Stable flies belong to the genus Stomoxys, Latin for “sharp mouth”; a fitting moniker, as these flies are the only members of the Muscidae family that feed on mammalian blood (particularly cattle and horses, humans, and pigs to a lesser extent). Although they are extremely common and exist worldwide, it is believed they first originated somewhere in Eurasia. Stable flies superficially resemble house flies, but are lighter in colour and have biting mouth parts. Both male and female flies feed on animal blood, and females must be engorged with blood to lay eggs. They are highly mobile insects and can travel up to 20 km in search of new hosts.
Life cycle and appearance of Stable Flies
Adult stable flies are between 5–7 mm in size and largely resemble the house fly, even having a four-striped thorax. However, they are light grey in colour; they have a broader abdomen that displays several dark spots in a “checkerboard” pattern; and, most significantly, they have long, piercing mouthparts that protrude from the front of the head. Additionally, Stomoxys flies will rest on surfaces in a characteristic head-upward position. Like the house fly, females tend to be larger and have more widely spaced eyes than males. Females require a blood meal just prior to egg laying and never lay eggs before their third-ever feeding (on average, a fly must feed four times before it can oviposit for the first time).
After mating the females lay their small (~1 mm long), white, sausage-shaped eggs (singly, or in bunches of 25–30) in humid areas with large amounts of decaying organic matter, such as compost piles, hay bales, spilled feed and near the edge of silage pits. As with all flies, development throughout the life cycle is temperature- and humidity-dependent, but yellowish-white larvae will generally emerge from the eggs in 1–4 days. They are typical maggots ranging in size from 5–12 mm long, and they pass through three larval instars within 10–30 (or more) days, depending on the environmental conditions. Larvae can even overwinter in livestock housing. At the end of the third instar, the larvae will move to drier sites nearby and transform into reddish-brown pupae 4–7 mm long. Adults emerge 6–20 days later. In summer, flies can complete their life cycle in 3–4 weeks.
Stable flies typically congregate on the legs of livestock to feed, and the population size can be assessed by counting the number of Stomoxys on the animals’ front legs. The flies become an economic concern when the average number per leg is as low as two. When populations are very large (> 25 flies/leg), the insects will begin to cluster on the animals’ undersides and flanks as well. Stable flies are persistent, intermittent feeders, returning time and again to feed off animals in short 2–5 minute sessions. The bites are extremely painful, but the flies will typically ignore swatting, stamping and other strategies used by animals to avoid being bitten. Damage caused by parasitism of Stomoxys spp. on livestock includes:
- Weakened animals and potential anemia from continual irritation, stress and blood feeding.
- Lower livestock feeding due to stress and avoidance behaviours (animals spend more time trying to escape flies than they spend eating).
- Open feeding wounds can develop secondary infections.
- Overheating if animals begin to bunch together (an avoidance strategy to reduce fly bites) when temperatures are high.
- Transmission of blood-borne pathogens between animals including the bacteria that cause anthrax, leptospirosis and anaplasmosis, the viruses that causes equine infectious anemia (EIA) and hog cholera, and the protozoan trypanosomes that cause surra.