Bibionid Flies

By Dr Kate Entwistle

There are two Bibionid flies that are common to amenity turfgrass areas. One is the St Mark's Fly and the other is the Fever Fly. These flies cause no damage in their adult form and are thought to be beneficial pollinators of fruit and other crops. However, their larvae do cause damage to turf by feeing on the roots and reducing water and nutrient uptake.

The adult St Mark's Fly (Bibio marci) and the slightly smaller Fever Fly (Dilophus febrilis) are relatively small (6 mm or less), stout, black flies. The St Mark's Fly appears between the end of April and May but the Fever Fly appears between March and October. After mating, the female lays around 30 eggs at a time and this results in small clumps or pockets of larval activity in the rootzone.

The eggs are laid between May and August/September and hatch from late summer, producing larvae that will feed on the organic matter in the rootzone or the roots themselves. Damage is often seen either in November/December or in January/February as small yellowing patches throughout the sward. If the damage to the turfgrass roots is extensive enough, the yellowed plants will eventually die. The adult fly will emerge from the pupal stage to initiate a new life cycle.

Although these pests are flies in their adult form, their larvae have a small dark head with well-developed mouthparts, similar at first glance to that of the chafer larvae. However, the Bibionid larvae do not have either legs or pro-legs and their body is relatively long and thin. The larvae will vary in size depending on their age but will grow to around 11mm long and 1.5-2.0 mm wide. Identification of the different Bibionid species requires microscopic analysis of certain details on the larval segments.

Insecticides that are used to control leatherjackets (larvae of the Crane Fly, Tipula sp.) will also have an effect on Bibionid larvae. However, timing of the insecticide application is important because during November/December, the larvae are relatively small and closer to the surface of the rootzone whereas during January/February, the larvae will have grown significantly in size and will start to move deeper in to the profile.

Dr Kate Entwistle, The Turf Disease Centre, Waverley Cottage, Sherfield Road, Bramley, Hampshire RG26 5AG. UK Tel: 01256 880246 Email: Kate@theturfdiseasecentre.co.uk