Does painting cows with stripes prevent fly bites? Researchers win Ig Nobel prize

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Europe pondered the types of pizza lizards preferred to eat

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The 2025 Ig Nobel prize in biology honored Japanese researchers who discovered that black cows painted with vertical white stripes were less desirable to biting flies than their counterparts that didn’t look like zebras.2 This work, led by Tomoki Kojima, a researcher at the Aichi Agricultural Research Center, may offer a more environmentally friendly way (than pesticides) to repel cattle pests. The researchers’ findings also support a leading theory on why zebras evolved stripes, an open question in wildlife research.

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To test this hypothesis, Kojima’s team recruited three black cows (Wagyu cows are often black). They painted one with white stripes, one with black stripes, and left one unpainted. The researchers touched up the painted cows’ stripes every morning for three consecutive days. Kojima's team noted that freehand painting of zebra stripes only took about five minutes per cow.

For 30 minutes twice a day (in the morning and evening), the researchers observed the cows and counted fly-repelling behaviors, such as head throws, leg stamps, and tail flicks. At the end of each observation period, the team took pictures of the cows so they could count the numbers of flies that landed on the cows' bodies and legs.

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Since Kojima’s work came out in 2019, Japanese farmers have put their unusual method to the test. Kojima’s team chose short-lasting paint in their experiment so they could easily switch between treatments on each cow, but the farmers used paint that could last for more than a month.

“I don’t think there are potential health hazards [from paint],” Kojima said, as researchers and farmers commonly paint cattle to be able to identify them from a distance. He added, “[Painting] is a cheap, easy, and animal welfare-friendly method.”

“I was skeptical at first, but I was really surprised that no insects came,” a cattle farmer said in their three-year survey response, reported by the Japanese daily newspaper Asahi Shimbun in 2024. “There was no bullying from other cows, which I was worried about.”

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