Steven Gaulin's answer has some merit, but Peter de Jong is right, no proposed mechanisms for moth 'attraction' to artificial lights have yet been examined experimentally.

Although an exact mechanism is yet to be determined, the 'attraction' is likely a result of a light-pollution-induced uncoupling of historical moonlight cues for navigation from the historical context in which the navigational adaptations evolved (Gaston et al., 2013).

Both van Langevelde et al. (2011) and Davies et al. (2013) have shown that lights with no UV components and a narrow wavelength spectrum are less ‘attractive’ to moths and other insects, and are also likely to be less detrimental to other animals. However lights with the aforementioned characteristics are being phased out in favor of lights which produce a more “pleasing” white light (broad spectrum LED) (Davies et al., 2013). Low pressure sodium lights that were common in the 1950s, appear to have the least harmful effects on moths and other animals overall, but have been slowly phased out because of the “ugly” orange light they produced (van Langevelde et al., 2011; Davies et al., 2013)

The area of land affected by light pollution has increased by 68% over the last two decades (Cauwels et al., 2014). This newly lit land represents an area more than half the size of the USA. Although 49% of this area represents dim light pollution (as opposed to bright city lighting), the harmful effects of light pollution are most pronounced when first introduced into an unlit habitat, as is the case with dim light pollution.

References

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This paper gives a very good overview of the progression of light pollution over the last few decades.

Cauwels P, Pestalozzi N, Sornette D (2014) Dynamics and spatial distribution of global nighttime lights. EPJ Data Sci., 3. http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2237410

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Davies TW, Bennie J, Inger R, de Ibarra NH, Gaston KJ (2013) Artificial light pollution: are shifting spectral signatures changing the balance of species interactions? Glob. Chang. Biol., 19. http://onlinelibrary.wiley.com/doi/10.1111/gcb.12166/full

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See this paper for options to reduce the effects of light pollution on ecosystems.

Gaston KJ, Davies TW, Bennie J, Hopkins J (2012) Reducing the ecological consequences of night-time light pollution: options and developments. J. Appl. Ecol., 49. http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2664.2012.02212.x/full

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Very good review of the ecological impacts of light pollution.

Gaston KJ, Bennie J, Davies TW, Hopkins J (2013) The ecological impacts of nighttime light pollution: a mechanistic appraisal. Biol. Rev. Camb. Philos. Soc., 88, 912–27. http://onlinelibrary.wiley.com/doi/10.1111/brv.12036/full

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van Langevelde F, Ettema JA, Donners M, WallisDeVries MF, Groenendijk D (2011) Effect of spectral composition of artificial light on the attraction of moths. Biol. Conserv., 144. http://www.cb.iee.unibe.ch/unibe/philnat/biology/zoologie/cb/content/e7117/e7118/e8764/e242959/vanLangevelde_BioCon2011.pdf

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Good review of nocturnal insect navigation.

Warrant E, Dacke M (2011) Vision and visual navigation in nocturnal insects. Annu. Rev. Entomol., 56. http://www.annualreviews.org/doi/abs/10.1146/annurev-ento-120709-144852

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They are not, strictly speaking , attracted to it; they are attempting to navigate by it using algorithms that evolved in a world where bright nocturnal objects (e.g., the moon) were at optical infinity. When moths try to fly at a fixed angle in relation to a light source that is NOT at optical infinity (e.g., a street lamp) they will inevitably circle it.

As for solutions, examples might include changing public perception away from everything needing to be lit all the time, and where it is needed, implementing night-sky-friendly lighting (plenty of hits if Googled). http://www.darksky.org/ might be of interest if you don't already know of it.

I like Steven's answer, but is it that definitive? I.e. is this theory (plausible as it sounds) or has it been experimentally proven? I am not aware of the latter, although quite some work has been done on this issue. Just 2 links below to shed more "light" on this matter (which will hopefully confuse neither moths nor scientists):

http://www.sciencedirect.com/science/article/pii/S000632071100231X

http://science.howstuffworks.com/zoology/insects-arachnids/question675.htm

Thank you very much for the illuminating answers. The darksky link is certainly very useful. The navigation theory, which is the most plausible explanation I have heard so far, is news to me. Thank you very much for that! The 'howstuffworks' link is useful, too.

Steven Gaulin's answer has some merit, but Peter de Jong is right, no proposed mechanisms for moth 'attraction' to artificial lights have yet been examined experimentally.

