It only takes five stable flies biting on the front legs of a cow to reduce weight gains and milk yields, according to Alec Gerry, a University of California – Davis veterinary entomology specialist.
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Julia Hollister is a freelance writer based in San Francisco, California.

Gerry, who spoke at the 2020 Golden State Management Conference in Modesto, California, has been researching flies for over 25 years. His most recent studies are in collaboration with researchers at the Veterinary Medicine Teaching and Research Center in Tulare, California.

The blood-loving, leg-biting stable fly may only be less than half an inch long, but it can cause major stress on dairy cows. They can be found resting on walls, hay bales, and other vertical surfaces and may be especially prominent in the morning in an area lit by the early sunlight. Stable flies have a long, bayonet-type proboscis (mouthpart) that sticks out in front of the head. The proboscis has rasping teeth at the tip that the fly uses to abrade the skin and create a pool of blood, on which it feeds over a period of about two to four minutes.

Behaviors caused by biting flies

Whether confined or on pasture, dairy cows are an easy target for the stable fly. The insect’s painful bite prompts specific behaviors by the cow to deter the pest.

“Bite-reducing behaviors include head tossing, foot stomping and tail flicking – all of which the cows do when they feel the flies biting them,” Gerry said. “When many flies are biting at the same time, cows will gather into a tight group, as cows in the center of the group receive fewer bites.”


This behavior is called “bunching” or “aggregative defensive behavior,” and when this behavior is observed in a herd, the cows are receiving many bites.

“All of these behaviors take energy to perform, and this costs the cows energy that they should be putting into weight gain when they are young or milk yielding in lactating cattle,” Gerry said. “In addition, all of these behaviors reduce the time spent feeding, consuming water, digesting and relaxing (as cattle are instead fighting off the flies), and this also reduces weight gain and milk yields.”

How to keep flies from biting cows

The most common question Gerry gets from his various talks is this: How can farmers keep flies from biting their cows? The answer is by making the environment less appealing to them.

Stable flies develop in decaying organic matter, particularly in old (not fresh) cattle feces or green waste (e.g., feed waste). These flies especially like to develop in cattle feces when it is mixed with cattle urine and waste hay or other plant material. The risk of having stable fly problems increases when your cattle facility has these materials available.

Producers can reduce these flies by composting cattle feces and green waste to reduce the attraction of these materials to egg-laying stable flies. Also, in a pasture setting, move feeding locations every couple of weeks to reduce the buildup of manure and hay waste these flies favor for development.

There is no cure for eradicating these blood-thirsty pests, just methods to control them. Scientists, researchers and corporations are inventing many methods to stop them.

Gerry explained, “There are insecticides that can be applied to cattle (spray on belly and legs), but this usually provides only temporary reduction in biting flies.”

There are traps sold commercially (Biting Fly Trap from Olson Products, and the Knight Stick from BugJammer Inc.) that can capture stable flies, which he said may provide some relief to cattle or other animals that are being impacted by these flies.

Ear tags are not effective for the stable fly, which bites mostly on the legs or belly of cattle. Fly baits do not work on this fly since these baits are formulated for control of house flies, not blood-feeding stable flies.

Ultimately, the best way to control these flies is to create an environment where they cannot thrive.  end mark

PHOTO: Stable flies have long bayonet-type mouthparts called a proboscis, which they use to tear through the skin causing blood to pool at the skin surface. Photo by Brad Mullens, University of California – Riverside.

Julia Hollister is a freelance writer based in San Francisco, California.