Video footage showed that the flies made uncontrolled approaches when faced with a striped landing strip.
The mystery of why zebras have black and white stripes may have been solved as scientists find the pattern "dazzles" blood-sucking flies.
"From distances of greater than two metres or so, a zebra would just look like a grey horse - they won't be able to see the stripes at all", Dr Martin How, co-author of the research, told the Guardian.
The evolution of the zebra's black & white coat has intrigued scientists for years.
They found that horse flies gathered around domestic horses and zebras at a similar rate - but landed on zebras a quarter as often. The zebra tail swishing, and at times running, means flies that land on zebras are tossed off more quickly.
The team said the research not only supported previous work suggesting stripes might act to deter insects, which can carry diseases, but helped unpick why, revealing the patterns only produced an effect when the flies got close.
Study leader Professor Tim Caro, from the University of California at Davis, US, said: "Once they get close to the zebras... they tend to fly past or bump into them". While horses are more low-key about the presence of flies, merely twitching and occasionally swishing their tails to ward off the insects, zebras are far less tolerant.
As a result, the exact cause of stripes in zebra remain unknown.
"We saw that these horseflies were coming in quite fast and nearly turning away or sometimes even colliding with the zebra, rather than doing a nice, controlled flight". Zebras, according to the study authors, are particularly susceptible to unsafe fly bites.
Researchers think that zebras may have evolved this way because where they live.
More recent research has suggested that somehow the stripes reduce the chances of a zebra being bitten by flies. The striped animals nearly continuously swish their tails during the day and will stop feeding if they feel bothered.