What Do Flies Do? Exploring Filth Flies, Types, and Control

Flies, often buzzing annoyances in our homes and surroundings, are more than just pests. Belonging to the order Diptera, or “true flies,” they are a diverse group of insects with significant impacts on both human and animal health. While some flies are mere nuisances, many, particularly those categorized as “filth flies,” pose genuine health risks by spreading diseases. Understanding what flies are, the types that matter, and how to manage them is crucial for maintaining a healthy environment.

Understanding Filth Flies and Their Impact

Filth flies are named for their breeding and feeding habits, which often involve decaying organic matter like garbage, manure, and carrion. This preference brings them into contact with a multitude of bacteria and pathogens. House flies, for instance, are notorious for transmitting diseases such as food poisoning and dysentery. Beyond disease transmission, flies like stable flies and mosquitoes (also classified under Diptera) can inflict painful bites as they feed on blood, with some species acting as vectors for serious illnesses.

The feeding mechanism of house flies further contributes to their role in disease spread. They regurgitate stomach contents to liquefy food before ingestion and can contaminate surfaces and food through defecation. This combination of habits makes filth flies efficient carriers of disease-causing organisms, highlighting the importance of controlling their populations.

Identifying True Flies: The Diptera Order

The order Diptera, meaning “two wings,” is one of the largest insect groups, characterized by having only two wings, unlike most winged insects with four. This defining feature helps distinguish true flies from other flying insects. A common trait among flies, especially filth flies, is their attraction to moist organic material for egg-laying. While this scavenging behavior can be beneficial in nature, it also brings them into close proximity with human environments, leading to potential health concerns.

Filth flies can be broadly categorized into large and small types based on their physical characteristics and food preferences, aiding in identification and targeted control strategies.

Large Filth Flies: Size and Breeding Grounds

Large filth flies are typically robust insects with stout bodies and shorter legs. Their larvae, commonly known as maggots, thrive in environments rich in manure, carrion, and garbage. Key examples of large filth flies include:

House Fly (Musca domestica): The common house fly is easily recognized by its dull gray body, approximately ¼-inch long, with four dark stripes on its thorax. They commonly lay eggs in animal feces and garbage. The larvae, white and legless maggots, grow to about ½ inch before pupating and emerging as adult flies. Adult house flies are capable of traveling one to two miles in search of breeding sites.
Blow Flies (Calliphoridae spp.): Often called “bottle flies” due to their shiny blue or green metallic bodies (though some are black or bronze), blow flies are similar in size to house flies or slightly larger. They are attracted to garbage and, notably, dead animals, where their larvae develop, causing bloating of the carrion. The presence of many blow flies indoors often indicates a dead animal within the structure.
Flesh Flies (Sarcophagidae spp.): True to their name, flesh flies are drawn to carrion and meat scraps for egg-laying. Adults are dark-colored, gray or black, and slightly larger than house flies, with a distinctive checkerboard pattern on their abdomen and three dark stripes on the thorax.
Stable Fly (Stomoxys calcitrans): Stable flies, while not exclusively found in filthy conditions, are significant as both males and females feed on blood, biting humans and animals, often around the ankles. Their bites are painful, though they are not known to transmit diseases to humans. Females lay eggs in decaying straw, manure, moist animal feed, and yard waste.
Cluster Fly (Pollenia rudis): Cluster flies are household pests, often mistaken for house flies, but they hold their wings parallel to their bodies (house flies hold them in a triangular shape). Covered in golden hairs and lacking thorax stripes, cluster flies are unique as their larvae parasitize earthworms. Adults seek shelter in homes, particularly attics, during fall and winter. Control primarily focuses on exclusion by sealing entry points.

