Stable Flies on Pastured Cattle

By Dave Boxler

Pastured animals that exhibit defensive behaviors such as tail flicking, skin twitching, leg stamping, lying with their legs tucked underneath, standing in water, and bunching in corners of pastures, are likely being attacked by the stable fly. Animals bunching to fight stable flies can damage forage, and on fragile soils, may create blow outs. Historically, stable flies had only been pests of cattle in dairies and feedlots.

Since the 1980’s, stable flies have emerged to be a serious pest of pastured cattle. This coincided with livestock producers transitioning away from small hay bales to large round bales in stationary feeders. Research studies indicated this transition increased stable fly numbers found in pastures because leftover piles of hay mixed with animal urine and manure create an ideal environment for stable fly larval development. However, other larval developing sites may also contribute to pasture populations.   

Both sexes of the stable fly require a blood meal, usually twice a day depending upon the weather. They prefer to feed on the legs and the belly area of pastured animals (Figure 1).

 After feeding the stable fly will seek a shaded resting site to digest the bloodmeal. In pasture windbreaks (coniferous or deciduous), walls of structures, windmill towers, and water tanks (Fig 2) serve as possible resting sites.

While the impact of stable flies on the performance of dairy and confined cattle has been well documented, the stable fly impact on pastured cattle has been less examined. A pasture study conducted at the University of Nebraska in the late 1990s saw a reduced average daily gain of 0.44 lb. in three, 84-day trials with cattle receiving no insecticide treatment versus cattle that received a treatment.  An economic injury level (EIL) of five flies per leg has been suggested and is often exceeded in normal pasture conditions. The EIL may be less based on the results of the above study, where stable fly numbers averaged 3.6 flies per leg.