Sep 18, 2012 – Got Blood?


Do Horse flies have nests and why do they attack people?


Horse flies and the closely related deer flies do not have nests, but deer flies, in particular, often hang out in groups waiting for food to stroll past them. That food, for the female horse and deer flies, will be the blood of warm blooded animals, and humans are perfectly acceptable. The males of these flies feed only on the nectar and other sweet plant fluids available, but the females also need blood in order to properly develop their eggs. They have mandibles similar to a pair of sharp scissors, and with these blades they slash open the skin, apply some saliva to act as an anticoagulant and keep the blood flowing, and lap up the blood rather than suck it out like a mosquito does. Attacks on livestock by large numbers of these flies can lead to serious blood loss, and when they bite people it seems they are not always kind enough to anesthetize the bite site first, so it can be really painful. 

Females hang out on vegetation, often in sunny locations, and home in on movement near them, indicating a potential blood source. They then quickly swoop in and find a comfortable place to feed. For deer flies this is often around the back of the neck and arms and for horse flies if the person is wearing shorts it is often the legs, but arms and other exposed skin work nicely too. 

The larvae of most species are predatory, living in holes in damp soils and damp, rotting plant matter on the soil. When they detect another insect moving past their tunnel entrance they lunge out, grab the prey, and pull it back into the tunnel to consume. This kind of habitat makes the control of deer and horse flies extremely difficult. You really cannot control or treat the vast areas where the flies are developing and must try to control the adult stage. Part of control is, of course, simply wearing long sleeves and long pants and perhaps even some head gear to protect the back of the neck. So, no they do not have nests and they attack people because we are food. 

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