Wednesday, August 4, 2010

Hoverfly heaven!

In the United States, hoverflies are called ‘flower flies’ and, at the moment, there are often clouds of these engaging insects around stands of flowers in our gardens and the wider countryside. Here are two species that are particularly noticeable and well worth examining in greater detail when you find them:

Scaeva pyrastri

There is little mistaking this relatively large and conspicuous hoverfly, with its three pairs of white comma-like markings. The first individuals that we see from about June onwards are migrants from southern and central Europe which breed and, in good years like this one, give rise to large numbers of offspring that turn up just about everywhere. We don’t know whether any fly south again as the year progresses, but those that remain can’t survive our cold winters, a situation that global warming could reverse over the coming years.

It’s reckoned that the larvae of this species probably consume as many as 500 or more of the aphids that they feed on, generally low down in the vegetation but, once they’ve become adults, they content themselves with nectar & pollen, which helps to mature the eggs (thankfully, they don’t require blood like the deerflies below!).

Episyrphus balteatus

This is probably our most familiar hoverfly – it’s even got an English name: the Marmalade Hoverfly. It’s smaller than Scaeva pyrastri but has that wonderful marmalade-coloured abdomen with the double black stripes. It’s a form of what is known as Batesian mimicry, the harmless hoverfly mimicking a more dangerous species, thereby gaining protection from predators.

The Marmalade Fly larva is also an important predator of various aphids, and a number of studies have been undertaken of their beneficial effect in controlling pests in various crops. One experiment established that female Marmalade Hoverflies ‘are able to evaluate and adjust oviposition rates according to different aphid prey densities’, i.e., the greater the number of aphids, the greater the number of eggs laid!

The thing I find most fascinating is the fact that they are drawn to the aphid colonies by the honeydew secreted by the individuals. So it’s not the number of aphids which attracts them so much as the concentration of honeydew and, even then, they are able to differentiate between the honeydew of various species so that they are choosing the correct aphid species', too.


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