As blood suckers their bites can be very painful and when large numbers are present they can be extremely annoying.
Some people develop allergic reactions to the bites which often swell and turn into nasty red sores.
There are somewhere around 4300 species of Tabanids in the world and over 300 in North America. Different deer fly species occur at different times during the year and often are found in high populations in local areas. When this is a back yard or favorite fishing spot, deer flies can become a real problem for people and pets. Most deer flies attack people and pets around the head, neck and shoulders. Yellow flies usually attack the legs.