A good exit strategy may explain why biting flies prefer some horses over others

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Photographs and thermographic images of sunlit black, brown, palomino and white horses. In the thermographic images, the black perimeters of the back and belly areas are shown where the surface temperature was averaged. Image: Horváth et al. https://doi.org/10.1371/journal.pone.0233038

Every dangerous pursuit needs a good escape plan, and it seems even biting flies that target horses instinctively recognise this.

Research has shown that blood-sucking female horseflies (tabanids) prefer warmer host animals, which typically will have darker coats, and generally attack them in sunshine.

Indeed, the attractiveness of sunlit brown horses to tabanids is about four times greater than that of sunlit white ones, and in comparison to a white horse, a brown horse spends two times longer in a tabanid-free shaded forest than in a sunny field with intense tabanid attacks.

Scientists have shown that these inclinations among horseflies appear to relate to a thermal preference. But why?

Hungary-based researcher Gábor Horváth and his colleagues hypothesized in a 2019 paper that blood-seeking female tabanids prefer elevated temperatures because their wing muscles will be quicker and their nervous system functions better in a warmer microclimate.

Thus, they can initiate prompt takeoffs more successfully to avoid the animal’s parasite-repelling reactions, whether it’s nibbling their skin, stamping their feet or swishing their tail.

It therefore follows that the escape success of horseflies that land on host animals increases with increasing surface temperature.

To test this prediction, Horváth and his colleagues, in fresh research, studied the escape success of tabanids that landed on black targets in relation to the surface temperature. The study team also measured the coat temperature of different-coloured sunlit and shaded horses with thermography.

In all, 46 thermographic images of horses were obtained, using two black animals, two brown, two with palomino colouring, and two that were white.

A tabanid horse fly from the United States. Photo: Bruce Marlin, CC BY-SA via Wikimedia Commons

Their field experiment, conducted over five good windless days on a Hungarian horse farm, involved using warm and cold sunlit barrels to attract the flies. Warmer air-filled barrels thermally imitated sunlit black horses, while the cooler water-filled barrels were cooler than either brown, beige or white horses.

Catch attempts were made by an experimenter using a hemispherical tea-strainer measuring 15cm across.

After each capture effort, the air temperature and the surface temperature of the barrel at the tabanid’s landing location were measured with a contact thermometer.

The results, reported in the open-access journal PLOS ONE, corroborated their prediction.

“We found that the escape success of tabanids decreased with decreasing target temperature — that is, escape success is driven by temperature.

“Our results explain the behaviour of biting horseflies that they prefer warmer hosts against colder ones.”

As expected, thermographic readings taken of the eight horses of different colours showed that the surface temperature of the sunlit back decreased in colour order from black to brown to palomino to white.

The maximum and minimum surface temperatures of black horses were 54.6°C and 30.9°C respectively. Brown horses had a maximum of 44.6°C and a minimum of 31.2°C. The two horses with palomino colouring recorded a maximum of 46.2°C and a minimum of 32.6°C, while the white returned a maximum of 46.6°C and a minimum of 31.0°C.

Since in sunshine the darker the host the warmer its body surface, the results also explain why horseflies prefer sunlit dark (brown or black) horses to those with a lighter coat, and why these parasites attack their hosts usually in sunshine, rather than under shaded conditions.

The authors say there could also be other reasons why blood-sucking horseflies might prefer to attack warmer host animals. For example, with increased sweating, the capillaries could be enlarged near the epidermis of warmer horses, which could be advantageous for blood-sucking insects.

The full study team comprised Horváth, Ádám Pereszlényi, Tímea Tóth and Imre Miklós Jánosi, all with Eötvös Loránd University in Budapest; and Ádám Egri, with the MTA Centre for Ecological Research, part of the Danube Research Institute, which is also in Budapest.

Horváth G, Pereszlényi Á, Egri Á, Tóth T, Jánosi IM (2020) Why do biting horseflies prefer warmer hosts? tabanids can escape easier from warmer targets. PLoS ONE 15(5): e0233038. https://doi.org/10.1371/journal.pone.0233038

The study, published under a Creative Commons License, can be read here

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