Insect Pests of Beef Cattle
Horse Flies
Horse flies are all too common pests of horses, cattle and sheep, and attack other animals and humans to some degree. Sizes range from slightly larger than house flies (the deer flies) to as large as 1 º inch long. Only the females are blood feeders - the males being nectar feeders.
Eggs of horse flies are laid in masses of 25 to 1,000 on vegetation. The eggs hatch in 5 to 12 days and the larvae drop off. It was long thought that swamps and other wet areas were necessary for larvae development. It has been discovered however, that the forest floor is the larval home of several species. The larvae live in mud, water or soil for varying amounts of time depending on species. All horse flies overwinter as larvae. Those with two generations per year may have the first generation larvae completing development to the pupal stage in 2 to 3 weeks while the second generations larvae will last months (over winter). Those with only one (1) generation per year will have larval stages of months, again, over the winter. Pupation occurs in drier soil and last 1 to 3 weeks. When adults emerge they mate and the females seek blood meals.
We have many species of horse flies in Mississippi although we don't have them all at the same time. Horse flies occur from late March through September with various species occurring at very predictable times of the year. This regularity is caused by their development time and by the fact that they all overwinter as larvae - which sets their clock, so to speak.
Horse fly females visit animals only to feed - they do not stay on the animal otherwise. Consequently they are hard to control because an effective insecticide would have to be on the animal in the right spot; at the right time; and of sufficient concentration to kill the fly. Even then, the fly may complete feeding on the animal before dying.
Horse flies are strong fliers being able to travel miles from their larval habitat. This also precludes good control programs, as any killed will only be replaced by those flying in from afar.
The mouth parts of female horse flies are strong and blade-like. They do not suck blood as most blood feeders, but rather slash a deep and painful cut. They then lap up the flowing blood with a sponge-like mouth part. This biting habit is responsible for the bloody animals when subjected to horse fly attacks. This loss of blood may be significant enough to reduce weight gain and milk production. Irritation caused by horse fly attacks may cause cattle to bunch-up or seek shelter in brushy areas. This behavior (ceasing to graze) may also reduce weight gain and milk production.
Horse flies interrupted while feeding may resume feeding on a different animal. This habit makes them efficient vectors of anaplasmosis in cattle and equine infectious anemia in horses.
Effective control procedures for pastured animals are lacking. However, animals that are handled regularly can be given some relief with repellants. Most of the repellants contain permethrin with a waxy additive to hold the permethrin in place for longer periods than a routine spray treatment. Show animals and horses that are handled regularly can be helped by frequent use of the repellants. Two (2) popular repellants are Swipe and wipe.
