Black-and-white stripes make a lousy landing strip for horseflies, study shows

Photo: Tim Caro/UC Davis

Zebra stripes disrupt the flight patterns of biting flies, researchers have shown in a study in which horses wore covers with the distinctive black-and-white patterning.

The findings, published today in the open-access peer-reviewed journal PLOS ONE, add to the growing weight of evidence that stripes found on Africa’s three species of zebra have evolved to thwart biting insects.

An international research team has explored the issue further, seeking to discover what it is about the stripes that actually disrupts a biting fly’s ability to land on a zebra and suck its blood?

University of California, Davis, Professor Tim Caro and Martin How, of the University of Bristol in England, led a series of experiments focused on the question.

The field trials took place on a horse farm in Britain that kept both zebras and horses.

The work involved:

  • Close-up observation of zebras as flies attempted to land on them;

  • Detailed videos to record flight trajectories as the flies cruised close to the zebras;

  • Dressing the horses and zebras sequentially in black, white and then black-and-white striped coats;

They found that stripes make lousy landing strips for tabanid horseflies.

In the study, flies were just as attracted to zebras as they were to horses, indicating that stripes do not deter flies at a distance.

“Once they get close to the zebras, however, they tend to fly past or bump into them,” said Caro, a professor in his university’s Department of Wildlife, Fish and Conservation Biology.

“This indicates that stripes may disrupt the flies’ abilities to have a controlled landing.”

University of California, Davis, wildlife biologist Tim Caro observes zebra behaviour in response to biting fly annoyance. Photo: Joren Bruggink/Aeres University

Compared to rates at which flies landed on the white coats and the black coats, hardly any landed on the striped coats.

“Stripes may dazzle flies in some way once they are close enough to see them with their low-resolution eyes,” How said.

The study also noted that zebras and horses respond very differently to the presence of flies.

Zebras swish their tails almost continuously during the day to keep flies off; they stop feeding if bothered by them; and if the flies are particularly persistent, the zebras will run from them.

As a result, very few tabanids are able to probe for a zebra blood meal, as shown by the data gathered in the study.

Horses, however, primarily twitch and occasionally swish to ward off flies. As a result, any flies that actually contacted zebras were soon dislodged compared to horses.

Researchers do not yet understand why zebras evolved these advanced defense mechanisms. A possible explanation is zebras may be highly prone to infectious diseases carried by African biting flies, although that hypothesis requires further study.

That stripes act to deter landings of biting flies has been suspected for over 75 years, the study team noted in their paper.

They acknowledged that the European horseflies observed in their study may differ in behavior from those in Africa but, nevertheless, there were a great many species of horse flies in Africa.

“Insect visual systems are highly conserved across taxa and there are no independent reasons to think that the visual system of European tabanids will be substantially different from those in Africa.”

The study’s co-authors include Yvette Argueta from UC Davis; Emmanuelle Sophie Briolat, Maurice Kasprowsky, Matthew Mitchell and Sarah Richardson of the University of Exeter; Joren Bruggink of the Netherlands’ Aeres University of Applied Sciences; and Jai Lake from the University of Bristol.