Behaviour
Episyrphus balteatus © www.photography.gordonengland.co.uk
In the UK, this species is mainly migrant, although adults can survive over winter if the weather is mild.
Occasionally there is a spectacularly large influx of these flies into the UK. The most recent case was in July and August 2004 when large swarms occurred at several coastal locations. This caused alarm among the public in some places, as swarms of hoverflies - perhaps mistaken for wasps - flew in off the sea.
Inland, they were more widely dispersed, but still numerous on the flowers of favoured plants - such as on the umbels of hogweed.
These events are caused by a large build-up of flies in continental Europe due to favourable breeding conditions, followed by suitable weather conditions for northward migration - fine weather with light southerly breezes - which helps to drift the migrating flies towards the UK.
The flies were observed on a yacht in the middle of the Mediterranean between the Balearic islands and Sardinia (Schmid, 1999), demonstrating that it will migrate across the sea.
The summer generation of flies migrates south in the autumn.
