Biting flies are strongly attracted to blue objects because they mistake the colour for an animal they want to feast on, scientists have said.
The finding may help the fight against diseases that are spread by flies, such as sleeping sickness, by making traps more effective.
Dr Roger Santer, from Aberystwyth University’s department of life sciences who led the work, said entomological field studies have long shown that flies are attracted to blue, which has led to traps across the world being made in that colour.
“But why biting flies are especially attracted to blue traps has been a real puzzle for researchers,” he said.
Theories have included that flies mistake blue for shady areas good to rest in because shadows can have a blue tinge to them, a phenomenon that can be seen, for example, when they are cast across snow. It had also been suggested that blue traps resemble animals to a fly’s eye – but this had not been proved.
In an effort to find an answer, researchers from Aberystwyth developed artificial neural networks that mimic the visual processing that goes on in the brain of flies such as the tsetse, stable and horse fly. Artificial neural networks are a form of machine learning inspired by the structure of real nervous systems.
The neural networks were trained to distinguish animals from leaf backgrounds, and shaded from unshaded surfaces, using only the responses of the five types of photoreceptor in a fly’s eye. Afterwards, the networks were challenged to classify blue flytraps.
The results, which have been peer reviewed and published in the Proceedings of Royal Society B journal, have cast doubt on the theory that blue objects represent shade to flies. The neural networks detected shade through lack of brightness and did not misclassify blue objects as shade.
But to recognise animals, the networks compared the relative responses of blue and green-sensitive photoreceptors, and because of this they commonly mistook the blue traps for animals. This led the team to conclude that blue objects, including traps, resemble potential animal hosts to flies.
Santer said: “If we can understand the mechanisms that attract flies to coloured traps, we can improve the colour of those traps so that they more efficiently catch flies. This is a really important aim because different species of biting flies spread diseases of humans and animals, so fly control is an important part of disease control.”
He said the work could be important in tackling diseases like human African trypanosomiasis (sleeping sickness), which is endemic in sub-Saharan Africa and without treatment is generally fatal. It could also be used to control the very common stable fly, which is a damaging global pest of livestock.
The team’s conclusions are backed by field experiments that have found that tsetse flies caught in blue traps tend not to have eaten recently, suggesting they are seeking hosts.
• This article was amended on 28 June 2023. An earlier version quoted Dr Roger Santer talking about “etymological field studies” instead of entomological field studies. (Etymology is the study of words, while entomology is the study of insects.)
