Tiny Detective Work

Small flies can cause problems in commercial accounts, but they aren’t generally the root of the problem. Instead, they are a symptom of a bigger problem. In the following article, we’ll examine the biology and behavior of two commonly encountered small flies — the moth fly and the fungus gnat.

MOTH FLIES. Moth flies, also commonly called drain flies because they breed in drains, belong to the family Psychodidae. They can breed in tremendous numbers in sewage treatment plants and then be carried by the wind to nearby buildings where they can crawl through fly screens. Indoors, moth flies primarily breed in drain pipes which bring them into contact with potential disease-causing microorganisms; thus, they are a concern when found in food-handling and hospital facilities.

Psychodidae are small dipterans with short, hairy bodies and wings which give them a furry, moth-like appearance, hence their name. The adults have long antennae (13 segments) with each segment having a bulbous swelling with a whorl of long hairs. The wings are leaf-shaped and feature a number of parallel veins without cross veins. The wings are held roof-like over the body when at rest. Larvae live in aquatic habitats and are long and cylindrical with eight ventral suckers on the flattened lower side.

Moth flies may develop in the muck or gelatinous materials that accumulate in sewage disposal beds, septic tanks, moist compost or dirty garbage containers. Or they may emerge from sink or bathroom drains, tree holes, rain barrels or bird nests with accumulations of moist fecal material. Eggs are laid on moist, wet or watery larval habitat and can hatch in 32 to 48 hours.

Moth flies become pests when they are abundant and begin to annoy people. They can enter nearby buildings and create a nuisance problem, as well as pose a potential health threat. Occasionally, these flies do emerge from drain pipes in structures. Broken or leaky sewer lines in substructural areas of buildings can cause polluted water, sewage and organic debris to accumulate, providing conditions for large numbers of moth flies to reproduce.

Psychodids, on average, complete a life cycle in one to three weeks. Generally, adults live for about two weeks. They are weak fliers and, when encountered, they are often found crawling on walls and other surfaces. Their flight is short and jerky and only lasts for a few feet at a time, but they can be carried by wind for some distance away from where they are breeding. Adults are attracted to lights and outdoor mercury vapor lamps can draw large numbers to a building. Being tiny flies, they can easily crawl through normal fly screens, making commercial accounts particularly susceptible to moth flies.

FUNGUS GNATS. Fungus gnats generally refer to members of the dipteran families Fungivoridae and Sciaridae. These are small, dark, delicate-looking insects with long wings and spindly legs. They are characterized by having long, elongated coxae, which are the first leg segments that are attached to the thorax.

Fungus gnats are weak fliers and are usually found not far from their breeding source indoors. They generally remain near potted plants and often run or rest on growing media, foliage or litter.

Females lay tiny eggs in moist organic debris or potting soil. Larvae have a tiny black head and an elongate, whitish to clear, legless body. They feed on organic mulch, leaf molds, grass clippings, compost, root hairs and fungi. When mature, larvae pupate in damp places where they have been living. There can be many generations per year, particularly indoors.

In commercial buildings, fungus gnats can be found breeding in the soil in atriums as well as in potted plants. Usually, the soil is overwatered which allows the growth of fungi on which the larvae feed. These flies are not known to carry any human diseases but they can be very annoying when found indoors. There is low tolerance for fungus gnats when they occur in shopping malls, their breeding source here often being interior plantscapes.

CONTROL. The key to solving infestations of small flies, including both moth flies and fungus gnats, is to use your knowledge of biology and behavior to locate all of the breeding sources. Then it’s simply a matter of mitigating or eliminating those breeding sites.

(Source: Mallis Handbook of Pest Control)