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There are various theories about how insects are drawn to light bulbs because of navigation mechanisms based on the sun and moon's position in the sky. It makes sense for that to work when the only major light source on Earth was high in the sky at a very slowly changing angle, but now that we have artificial lights it seems like that would severely interfere with the regular life cycle of insects. Being stuck buzzing around a light bulb for almost half of every day seems like it could be damaging to the life expectancy of each bug, and therefore damaging to the species as a whole.

But the reality is, most of the "livable" surface on Earth is not covered in cities, so most of the bugs aren't affected by these artificial lights.

So that leaves me wondering, if we expanded our cities to cover most of the earth livable surface, would we potentially kill off a large portion of the bugs on earth that are drawn to light or rely on bugs drawn to light as a food source?

That's sort of a hard question to answer though, because of so many factors, however I think the root of the question is a simpler one:

Do artificial light sources cause enough fatalities in light-attracted species to prevent them from successfully reproducing at the necessary rate for survival? (assuming the whole species were subjected to such light sources)

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Short answer: Yes, it is highly likely that artificial light sources may cause such high mortality that insects become (locally) extinct.

Long answer: As you will understand, it is almost impossible to perform an experiment where lights cause extinction of a species. However, we have a lot of 'circumstantial evidence'; cases where it is clear that lights have dramatically decreased species richness (which means local extinction). If lights can cause local extinction, they can also cause global extinction if, as in your question, their entire population would be exposed to light.

Percentage of insects attracted/killed by light
Why insects are attracted to light remains unanswered, but that they do is clear. Also, it is known that insects that are attracted suffer an increased mortality, due to the light itself and due to predators actively hunting the blinded and inactive insects. The reported estimates of the number of insects that are attracted and killed by a light source differ and depend on the light source (color mainly), the background lighting (moon!) and other factors. A fair estimate is that about a third of the insects that are attracted by a light source are killed A third is a very high mortality, that is off course added onto the other (natural and man-made) mortality insects suffer from. It is very likely that this would cause insect populations to become extinct, especially those already at risk.

Based on observations
Eisenbeis&Hänel give a number of examples that show that insects populations dramatically decline when exposed to lights:
- the number of insects drawn to the lights of newly build fuel stations is high in the first years, than rapidly declines.
- the butterfly fauna in remote Austrian valleys declines after introduction of lighting (for tourism).
- The number of caddisflies drawn to lights exceeds that of the number found in a stream.

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  • $\begingroup$ Nice answer. Note that it's a third of the insects attracted to a particular bright light source that are killed estimated from that paper, not a third of all insects. $\endgroup$ – Bryan Krause Jun 22 '17 at 16:06
  • $\begingroup$ @Bryan. Good observation. I've edited that in. $\endgroup$ – RHA Jun 25 '17 at 18:34

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