then dart forwards for a metre or two before hovering again. They sometimes hover in front of flower heads when they're sipping nectar but sometimes they actually land on the flower head. Most Irish species hover in a horizontal position, like a helicopter. But there are a few species that hover in a vertical mode, with their tails down, which looks even more odd.

About a third of the species in Ireland have predatory larvae like the marmalade fly, about another third have larvae that are saprophages, living on dead and decaying organic matter and the final third eat living plant tissue. The larvae of this last group are the only ones that pose a threat in the garden because they tend to eat bulbs, tubers and roots from the inside and can do quite a lot of damage. The larva of Syritta pipiens is a saprophage and, in contrast to the plant eaters, it's very valuable to gardeners. It's fond of living in compost heaps and bins where it helps to break down garden rubbish and convert it into valuable organic fertilizer.

The larvae of some other hoverflies may live in your garden pond, particularly if the pond has some mud in its base that is rich in organic matter. These larvae include a group called the rat-tailed maggots which have very long, thin tails. These are hollow and the larva use them as breathing tubes, sticking the end of them out of the water or mud and sucking in air to get oxygen. Eristalis tenax starts life as a rat-tailed maggot and ends up as a hoverfly that mimics a bee. Learning more about the life cycle of these fascinating insects, you begin to realise the immense diversity of the solutions that evolution has come up with to cope with the challenges of living on this planet.



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