Mayflies collectively "attack" the bridge, is it a sign of disaster or serious pollution?

Tuchong Creative

Recently (September 2024), a group of videos have been circulating on the Internet. In the video, you can see that the ground of the bridge is covered with a large number of insects, just like heavy snow. At the same time, there are also a large number of flying insects gathering in the air and flying around the street lights. Some netizens can't help but ask: What kind of insects are these? Why are they gathering together? Is it a precursor to a major disaster? Today, let's talk about the sudden appearance of a large number of insects. Let's listen to the analysis.

The insect in the video should be some kind of mayfly, which is a relatively primitive insect. There are about 2,000 species in the world, and more than 200 species have been discovered in my country. In my country, the ancients knew about this insect very early, and there are descriptions of them in many poems. For example, the famous Song Dynasty writer Su Shi’s famous work "Red Cliff Fu" contains the following lines: "Mayfly is in the universe, a grain of sand in the vast ocean."

Adult mayflies generally do not feed and have a short lifespan of usually a few days, or only a few hours for some species. They will die after mating and laying eggs, hence the saying "born in the morning and dead in the evening". Many species of mayflies often emerge in large numbers at a specific time of the year and then mate. The males will die shortly after mating, and the females will also die after laying eggs. This behavior is called "nuptial flight."

The larvae (young bodies) of mayflies live in water and feed on various organic debris and aquatic plants. Although the adult lifespan of mayflies is very short, the larval stage can reach 1-2 years, and some species can even last for several years. Mayfly larvae have very high requirements for the water quality of the waters they live in and will die if there is any pollution. Therefore, they are also called "indicator organisms" and can be used to monitor the water quality in waters.

So why do mayflies gather on the bridge? Clustering is a common phenomenon among insects. Some, such as caterpillars, bugs, and maggots, gather together just to share food. The collective behavior of some insects is an instinct controlled by genes, such as the nuptial flight of termites, ants, and mayflies. Gathering together can provide protection for insects, and the chance of being attacked when encountering natural enemies will be lower.

Mayflies need to lay their eggs in water. Many species of mayflies will choose to start their mating flights at night to avoid being caught by natural enemies such as birds. However, in the dark night, mayflies need to fly to the water surface to lay eggs. How can they distinguish between the water surface and the land? That requires light. In the wild without lights, the river water will reflect the moonlight and starlight. Mayflies will fly towards the bright places according to their instinct. This behavior is called "phototropism". After we built a bridge over the river, there will be very bright lights on the road, and the car lights are also very bright. These lights hit the asphalt road and the car surface and are reflected. In the eyes of mayflies, they look like water surface. As a result, they gather in large numbers, but they cannot lay eggs here. Mayflies will die after they are exhausted, and the insect corpses on the bridge surface will gradually pile up into a mountain.

According to news reports, the incident occurred at the Hanshou Yuanshui Bridge in Changde, Hunan. In fact, at this time in 2022, a large number of mayflies gathered in Yuanjiang City, not far from the incident, and it was on the news. Not only in my country, but also in the United States across the ocean, a huge number of mayflies gather on a bridge over the Susquehanna River in the United States at certain times of the year, affecting the passage of people and vehicles. So this is just a normal natural phenomenon and has nothing to do with disasters. You can put your mind at ease.

If we suddenly encounter a large number of insects gathering, we can find items such as headscarves and handkerchiefs to cover our faces and try to avoid facial injuries. If conditions permit, other exposed parts of the body should also be covered to fully protect our bodies. At the same time, do not get excited or shout loudly, and try not to move too quickly. Move steadily and quickly out of the gathering range of insects, because if you encounter a swarm of bees, fast movement and sharp shouting are likely to irritate the bees and trigger their instinctive attack behavior. If there are insect repellents, floral water and other insect repellents, spray them on your body in appropriate amounts. If conditions permit, go indoors as soon as possible to avoid them, close doors and windows to prevent flying insects from entering. If you are bitten by mosquitoes, you can wash the wound with soapy water and apply an appropriate amount of medicine. If the injury is serious, you need to seek medical attention as soon as possible. If you encounter a large number of insect corpses on the road while driving, it may cause a decrease in ground friction and cause a traffic accident. In this case, you can park the vehicle in a safe place and continue to move forward after the road is cleaned. If the car is parked in the emergency lane, turn on the warning lights, put out warning signs, wear reflective clothing and wait outside the road guardrail.

This article is a work supported by the Science Popularization China Creation Cultivation Program

Author: Li Weiyang Popular Science Writer

Reviewer: Yin Haisheng, Senior Engineer, Researcher, Shanghai Insect Museum, Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.