Small Filth Flies: Delicate Bodies, Diverse Habitats

Small filth flies are characterized by their smaller size, delicate bodies, and longer legs compared to large filth flies. Their larvae may be maggots or worm-like and thrive in drain sludge, organic debris, and rotting plant material. Examples include:

Fruit Flies (Drosophila spp.): Also known as vinegar flies, fruit flies are attracted to fermenting liquids like alcohol, syrups, soda, vinegar, and overripe fruit. Females lay eggs near these substances, providing food for their tiny larvae. Adults are small, gnat-sized, typically tan with red eyes.
Phorid Flies (Phoridae spp.): Phorid flies, or humpbacked flies (due to their arched thorax), are small and dark. Their larvae feed on a wide range of decaying organic matter, both plant and animal. They can rapidly multiply in environments with liquefied garbage, sewage, or carrion, often in hidden, inaccessible locations.
Drain Flies (Psychodidae spp.): Drain flies, also called moth flies or sewer flies, are around 1/8-inch long, slightly larger than other small filth flies, with broad, hairy wings. Adults are often seen resting on bathroom walls. Larvae develop in the gelatinous muck in drains, using breathing tubes to access air. Control involves cleaning drains with brushes and drain cleaners.
Fungus Gnats (Sciaridae and Fungivoridae spp.): Fungus gnats are delicate, long-legged flies resembling tiny mosquitoes. Their larvae live in moist environments where fungi grow, such as overwatered potted plants or areas with pigeon droppings. Indoor infestations can be linked to these sources. When cleaning bird droppings, it’s important to pre-wet the area with disinfectant to minimize the risk of disease transmission.

Managing Filth Flies: A Multi-faceted Approach

Effective filth fly management hinges on a combination of strategies, with sanitation being the cornerstone.

Sanitation: Eliminating Breeding Sites

Sanitation is the most critical step in managing all types of filth flies. Eliminating their breeding sites – the organic materials that attract them for feeding and egg-laying – is usually sufficient to control and prevent infestations. Without good sanitation, other control methods are largely ineffective. Key sanitation practices include:

Storing trash in sealed containers, using trash bags and bins with tight lids.
Keeping dumpsters clean, emptying them regularly, and locating them as far from buildings as possible.
Promptly removing manure and decaying plant or animal matter.
Eliminating sources of excessive moisture.

Inspection: Finding the Source

Inspection is as vital as sanitation. To eliminate breeding sites, you must first identify the attracting material. This often requires a thorough inspection of the premises, knowing what to look for, and where to look. The process involves:

Identifying the type of fly.
Inspecting for materials that attract that specific fly type.
Eliminating the identified material.

Exclusion: Preventing Entry

Exclusion is another crucial step in preventing flies from accessing buildings. This involves:

Keeping doors, windows, and vents closed as much as possible.
Screening windows and vents.
Sealing gaps and cracks around doors, windows, and other entry points.
Using automatic door closing devices and air curtains to deter flies at doorways.

Mechanical Control: Trapping and Physical Removal

Mechanical control methods can supplement other strategies. These include:

Fly swatters for manual removal.
Sticky fly paper traps.
Ultraviolet light traps, particularly useful in commercial settings. Light traps should be placed strategically, out of sight from outside, no more than 5 feet above the floor, away from competing light and food preparation areas. Bulb replacement should occur at least annually.

Chemical Control: Pesticides as a Last Resort

While pesticides are generally not the primary solution for filth fly problems, they can be a component of an integrated management program in certain situations.

Pesticide-releasing fly strips can be used in unoccupied enclosed spaces like attics.
Contact pesticides (non-residual) can be used as space sprays (“fogging”) for temporary adult fly knockdown.
Residual pesticides can be applied to outdoor surfaces where flies rest, such as walls of barns, stables, restaurants, and homes.
Pesticide baits are also available for outdoor fly control, including around dumpsters.

Important Note: Always read and follow pesticide label directions carefully. It is a legal requirement and ensures safe and effective use.

Conclusion: A Clean Environment is Key to Fly Control

Understanding what flies do, particularly filth flies, and their habits is essential for effective management. While they play a role in nature as scavengers, their presence in human environments poses health risks. The key to controlling filth flies lies in diligent sanitation practices, coupled with inspection, exclusion, and, when necessary, mechanical and chemical control methods. By focusing on eliminating breeding sites and preventing access, we can significantly reduce fly populations and create healthier living spaces.

Source: University of Illinois Extension, Illinois Department of Public Health. http://www.idph.state.il.us