Prevention and control

The larvae, which are scattered in the soil, cannot be eradicated. Insecticides cannot effectively control the adult flies, flying around, either.

Adult horse flies can be trapped in traps, which consist of a rather large, dark object, which can be seen and is heated by the sun. Carbon dioxide (from dry ice) may increase the attractiveness of the trap. The horse-flies that are attracted, are trapped in a trap or in an adhesive that is placed in the trap. If strategically positioned, the trap can help the problem for grazing cattle. As mentioned, horse flies which have strayed indoors – do not bite, but can be controlled with a fly spray with pyrethrin, if you do not just let them out of the windows.

Horse-fly bites

Only the female horse flies suck blood. The males feed on nectar. A horse fly, which is about to suck blood, is almost impossible to get off, and is therefore easy to kill. Their eyes are well developed, and they often use their eyesight when finding their hosts. They move towards dark silhouettes in the landscape, and there is a greater risk of being bitten by them, when wearing dark clothes. The horse fly prefers to bite white people through dark colored clothing rather than the pale skin. Horse flies only bite in daylight. The risk of getting bitten is greatest near marshes and river valleys. They often stray into cars and houses, which can then act as traps. They do not bite inside; however, they just sit by the windows and wait to get out into the light again. Horse fly bite hurts , and it is quite bloody. The horse-fly pumps rather large amounts of saliva into the wound. For some people, this results in severe allergic reactions.
Meaning. Locally, horse-flies can be a serious nuisance because of their painful bite, and they can – at times – prevent a normal outdoor life for those who are allergic to the horse-fly saliva.

Life cycle

Horse-flies lay their eggs on the plants that grow in damp places. When the larvae hatch, they crawl down into the moist earth or into the water where they live as voracious predators of the prey they can overpower.

The adult horse-flies are active and fast fliers, and they can fly far away from their breeding site. They are common in the high summer, and statistics from the Danish National Pest Control Laboratory show that 70% of all horse-fly encounters take place in July, the other in June and August.

Horse-flies

Giant horse fly

Fig. 42. A horse fly (top), a deer fly (left) and a common horse fly (right). (Lindner, Seguy and Rietschel, respectively)

In Denmark, there are a dozen different horse-fly species of the family Tabanidae.

The biggest fly in Denmark is a horse-fly, namely the 3 cm long pale giant horse-fly, Tabanus bovinus. However, it is mainly the smaller species which can become so numerous that they become troublesome: deer flies of the genus Chrysops, which has yellow markings on the abdomen and brown-spotted wings and the common horse fly, Haematopota pluvialis, which is one centimeter long and characterized by its grey, white-spotted wings.