Bibionids

There are several species of flies commonly referred to as bibionid flies. The most common in Scotland are the St. Mark’s fly (Bibio marci (L.)) and the fever fly (Dilophus febrilis (L.)). Bibionid flies are stout, black and often hairy flies, and numbers can be very high particularly in early spring and persist throughout the summer.
Adult St. Marks flies usually appear toward the end of April and often form large swarms which may be a nuisance to agricultural workers. They may also congregate around houses and cause concern.  Fever flies appear in early spring and can often still be seen in October.
Adult bibionid flies prefer to lay eggs in grassland or soil high in organic matter such as soils manured the previous season. They also lay eggs directly into manure heaps, which may then be applied onto the soil. Rotting vegetation such as straw stubble also attracts adult flies for egg laying.
Eggs will be laid from May through to August (or even later in the case of fever flies) and over 30 eggs may be laid at a time which leads to the clumps of bibionid larvae usually found in the soil.
Adult flies pollinate fruit and other crops and feed on nectar from flowers and are considered to be the second most important pollinators after bees. Egg hatch does not occur until the late summer, and initially the larvae (Fig. 8) will feed on organic matter in the soil. Damage is most often noticed in grassland and crops under some form of stress such as low fertility, shallow roots or excessive application of manure/slurry.
Damage to grassland is usually noticed in November/December or early in the new year when patches of yellow plants are noticed. The larvae sever the roots of grasses, causing reduced uptake of nutrients and water and eventual death of the plant. Damage may often be confused with that of leatherjackets, and insecticide treatments to kill leatherjackets will also have an effect against bibionids.
Damage to cereals is usually noticed by gaps in the drills, and pale plants in patches throughout the field. Seed may be hollowed by larvae or more commonly the roots trimmed so that nutrient uptake is disrupted.
Deep ploughing, harrowing and rolling can reduce larval numbers in crops after grass.
Insecticides used to control leatherjackets will have some effect on bibionid larvae, but timing may be important. In November/December the larvae are small and near the soil surface but from February onwards the larvae may be deeper in the soil and more difficult to kill with an insecticide.