Beware the next invasion of the marmalade flies

Suddenly, around about the third and fourth of August we were inundated with Episyrphus balteatus, the Marmalade Hoverfly.

It is a relatively small hoverfly, a member of the Syrphidae family, and generally about 9–12mm long.

It is globally widespread and abundant, and like most hoverflies mimics more dangerous insects, ie wasps.

In other words, rather than hiding away, it announces itself to the world as a “wasp” and to be avoided – judging by its recent abundance, a successful strategy.

However, despite the stripey wasp-like appearance they are totally harmless. This little insect is easy to identfiy too; the upper side of the abdomen being patterned with striking orange and black bands.

Look closely and you see two further identification characters: secondary black bands on the third and fourth dorsal segments of the abdomen, and faint greyish longitudinal stripes on the thorax (the middle bit between head and abdomen).

Like most hoverflies, the males have holoptic eyes – left and right compound eyes touching at the top of the head.

This interesting species is found throughout the year in various habitats, and especially in southern England.

As well as being found in the wider countryside they are often seen in urban gardens, where they visit flowers to feed on pollen and nectar. It is is one of the few flies that crushes pollen grains to feed on; the terrestrial larva eat aphids.

What we experienced across Yorkshire in early August, and I received many reports, was the most spectacular part of their story.

These tiny flies often form dense migratory swarms, perhaps when conditions are just right with warm dry weather, plenty of food, and a sudden increase in numbers.

Then they are off to wherever the atmosphere takes them, in clouds of thousands or probably millions.

Our recent visitation probably originated along the western coast of Europe where an increase in numbers triggered migratory behaviour.

The tiny hoverflies ascended en masse into the atmosphere to drift like aerial plankton. When they arrive here, they often cause panic as they descend from high into people’s gardens.

Thousands of what look like tiny wasps filling the air certainly puts the wind up people. There may be more soon so watch out.

Ian Rotherham is a writer and broadcaster, Professor of Environmental Geography and Reader in Tourism and Environmental Change.