Common pasture pests

Pasture pests are different from those you find in confinement operations, and how knowledge of pest biology is crucial to effective control.

The local agricultural extension agency, Cornell Cooperative Extension, typically conducts “Pasture Walks” during the spring, summer and fall months. These are short, on farm sessions focused on topics relevant to pasturing. I attended a local Pasture Walk on Common Pasture Pests in August 2004. Attendees were a mix of organic and conventional dairy producers, beef producers and agricultural advisors- about 20 people in all.

The farm owners, Bill and Joanne, milk approximately 45 cows on their small, organic farm in Central NY. They own about 200 acres, with about 65 acres devoted to a managed grazing system. They have been organic for about 3 years.


The common pasture pests

Keith Waldron began the discussion by pointing out how pasture pests are different from those you find in confinement operations, and how knowledge of pest biology is crucial to effective control. The most common flies in pasture situations in the Northeastern United States are the face fly, the horn fly and the stable fly.

It is important to inspect your pastured animals on a regular basis. This includes looking for the presence of flies (note their location- are they on head, back and sides or legs), and looking for evidence of flies (bites, sores, weeping eyes).

He noted that sometimes specific animals or breeds are more resistant to pests, so several animals should be inspected. (ie. Bos indicus breeds evolved in the same geographic area as horn flies, and thus have deeper blood vessels- making them less susceptible to horn fly bites)

Flies may also be distinguished by what kind of habitat is available for them. Horn and face flies tend to lay eggs in very fresh manure- under the edges to protect eggs from ultraviolet light. Stable flies lay eggs in moist organic matter- ie. around waterers, feeders, bales- and they don’t like dry conditions.


He then discussed the concept of thresholds- what level of pest population will affect the health and milk conversion efficiency of the animals?

  • Face flies are found on the face and concentrate on eye and nose secretions. The threshold is roughly 10flies per face (observe about 10 animals).
  • Horn flies are found on the back, sides or belly of animals (hot days). Threshold is roughly 50 flies per side of animal (dairy) or 100 per side (beef).
  • Stable flies are found on the lower legs (indicated by bunching of animals, stomping and tail switching). The threshold is roughly 10 per animal.

Control methods

There are many methods of pest control available to organic farmers.

  • To decrease growing conditions, manure clods may be broken up via a harrow, drag roller or birds (bear in mind that harrowing/rolling can be undesirable in pastures). Many organic producers pasture chickens in rotation with their cattle so that the chickens “clean up” pest larvae.
  • Biological control such as parasitoids, beneficial insects (there are not many for pastures, as the area that needs to be covered is large and there is little control over where the biological control organisms will end up)
  • Mechanical controls such as the horse fly trap, sticky traps and electrical traps. One problem with these devices is that flies frequently learn to avoid them.
  • Management methods include providing shaded areas, managing stocking rates and managing the times that animals are on the pasture.


Pest Biology

Dr. Phil Kaufman took over the discussion with information about the biology of the different types of flies.

Face and Horn flies have been pasture pests for a long time, stable flies more recently became pasture pests. Stable flies travel on wind currents, and generally arrive in the northeast after a severe weather front in early June.

Other flies such as horse and deer flies, have a different biology- they are semi aquatic, and feed on many animals- making it difficult to find a method to control them.

Dr. Kaufman listed several types of common flies and covered their basic biology and habits.

Face Flies:

  • Primary pest in Northeast US on cows and horses
  • Typically >100 per animal in summer
  • Females gathering proteins for eggs
  • Feed a max of 2 hrs per day :. Hard to control
  • Males get food from flowers
  • Flies live several weeks
  • Female feeds on host, lays eggs in dung pats, leaves
  • If dung is undisturbed, eggs to adults in one week
  • Breaking up of dung pat suffocates larvae
  • They move on a daily basis (another reason they are hard to control)
    *Every day you get your neighbor’s flies
  • Serve as mechanical vectors for pinkeye

Horn Flies:

  • Have spear-like mouthparts
  • Not as common in Northeast US
  • Very resistant to pesticides
  • Spend entire life cycle on the animal (unless the animal comes in barn)
  • Found on shoulders, back, sides and belly (when its very hot and sunny)
  • Lay eggs in fresh, undisturbed dung

Stable Flies:

  • Develop in moist organic matter
  • Found on legs of animals
  • Signs include stomping and clustering
  • Similar to tse tse fly in Africa

Horse Flies:

  • Feed on the animal
  • Slash open a wound and lap up blood
  • Spread disease quickly because they feed so frequently (vector for anthrax)
  • Pesticides don’t work well with them b/c they’re so big and they’re on the animals such a short time
  • Larvae develop in mud along streams and ponds (larvae are actually beneficial b/c they are predatory)
  • Mechanical traps work ok
  • Huge eyes- they are very visual.
  • They are attracted to color contrasts—traps are designed with this in mind.

Control strategies

In an organic farming situation, the use of pesticides is not allowed. Good barnyard management is the best basis for control, followed by use of biological controls and trapping.

Common biological controls are:

  • Parasitoids- don’t work well in pastures, used mainly in barns
  • Dung Beetles- very good competition

* Note: use of Ivermectin products kills beneficials, too

Trapping is another common method of control

  • “Horse Pal” trap is one type- basically a hanging target with a capture bottle, which the flies enter and cannot get back out
  • New “tent” design with blue and black panels is being tested by Cornell

Other pests

There was a quick discussion of another common pasture pest- the cattle grub.

These lay eggs on animals’ lower legs or bellies and migrate towards the animal’s back as they move through their life cycle. The primary caution regarding cattle grubs was that one must adhere to recommended grub treatment deadlines when treating with a pesticide. If the grubs die in the area of the spinal cord or the larynx, it could cause serious problems

Out to pasture

The group then moved to the pastures, where they

  • Poked through cow pats with a trowel, observing several different species of dung beetles at work
  • Viewed several types of mechanical traps
  • Observed flies on the cows
  • Discussed the fact that there are very different allowable practices when the farm is organic

This was an enjoyable way to learn the tips and tricks of basic fly identification and control.

Related Links:

Veterinary Entomology