Although an exact mechanism is yet to be determined, the 'attraction' is likely a result of a light-pollution-induced uncoupling of historical moonlight cues for navigation from the historical context in which the navigational adaptations evolved (Gaston et al., 2013).

Both van Langevelde et al. (2011) and Davies et al. (2013) have shown that lights with no UV components and a narrow wavelength spectrum are less ‘attractive’ to moths and other insects, and are also likely to be less detrimental to other animals. However lights with the aforementioned characteristics are being phased out in favor of lights which produce a more “pleasing” white light (broad spectrum LED) (Davies et al., 2013). Low pressure sodium lights that were common in the 1950s, appear to have the least harmful effects on moths and other animals overall, but have been slowly phased out because of the “ugly” orange light they produced (van Langevelde et al., 2011; Davies et al., 2013)

The area of land affected by light pollution has increased by 68% over the last two decades (Cauwels et al., 2014). This newly lit land represents an area more than half the size of the USA. Although 49% of this area represents dim light pollution (as opposed to bright city lighting), the harmful effects of light pollution are most pronounced when first introduced into an unlit habitat, as is the case with dim light pollution.

References

----------------------------------------------------------------------------------------

This paper gives a very good overview of the progression of light pollution over the last few decades.

Cauwels P, Pestalozzi N, Sornette D (2014) Dynamics and spatial distribution of global nighttime lights. EPJ Data Sci., 3. http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2237410

-----------------------------------------------------------------------------------------

Davies TW, Bennie J, Inger R, de Ibarra NH, Gaston KJ (2013) Artificial light pollution: are shifting spectral signatures changing the balance of species interactions? Glob. Chang. Biol., 19. http://onlinelibrary.wiley.com/doi/10.1111/gcb.12166/full

------------------------------------------------------------------------------------------

See this paper for options to reduce the effects of light pollution on ecosystems.

Gaston KJ, Davies TW, Bennie J, Hopkins J (2012) Reducing the ecological consequences of night-time light pollution: options and developments. J. Appl. Ecol., 49. http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2664.2012.02212.x/full

-------------------------------------------------------------------------------------------

Very good review of the ecological impacts of light pollution.

Gaston KJ, Bennie J, Davies TW, Hopkins J (2013) The ecological impacts of nighttime light pollution: a mechanistic appraisal. Biol. Rev. Camb. Philos. Soc., 88, 912–27. http://onlinelibrary.wiley.com/doi/10.1111/brv.12036/full

--------------------------------------------------------------------------------------------

van Langevelde F, Ettema JA, Donners M, WallisDeVries MF, Groenendijk D (2011) Effect of spectral composition of artificial light on the attraction of moths. Biol. Conserv., 144. http://www.cb.iee.unibe.ch/unibe/philnat/biology/zoologie/cb/content/e7117/e7118/e8764/e242959/vanLangevelde_BioCon2011.pdf

--------------------------------------------------------------------------------------------

Good review of nocturnal insect navigation.

Warrant E, Dacke M (2011) Vision and visual navigation in nocturnal insects. Annu. Rev. Entomol., 56. http://www.annualreviews.org/doi/abs/10.1146/annurev-ento-120709-144852

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Thank you, Corneile, for the excellent review. So the consensus seems to be that we still don't really know exactly why moths are attracted to artificial light. In this context, it is interesting that there are many species that are never attracted to light. If the current belief is that artificial light is a substitute for the moon and the moths attracted are confused, then what sort of navigation system are the other moths using? The moths that are not attracted do not belong to a particular group but are scattered among genera of moths that are often attracted to light.

The orientation hypothesis is dominant but not convincing to me as this would suggest that moths fly towards light in a spiraling fashion. I've spent a lot of time trapping moths and they do not do that in general.

A lot of moths near light sources are not attracted while conspecifics are (about 75% is not attracted, unpublished). This probably has something to do with there behaviour at that moment, for instance feeding vs. dispercing.

The knowledge on the impact of artificial light on population levels and ecosystems is extremely limited. However there is a project in The Netherlands targeting that question (I'm a post-doc in that project). For details see www.lichtopnatuur.org this site will have a lot more results soon and is in english.

Nocturnal insect navigation is likely more complex than we currently understand. More and more research is revealing new mechanisms of navigation among nocturnal insects, suggesting insects likely use a combination of different mechanisms to navigate (Warrant & Dacke, 2011). Moths in particular use the moon directly as a cue but may also use polarized light from the moon and possibly starlight (Warrant & Dacke, 2011). A recent paper found navigation using the milky-way as a cue in dung beetles (Dacke et al., 2013).

