Thursday, 11 August 2011

Moths to a flame: The attractiveness of short-wavelength artificial light

(Kampina, a nature preserve in Noord Brabant in The Netherlands, where the current research was conducted.)

Just as one type of sound may elicit an evasive behavior among animals while another type is completely ignored, so, too, can different types of artificial light be more or less attractive to moths—which are drawn to human-made “flames” just as much as they are to natural ones. Night, street, and security lights are all common forms of artificial illumination common in anthropogenic areas and, cumulatively, are thought to be one of the major threats to moth populations, given that most moths do not survive their encounters with artificial light.

Different light sources differ in intensity and spectral composition (e.g., the wavelengths present in the light, which determine what color the light will appear). Both of these characteristics determine how attractive insects will find the light sources. Light with a large ultraviolet (UV) component is thought to be particularly harmful, since it may mislead moths into thinking they are flying toward one of the UV markers that commonly act as nectar guides on plants. The luminosity of UV light may also mimic the moon, thereby disorienting the moths as they wing their way through the landscape.

Until recently, there had been no research investigating which spectral characteristics of artificial light make it most attractive to moth species, and whether different types of moths have different light “preferences.” Earlier this summer, though, collaborators from Wageningen University, the Dutch Butterfly Conservation, and Philips Lighting published the results of a study in which they tested the “popularity” of 6 different light sources dominated by different wavelengths. During the study, they set light traps just before sunset and checked them just after sunrise. All moths attracted to the lights fell into containers of ethyl acetate, preserving them until the researchers could measure their morphological characteristics and identify them to the species level.

During the 6-week study period, 112 moth species were caught across the 6 different types of light. The highest richness and abundance of species were recorded at lamps that had shorter wavelengths—in other words, those that contained the highest proportion of UV. Of the four major families of moths identified (Noctuidae, Geometridae, Arctiidae, and Pyralidae), only one (Pyralidae) did not have richness or abundance patterns closely linked to the type of light.

(One example of many Pyralidae species.)

Regardless of which family they are in, larger moths tended to have bigger eyes, a characteristic which previously had been hypothesized to increase their attraction to artificial lights. Indeed, the current study showed that these species were preferentially attracted to light dominated by UV.

Previous work has found that LED lights are associated with the lowest insect capture rates; this type of artificial lighting contains no UV. On the other hand, high-pressure mercury and sodium vapor lamps are known to be most beguiling. In the current study, the most “popular” lamp was an Actinic lamp with an average wavelength of approximately 382 nm; this contains a large UV component. Clearly, then, it would be beneficial for insects if anthropogenic lights were to avoid these shorter wavelengths (with UV, purple, and blue tones) and instead aim for wavelengths of approximately 620 nm (with yellow and orange tones), which were found to attract fewer species. Unfortunately for the average light bulb buyer, information on wavelength is not generally included on packaging; thus, it will be up to civil engineers to select insect-safe products when installing artificial lights.

(Wavelengths and the colors they correspond to.)

If changes are not made to the artificial light environment, there may be ecosystem-wide repercussions, since moths—particularly large ones—are important pollinators for many species of plant, regulate plant growth by engaging in herbivory, and are themselves food for nocturnal predators such as bats and nightjars. The authors of the current study plan to investigate whether these ecological changes are already underway in brightly lit anthropogenic environments. Hopefully their research can shed some light on this topic before it is too late for the moths.

van Langevelde, F., Ettema, J.A., Donners, M., WallisDeVries, M.F., and Groenendijk, D. 2011. Effect of spectral composition of artificial light on the attraction of moths. Biological Conservation 144:2274-2281.
Thanks to the following websites for providing the images used in this post:

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