Are you observing small black flies in the kitchen or dining area of your food service establishment? 

Several common varieties of small black flies can affect food facilities, and treatment and elimination approaches may differ depending on the species.

1. Adult Drain Moth Flies, also known as Sewer Flies or Filter Flies, range from 1/16″ to 1/4″ (1.6 to 6.4 mm) in length and are light gray to black with heart-shaped wings. They possess a delicate, fuzzy appearance and usually fly short distances before crawling on surfaces. These flies breed both indoors and outdoors on the gelatinous bacterial film found in drains, filters, sewers, lift stations, pits, and sump pumps.

 

2. Phorid Flies

Adults measure between 1/64″ and 1/4″ (0.4 to 6.4 mm) in length and can be black, brown, or yellowish. They have a humped back and display erratic movements, especially when running. Phorid flies breed in moist, decomposing organic matter both indoors and outdoors, often indicating potential plumbing or structural issues.

 

3. Fruit Flies (Vinegar Flies)

Two common types of fruit flies include:

• Red-eyed: Smaller, tan or light brown with distinctive red eyes.
• Dark-eyed: Larger, dark brown with dark eyes.

 

Biology and Behavior

The term “fruit fly” commonly refers to the small flies often found in kitchens and foodservice environments. The two most prevalent species are the Red-Eyed Fruit Fly, Drosophila melanogaster, and the Dark-Eyed Fruit Fly, Drosophila repleta. Additionally, establishments may encounter other types of fruit flies, such as Moth flies and Phorid flies, frequently referred to as “drain flies.” While there is some debate among taxonomists regarding the classification of these flies as ‘fruit flies,’ we will refer to them as such.

In recent years, the occurrence of fruit flies in foodservice facilities has increased due to a combination of factors, including reduced pesticide use and weakened sanitation practices. Both the Red-Eyed Fruit Fly and the Dark-Eyed Fruit Fly are present in these settings, but the Dark-Eyed variety is likely more abundant due to the availability of suitable breeding sites. Consequently, this review will primarily focus on the Dark-Eyed Fruit Fly.

Despite their small size, Dark-Eyed Fruit Flies can significantly impact foodservice operations. They can quickly become a nuisance for customers and harm a facility’s reputation or brand image. Moreover, fruit flies can adversely affect health inspection ratings and potentially transfer bacteria to food and food contact surfaces. A deeper understanding of the Dark-Eyed Fruit Fly will facilitate the development of more effective strategies to manage this pest and reduce the associated risks in foodservice environments.

The Dark-Eyed Fruit Fly is exceptionally adaptable, thriving year-round in foodservice facilities where they can live, feed, and breed. Their small size, relatively brief life cycle, and preferred food sources contribute to their persistence as a pest. 

Fruit flies undergo complete metamorphosis, which includes four developmental stages: egg, larva, pupa, and adult. The time from egg to adult for the Dark-Eyed Fruit Fly is approximately three weeks, which is longer than that of the smaller Red-Eyed Fruit Fly. Adult Dark-Eyed Fruit Flies can live for over a month, making them relatively long-lived for their species. They can be identified by their slightly larger size and darker coloration compared to the Red-Eyed Fruit Fly.

 

Breeding Sites

Once an adult female Dark-Eyed Fruit Fly enters a foodservice facility, it is just a matter of time before she discovers optimal locations to lay her eggs. Equipped with specialized sensory capabilities, these flies are adept at locating suitable breeding sites. Once inside, they perform meandering flights to thoroughly explore every corner of the facility, seeking even the tiniest amounts of organic material suitable for breeding.

The ideal breeding sites for the Dark-Eyed Fruit Fly are typically moist, decaying organic matter that often forms biofilm. Contributing factors in biofilm formation include bacteria, yeast, and fungi. Familiar biofilm sources include “sugar snakes” in drain lines, slimy deposits in drains, and loose aggregates of organic material beneath and around equipment. Due to the constant presence of moisture, typical breeding locations include the interior and exterior of beverage service equipment, cracks and crevices in bar areas, underneath and between worn floor tiles, in beverage drain lines, and within and around floor drains. Additionally, decaying fruits and vegetables serve as breeding grounds. Structural issues that create sustained wet areas, such as beneath improperly sealed mop sinks, can also foster breeding conditions. Cleaning practices, such as nightly power washing, can inadvertently push water and food debris into wall voids and behind floor tile cove molding, further creating breeding sites.

Once a suitable breeding site is identified, female fruit flies will lay eggs and establish a population of larvae there. Depending on the availability of breeding material, these locations may attract additional flies, becoming a hub for reproduction and feeding. As long as breeding materials are present, sites will continue to draw in an increasing number of adult flies. The growth of fruit fly populations follows a logarithmic pattern, meaning that, if left unchecked, the number of fruit flies can escalate rapidly. What begins as a few flies one day can quickly turn into hundreds the next.

The most effective long-term strategy for maintaining a fruit fly-free environment is—and will always be—the implementation of excellent sanitation practices. Identifying and removing breeding sites, along with any organic material that could become breeding sites, will disrupt the fruit fly life cycle and prevent rapid reproduction.

 

Resting and Foraging Sites

Adult Dark-Eyed Fruit Flies exhibit specific daily behavioral patterns, with two peak activity periods occurring at dawn and dusk. These behaviors are influenced by internal timing mechanisms (circadian rhythms) and by external factors such as light and dark cycles. However, when flies inhabit indoor facilities, these activity patterns may become less pronounced due to altered lighting schedules and human interactions. Nonetheless, general activity trends for Dark-Eyed Fruit Flies can still be observed.

These flies tend to rest when the facility is quiet and the lighting is subdued. This resting behavior persists until light levels begin to rise. During the morning, the flies become highly active, focusing on locating and visiting breeding sites, feeding, and engaging in reproductive activities. In the evening, there is another surge of activity when many flies relocate from breeding sites to find quieter spots to rest. These resting sites can be located on walls, ceilings, cords, pipes, cardboard boxes, and other surfaces. While dark surfaces are preferred for resting, fruit flies will settle on any available surface.

Due to these daily behavior patterns, visual inspections for fly activity should be conducted at the same time each day to minimize the chances of false negatives. Adult flies are not always clustered near known breeding sites; during foraging, they can be found in areas with appealing odors where larval development opportunities do not exist. Common foraging locations include bars, waste receptacles, mop sinks, and other easily noticeable regions. 

While recognizing resting and foraging sites is essential for managing fruit fly populations within a foodservice facility, the primary focus must be on identifying breeding sites and eliminating them through consistent, thorough sanitation practices.