Moths are nocturnal creatures and thrive in darkness. But when is it really dark anymore in the Netherlands? Artificial lighting is everywhere, exposing both cities and countryside to continuous light – a phenomenon known as light pollution.
Van Geffen focused first on how light influences the growth of caterpillars. To do this he exposed young cabbage moth caterpillars to light of different colours: white, red and green. Some caterpillars were kept in darkness for comparison. No effect was noticed. Van Geffen: ‘Cabbage moths are nocturnal, so I expected that they would grow more slowly under light. That was not the case.
They grew just as fast as the caterpillars that were not exposed to light.’But there was a sting in the tail. Caterpillars pupate and a butterfly hatches out of the pupa, or cocoon. Under normal conditions, cabbage moths produce two generations per season. The pupae of the autumn generation go into what is known as the diapause, a dormant phase in which they wait for spring. The trigger for this process is the declining daylight hours.
Van Geffen made the surprising discovery that artificial light confuses the cabbage moths. ‘If you hang a lamp above them, they come out of the pupa stage months earlier. They think they are the first generation rather than the second one.’ As a result, the moths die of cold. This only happens after exposure to white or green light. Which is only logical, says Van Geffen: the creature’s eyes are not sensitive to red.
But there is more to it than this. Light plays a decisive role at a much earlier stage in the reproduction process. Cabbage moth females use sex pheromones to attract mates. Van Geffen studied the effect of light on the production of these attractants. His results are worrying. Not only does production come to a halt: light also influences the composition of the range of pheromones the females produce. Van Geffen: ‘Under light one of the attractants was produced in smaller quantities, while another substance, which repels cabbage moths, is produced in larger quantities.’ It is not hard to guess what effect this will have on nature. If females lose the capacity to attract a partner, the reproduction process is doomed.
Van Geffen did his experiments in the lab, but this effect seems to be present outside the lab as well. He investigated this by looking at the mating success rate of the winter moth. He caught some females of this species in the Dassenbos copse on campus. They have no wings, so mating has to take place on the tree trunk they live on. In the dark, the eggs of half of the females caught were fertilized. If a green light was hung up in the tree, fertilization took place in only a small fraction of the females. Van Geffen’s results are alarming, but he is wary of jumping to conclusions. ‘I must be cautious about blaming artificial light for the sharp decline in moth numbers. But there are strong indications. These are mechanisms that could explain the current decline.’
Light on Nature
Koert van Geffen’s light experiments are part of the Light on Nature project, in which researchers from institutions including Wageningen UR and the NIOO investigate the effects of artificial light on nature. Around the edges of nature reserves in several parts of the Netherlands, rows of lampposts have been erected which give off white, red and green light. The influence of this light on insects, mammals, birds and amphibians is being monitored closely.