Advances in materials science and specifically the development of ultraviolet light-emitting diodes is allowing biologists to undertake studies with standardised light sources that were not previously feasible. One particular area in which they might help improve our understanding of life on earth is in answering an age-old question - why are moths drawn to a flame? New insights into this phenomenon might help us improve the nocturnal environment for the vast range of moth species that fly at night.

There are numerous theories that have been put forward to explain why moths are drawn to light. One of the most well-known explanations is that they navigate by moonlight and that any artificial light simply confuses their biological navigation systems. Another is that moths are somehow stimulated by the light and it simply confuses them. Lots of species are drawn to light, but not all, day-flying moths are, perhaps by definition, not interested in artificial light. There is anecdotal evidence that it is mostly male moths that are drawn to light, but this seems to apply to only a few specific species, and males and females of many more are equally drawn.

Gunnar Brehm of the University of Jena, Germany, and his colleagues have pioneered the use of ultraviolet LEDs in the study of moths, particularly in the field where the low power consumption is important for remote and mobile applications. "Unfortunately, there is no answer to the moth to a flame problem," he told me, "It would be great to have one," he said. "I agree that the moon theory is not likely to apply and some sort of confusion happens." He points out that "Short wavelengths at night have simply not been there before humankind invented artificial lights."

He and his research team are currently working on the problem. "We are carrying out choice experiments with different wavelengths - four lamps in four corners of a hall. When moths are offered white, green, blue, and UV, the vast majority is attracted to UV," he explains. "However, when we replace UV by red the next night, the majority of moths is then attracted to blue."

This would seem to suggest that short wavelengths are always the most attractive ones. When there is no short wavelength source, moths will be drawn to the yellow light of even a source as dim as a candle. Of course, the shorter the wavelength of light, the higher its energy. But, with modern LEDs it is possible to control the wattage output and so standardize experiments with different wavelengths. [Brehm, G.,  40(1): 87-108; DOI: 10.3897/nl.40.11887 and Eur. J. Entomol. 114: 25-33; DOI: 10.14411/eje.2017.004]

With these standardized LED sources, the team can use different colors and different wavelengths of ultraviolet. "We number each specimen, determine the sex and count each day what is caught and not caught," he explains. They have found significant differences between species: in some, males and females are attracted in similar quantities, in others far more males are attracted, but the attraction of short wavelengths appears to be rather universal," he adds. "It seems that short wavelengths are always the most attractive ones." Brehm says. This does not explain why moths are attracted to light in the first place. "We're working on the problem…" Brehm told me.