Cheilosia grossa (Fallén, 1817)
Cheilosia corydon (Harris, 1776).
Biology & ecology:
The larva tunnels the stems and roots of a wide range of thistles (Cirsium and Carduus spp.), especially Marsh Thistle Cirsium palustre, which then tend to become stunted and multi-stemmed. Females oviposit on small, young, flowering spikes growing from the centre of the rosette. Adults are found visiting flowers such as Salix catkins in the early spring, usually in sheltered situations such as woodland rides or edges. Males typically hover at considerable height.
Adult records are scarce, probably because this early-flying species is missed by recorders, but also because individuals tend to fly high and out of reach. The best way to record this species is to look for larvae which occur in the base between the roots and the stem. Recording in this way has shown that this species is widely distributed across the uplands in England and Scotland, but is less common in Wales and western England.
Status & conservation:
The frequency with which this species has been recorded has varied considerably. This probably reflects the waxing and waning of the popularity of larval searching as a method of recording.