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Why do Horseflies Avoid Stripes?

In recent years there have been several fly sheets for horses that have unusual patterns or mimic the stripes on zebras. Many of these fly sheets purport the irregularity of their pattern or stripes deters flies, similar to the effect of zebra's stripes, but is there any scientific basis for these claims?

There have been several studies that have demonstrated horseflies do indeed have an aversion to zebras. Many hypotheses have been developed to explain why. Researchers from the University of Bristol in England set out to determine if they could validate any of these hypotheses, and the results are quite interesting.

First, a little about horseflies. The horsefly comes from the Tabaniade family in the insect order Diptera. They are relentless, agile, and aggressive in flight and have previously demonstrated that they are attracted to dark-moving objects. Horseflies can travel more than 30 miles and are most active during daylight. Only the female horseflies bite to supply protein used when producing her eggs. Her mouth is designed with a skin-piercing apparatus and two cutting blades, which are used to cut through and spread apart her host's flesh and ingest the exposed blood. While biting, she injects an anticoagulant into her host, which ensures the blood flows easily. Watch the video below for a close-up of a female horsefly biting its host.

Horsefly bite. Source: makroskooppi, YouTube.

Unfortunately, when injecting the anticoagulant, viruses such as equine infectious anemia, anthrax, and bacteria and parasites can also be spread. Horsefly bites are very painful, and some horses are allergic to them. The female horsefly prefers to lays her eggs in the grass near moist soil close to the water in the fall, and they will hatch the following summer. While the lifespan of horseflies is incredibly short, just 30 to 60 days, they are unbearable pests during summer.

Scientists and naturalists have previously observed that horseflies are deterred from landing on zebras, but no theories which explain this aversion have been proven until now. Researchers from Bristol University investigated three possible hypotheses: aliasing (misperception of motion), the polarization of light, and contrast.

Ultimately, only one of the three hypotheses could be proven: contrast. The study demonstrated that strongly contrasting striped coats had a strong effect in thwarting horsefly attacks. Weakly contrasting stripes we considerably less effective. The researchers also showed that uniform coat colors were associated with the most landings. The reason for this appears to be the solid outline of the coat's color against the sky, as seen by the horsefly from far away, making it easier for the horsefly to identify its target.

The researchers also speculated that the size of the stripes on a zebra, which are relatively thin, minimize the features horseflies are searching for- large, dark, moving objects. Thus, it can be surmised that the size of the contrasting dark stripe (small vs. large) is essential in deterring horseflies.

In summary, the study's authors concluded, "Our working hypothesis now is that horseflies are attracted to equid hosts owing to a combination of odour at a distance, then size of the animal contrasted against the sky or vegetation at a middle distance. But at close range, where they can no longer see the body's outline, flies make a visual switch to local features. If these are small dark objects contrasted against a light or white background, the horsefly no longer recognizes this as a host target and veers away. The contrast of stripes and their relatively small size are therefore the key elements of how stripes operate to thwart fly landings."

One curious question that remains for researchers is - why did zebras evolve to have coat colors with contrasting stripes, and why did other equids and hooves animals not? For now, that question is still unanswered.

In the meantime, this information may help you more accurately choose your horse's next fly sheet. If you live in an area where horseflies are a problem, a fly sheet that closely mimics zebra stripes may be a worthwhile investment!

Reference: Tim Caro et al, Why don't horseflies land on zebras?, Journal of Experimental Biology (2023). DOI: 10.1242/jeb.244778.

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