Bibionid flies (Fever fly, St Mark’s Fly) are stout, black, hairy flies with short wings, ranging from 6 to 10mm long. Swarms of Bibionid flies can sometimes be seen hovering above the lawn on sunny days. Adult Fever Fly can be seen from March to October. Another common species, the St Mark's Fly (so-called because they emerge around St Mark's day) appear between the end of April and May.
Bibionid flies lay around 30 eggs at a time, resulting in small clumps of larval activity. Eggs are laid in grassland or soils high in organic content between May and August and hatch in late summer. The larvae can be pale or dark grey/brown with a dark brown/black head and are between 13 to 15mm long with no legs. They are similar in appearance to chafer grubs, however bibionids do not have legs and their body is relatively long and thin compared to chafer grubs.
The larvae usually feed on organic matter around the base of the grass plants. Occasionally, small yellowing patches can occur during November & December where the roots have been severed. Secondary damage may also be caused by birds searching for grubs.
There are currently no chemical controls for use against Bibionid flies, however reducing organic matter by removing clippings and managing thatch levels can help to control them.
Image courtesy of the STRI.