Street lighting is not good for insects. In addition to disrupting their behavior, it can go so far as to directly reduce their number, according to a study in southern England whose findings were published Wednesday in the journal Science Advances.
The researchers studied in particular the larvae of moths. They move only a few meters around their hatching place and so their numbers can easily be studied very locally.
A total of 26 sites that were illuminated at night – hedges or roadside grass areas – were studied. Each of them has been compared to an unlit and not too remote location, presenting the same characteristics (vegetation cover, town planning, etc.).
Larvae were sampled, either by shaking hedges and recovering fallen larvae, or using a net.
As a result, caterpillars were on average 47% less numerous in hedges, and 33% less abundant in weeds.
“We were all surprised because it was so amazing,” Douglas Boyce, lead author of the study and a researcher at the UK’s Center for Environment and Hydrology, told AFP. Instead, a drop of about 10% was expected.
“The most likely explanation is that females do not lay eggs in these areas,” he explains. “It is not a behavior they have in the light.”
These nocturnal animals have been accustomed to darkness “for millions of years” so “this light is unusual, it’s new,” he says.
It also disrupts its life cycle: each caterpillar was weighed and those taken from the lit areas were heavier.
“They speed up their growth” by eating more food faster, explains Douglas Boys, because the situation seems “dangerous or unusual” to them.
The researchers also showed that the reduction in larvae was greater under street lights with LEDs—which are whiter and therefore more similar to daylight—than under conventional sodium lamps (HPS).
However, LED lights are being used more and more due to the better energy performance.
The study acknowledges that “street lighting may have contributed only in a minor way to the long-term decline of mites at the national level.”
But this light “has the same very important local effect.” And the cascading results: fewer larvae means less food for birds or bats.
However, there are “very accessible solutions,” assures Douglas Boyce. By installing color filters or screens, for example, to direct light only on the roads.
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