The differences in attraction to light among moths may be explained by eye size. Larger moths (with larger eyes) are more attracted to lights than small moths, especially lights with UV components (van Langevelde et al., 2011). This may be due to increased light sensitivity of larger eyes, but may also be a result of different navigational mechanisms for large and small moths. Many large moths are pollinators and have increased color vision, relative to non-pollinating moths (van Langevelde et al., 2011). Apart from light cues, insects probably also use terrestrial landmarks to navigate. This mode of navigation is especially common in forest-dwelling insects, which do not have a clear path of vision to nocturnal light cues. Instead these insects use cues such as forest canopy patterns to find their way around (Warrant & Dacke, 2011).

I personally think the mechanism for light attraction is more complicated than moths flying at an angle relative to a very distant light source (transverse orientation) which is then disrupted by a nearby, artificial light source. Insects with various modes of navigation are attracted to light, including insects using starlight and diurnal insects which use polarized sunlight. In my experience, the pattern of flight around light is not always circular, which may contradict the transverse orientation hypothesis. Insects also appear to engage in day-time behavior once they are close to lights: moths and other insects appear to stop flight activity and remain stationary next to wall- or roof-mounted lights. This suggests they revert back to daytime behavior once they are close to a light source.

References

Dacke M, Baird E, Byrne M, Scholtz CH, Warrant EJ (2013) Dung beetles use the Milky Way for orientation. Current biology : CB, 23, 298–300.

See my previous answer for other references.

I remember once seeing a flowering tree at the edge of our garden swarming with moths, at least over 10,000 individuals, at dusk. I took some with a net for identification. That evening, only 3 turned up at the MV light. These were Mythymna sp, which are normally common at lights.

As you mention, most settle down sedately near the light. So it is not only a question of navigational error. I shall be glad to see the results of the lichtopnatuur project when it is translated. Sounds interesting and certainly worth doing in other parts of the world!

Just a brief experience of my own: Moths seem quite not often apprach light spirally but come towards it even from a distance like a bullet. Some catytids do not even seem able to spiral but rather directly cast themselves to the light. This suggest that there are really also other mechanisms than the generally plausible moon/sky-polarization-orientation that indeed should lead to spiralled approach. How about scorpions and other walking creatures that are also attracted to light? I haven't seen any spiralling approach (to be honest, not much else either, as they usually appear to have come out of blue under your feet). But the point is that light attraction is a common phenomenon across nocturnal arthropods, flying or flightless, though others seem immune to it. Also, if the light source is too strong, some moths (often geometrids and microleps), even though strongly attracted to light, keep a distance from it.

Van Langevelde et al found a relatively large number of larger moths in their traps. They claim that this has to do with their large eyes. I believe this to be incorrect. They did not know the population densities of small and large species but I do agree it is likely that the number of individuals of small species is larger and this pattern deserves an explanation . However they also did not consider differences in behavior between large and small species. If large species cover larger distances in a night they have a higher chance of coming near the trap and are therefore over represented, without a difference in the attraction.

We have done trapping experiments with marked moths and did not find a difference between large and small species (unpublished)

Thank you very much for sharing your experiences! Clearly, much needs to be understood and the truth is probably a combination of much of what has been discussed above, as well as some new insights. I feel that the factors vary from group to group and perhaps even species to species in some cases. There was the case of a new Darna species (Limacodidae) which I still have to describe, bred from 2 larvae found in different years on the lawn where we had been continuously monitoring moths for 30 years. Adults never visited the light, although other members of the genus are known to do so. Ditto for the Catocalinae (=Erebidae now), many individuals never visit the light but are common at bait, while others are impartial about what attracts them. The moths with atrophied mouth parts, of course, would be navigating, but.....

Thanks for this interesting discussion. I have nothing to add, except that my (inexpert) feeling is of course that there is a mix of patterns: interference in navigation, escape (through an inexpectedly light 'hole')... I have seen hawk-moths crashing repeatedly against a white ceiling above themselves when this was illuminated by a near light (instead of flying towards the light itself), a clear symdrome of disorientation.

I have heard that LED technology lights are not attarctive to moths... which is at least partially true after my experience.

Light pollution is of course a problem. It has an not-expected implication: in most habitats, artificial lights provide an interesting resource for insectivores such as bats and gekkos. The concentration of insect prey is likely to represent a very important saving in the energy they have to invest to catch insects (especially bats whose flight is energetically expesive). Some species/populations might have been enhanced by this during the last decades...