While it has been known for millennia that moths (and other nocturnal insects) are attracted to light, the impact on insect populations of the ongoing global increase in night-time lighting1 over recent decades is unknown. Research into the impacts of artificial light at night on moths has proliferated in recent years, revealing effects on behaviour and development throughout the life cycle2. However, until now there has only been very limited evidence for a direct link between light pollution and population change in any moth species or communities3 and none in real-world situations.
A new ground-breaking study from Butterfly Conservation, the UK Centre for Ecology & Hydrology and Newcastle University has changed all that. The research, led by PhD student Douglas Boyes, shows that streetlights in southern England reduce the abundance of moth caterpillars in grass verges by one-third (33%) and in hedgerows by almost a half (47%) compared to comparable unlit roadside habitat4.
Douglas spent over 400 hours sampling for caterpillars along roadsides at 27 pairs of lit and unlit sites in Berkshire, Buckinghamshire and Oxfordshire over the past three years. The grass verge sampling was done by sweep netting at night when the caterpillars climb up vegetation to feed and could have been biased by changes in behaviour due to the presence of streetlights. Indeed, in another part of the same study, Douglas proved that artificial light at night does make it less likely that caterpillars will come up to feed. However, the hedgerow sampling was carried out during the daytime (by beating) and therefore cannot be subject to such bias.
This is the first study to show major, long-term and real-world (as opposed to experimental) negative impacts of light pollution on moth populations. Such impacts, i.e. the loss of one-third to one-half of caterpillar abundance, can be expected when streetlights are installed as part of new road or house building and should be given proper consideration in planning and mitigation as part of urban development. Nevertheless, the findings do not reveal much about the importance (or otherwise) of artificial light at night as a driver of the substantial decrease in moth abundance in Britain over the past 50 years5. Even in the crowded landscape of south-east England, where the study was undertaken, streetlights directly illuminate a very small proportion of the landscape that is available as habitat for moths – estimated at only 1% by Boyes and colleagues. It is possible that the much lower levels of diffuse light pollution (‘skyglow’) which affect much more of the British landscape are also impacting moth populations, but we don’t yet know if this is the case.
Another extremely important finding of the study is that the white LED streetlights that are rapidly replacing older sodium lamps (which produce a yellow-orange light) across the UK had a more detrimental impact on caterpillar abundance. Populations under LED streetlights were reduced by 52% in hedgerows and 43% on grass verges compared to unlit comparisons. This was surprising because studies on adult moths have generally suggested that LED lights are similarly (or perhaps slightly less) attractive to moths2. Thus the shift to LED lighting, driven in part by a desire to reduce energy consumption (to benefit both the climate crisis and save public money) is ironically having a greater impact on biodiversity. One silver-lining that we can take from this, however, is that it is very easy, in principle, to change the intensity and colour of LEDs, which might offer affordable opportunities to reduce the impact on moth populations in the future.
We don’t yet have a clear understanding of how streetlights cause the substantial reductions in moth populations found in the study. The gut feeling of the researchers is that female moths may lay fewer eggs in habitat illuminated by streetlights leading to lower caterpillar abundance, but this requires further study.
Associate Director Recording and Monitoring, Butterfly Conservation
Follow me on Twitter: @RichardFoxBC
Images courtesy of Douglas Boyes.
- Kyba CC, et al. (2017) Artificially lit surface of Earth at night increasing in radiance and extent. Science Advances 3: e1701528. DOI: 10.1126/sciadv.1701528
- Boyes DH, Evans DM, Fox R, Parsons MS & Pocock MJO (2021) Is light pollution driving moth population declines? A review of causal mechanisms across the life cycle. Insect Conservation and Diversity 14: 167-187. DOI: 10.1111/icad.12447
- van Grunsven RH, van Deijk JR, Donners M, Berendse F, Visser ME, Veenendaal E & Spoelstra K (2020) Experimental light at night has a negative long-term impact on macro-moth populations. Current Biology 30: R694-695. DOI: 10.1016/j.cub.2020.04.083
- Boyes DH, Evans DM, Fox R, Parsons MS & Pocock MJO (2021). Street lighting has detrimental impacts on local insect populations. Science Advances 7: eabi8322. DOI: 10.1126/sciadv.abi8322
- Fox R, et al. (2021) The State of Britain’s Larger Moths 2021. Butterfly Conservation, Rothamsted Research and UK Centre for Ecology & Hydrology, Wareham, Dorset.