What does the Specialist Say: The deer fly population is larger than average this year, the worst in five or ten years.  Hatches are cyclical and based on a variety of environmental factors.  The yellow fly is the nastiest offender in this category of flying pest which numbers over 40. These insects require a blood meal for reproduction, hence the aggressive biting.  In years of low to moderate populations, the yellow flies stay close to the ground and victims will experience bites on the lower legs and ankles.  In heavy years, like 2010, the flies and biting may occur anywhere.  As a means of control, a blue cup with an adhesive can be located on a vehicle or person.  The flies will stick to the cup and die.  Alas, the blackberry crop has no influence on the population.  Russ Mizell, Assistant  Program Director and Professor of Entomology, North Florida Research and Education Center