Entomology What is the fastest flying insect

Insect death : Deadly buzz

Lights on, balcony door open. So one thing is certain on mild summer evenings: Not only does fresh air blow in, but one or the other insect will soon be buzzing around the room lamp. The Totenkopfschwärmer, for example, a stately moth that has penetrated from Africa to Germany. Because of his characteristic skull-like drawing on his body, he is considered the epitome of the ominous, also in the novel "The Silence of the Lambs".

But that's superstition. It is more the human being who brings mischief to the butterfly - because he leaves the lights on everywhere. Like all moths, the hawkmoth flies around the lamp, tears away, and then - learned nothing - to dare a new approach. At some point the animals end up exhausted on the ground.

Easy prey after the buzz

You don't need to be an entomologist: artificial light is a trap for many insects. Hundreds of them buzz around street lamps every night. An average of 450 researchers counted on a single mercury lantern per night; 180 on a sodium vapor lamp. About a third of them die from the light source. "They rarely burn, however," says the butterfly specialist and environmental expert Stefan Birrer from Reinach, Switzerland. “Most of them turn their circles until they sink to the ground, exhausted.” Then they are easy prey.

Light harms the insects mainly by preventing them from their natural behavior. They circle instead of eating; they circle instead of mating; they circle instead of chasing. This is what happens to beetles and ladybirds, hornets and housemothers, mayflies and lace-ups. Night for night.

Without question, the loss of the night goes hand in hand with the death of many insects. But: Are the many street lamps also to blame for the massive insect deaths? According to a study published in 2017 by Krefeld's natural scientists in the journal “Plos One”, the number of flying insects in parts of Germany has plummeted by more than 75 percent. Although the methodology of the survey and thus the drastic number are controversial, the trend towards insect decline is not. Climate change and agriculture, especially monocultures and spray poisons, such as the controversial glyphosate, are often blamed for this. But what part does the nocturnal artificial light have?

Deadly street lights

Alessandro Manfrin investigated the effect of street lamps in a unique experiment over a period of two years. He went to the Westhavelland Nature Park in Brandenburg, a large contiguous nature reserve and one of the few "dark field reserves" in Germany. In the middle of it he placed twelve street lights with sodium vapor lamps as part of his doctoral thesis at the Leibniz Institute for Freshwater Ecology and Inland Fisheries in Berlin. He also equipped a comparative test field with lanterns, but these remained switched off to check whether the buildings themselves might be an insect magnet. "In the first year we found 76 times as many insects on the burning lamps and in the second even 267 times as many," says Manfrin. “The lamps are like a vacuum cleaner in the ecosystem. Water-loving insects in particular are attracted by the light. ”But the light is not only disadvantageous. A number of insects benefit from this because they find easy prey on the lamp, namely exhausted animals that have been circling for a long time.

"Many people think spontaneously that the attraction of light doesn't matter," says the biologist. "But with thousands and thousands of street lamps in Germany alone, which shine night after night for many decades, there is an impact on the ecosystem to the same extent as that of insecticides."

Effects of light are difficult to research

This is not clearly proven. Few researchers go out at night to count both dead and living insects on lamps. A study from the 1960s reported that the insect population at one location plummeted within two years after an intensely lit gas station was opened. “But other environmental factors have always changed in such studies, too. The gas station has been added and with it noise and cars, so that one cannot say how big the effect of the light alone is,” objects Birrer.

However, one thing is beyond dispute: the artificial brightness has a massive impact on the behavior of the insects. One study after another demonstrates this. Manfrin also observed that nocturnal spiders in the area of ​​influence of the lanterns extended their hunt into the daytime because they filled their stomachs with the masses of prey lying around. So there is likely to be a shift in the balance between the species. Those who eat insects have an advantage, other species suffer.

For example, the lamps are a tragic attraction for nocturnal flower visitors, noted the ecologist Eva Knop from the University of Bern. She went to the dark foothills of the Alps and, like Manfrin, set up LED lamps on seven surfaces. Then she concentrated in her research on an ubiquitous growing plant, the cabbage thistle. Around 300 different pollinators swarmed out at night to visit their white, brush-like flowers. “The pollination performance of nocturnal insects is often disregarded. It is very significant, ”says Knop.

Nocturnal pollinators avoid brightly lit fields

The night shift workers, however, do not like light. 62 percent fewer insects flew on areas illuminated at night. This also had an effect on the cabbage thistle. It produced fewer seeds. As a result, the plants produced thirteen percent less fruit. “So it should be as dark as possible at night in the vicinity of fields, vegetable and fruit plantations that are not pollinated by wind. That would be good for the harvest, ”says Knop.

“The light can work in two ways,” says Freiburg entomologist Michael Boppré. “On the one hand, it attracts the insects away from the flowers. On the other hand, it manipulates the biological clock of insects and plants, which can reduce flower visits. "

There is evidence for both effects - the attraction of light and the adjustment of the biological clock. The production of fragrance and nectar in many plants follows a day-night rhythm. Light sets the pace. But the insects themselves also react to light. In 2017, Dutch researchers documented that various species of moths ate significantly less food under artificial light than in the dark. Light pollution could therefore explain the decline in some species of moths via the subdued appetite.

According to older experiments, the harmful cone of light from a street lamp can reach up to two hundred meters. However, it depends on how intensely the artificial light source shines and how dark the surrounding night is. The stronger the contrast, the stronger the attraction.

It depends on the type of lamp

It is controversial which lamps are the worst for insects. In her experiments, Knop worked with modern LED versions, as can also be found in common street lighting. You and Manfrin describe that these attracted many more insects than older sodium vapor lanterns. But the moth specialist Birrer contradicts: When inspecting lanterns at night, he observes that “there is almost no life on the LED lamps. In contrast, we find hundreds of insects in sodium lamps. ”Presumably, it depends on the exact spectrum of the LEDs. The more light they contain in the ultraviolet light, the stronger the suction effect on insects. Indeed, entomologists use LED lights with a high UV content to attract different species to their research.

So much is still in the dark. But even if it turned out that light pollution was a serious threat to many insects, humans would hardly turn out all the lights outside at night - after all, in many cases it is also about their safety. In the light, drivers recognize passers-by more easily and, above all, many people feel safer.

That is why Birrer advocates a pragmatic strategy: Do more research and take precautionary measures. As few lanterns as possible should be lit, especially in small-scale habitats, such as moors or rough meadows, which are rare and in which specialized rare insects such as the nocturnal copper hen live. If possible, they should be more than 200 meters away from the habitat of the species that are worth protecting. Elsewhere, a compromise between security needs and environmental protection would be conceivable: On roads with little traffic, motion detectors could briefly light up the darkness for human night owls so that animal night owls are disturbed as little as possible.

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