Damage symptoms

While the lesser house fly and other Fannia spp. do not cause any direct damage to animals or buildings, they are considered a serious pest because:

• The males form swarms above the ground at head height inside barns and hen houses, making them a particularly annoying deterrent for employees.
• Adult flies can spread many different pathogens from contaminated breeding sites to healthy animals over large distances.
  • Fannia flies have been confirmed to carry the viruses that cause Newcastle disease and hog cholera, protozoan parasites such as Cryptosporidium and Toxoplasma, and bacteria such as Salmonella

Life cycle and appearance of the Lesser house fly

Lesser house flies and other Fannia spp. are medium-sized to small flies with dark bodies and legs. When the flies mate, adult males will form swarms in areas with still air out of direct sunlight where they remain in flight as they wait to be visited by an unmated female. Female flies are less active and tend to remain close to breeding sites, where they deposit their small white eggs in batches of up to 100 on animal feces, moist organic matter or carrion.

The brown larvae that emerge after a few days are typically flattened, with a fringe of spiny lateral protuberances. These maggots feed by scavenging bacteria and other microorganisms in the surrounding decaying matter. They can tolerate a wide range of moisture levels and are adapted to stay near the surface of near-liquid waste. Depending on temperatures and environmental conditions, the maggots complete three larval stages and pupate (inside a puparium) in a week or more. Fannia puparia resemble the larval form, and the flies develop for another week before emerging as new adults. The lesser house fly’s full life cycle generally takes between 18–22 days to complete but is prolonged by low temperatures.