Pasolini wrote a famous essay in 1975, "The Disappearance of the Fireflies," which, at that time, was already starting to become very noticeable. Of course, the essay was really about capitalism.
Personally, outside my childhood in the countryside, I noticed fireflies in the outskirts of a largish city some 20 years ago, then nothing for a very long time, and then I saw a few when I lived for a brief period of time in a really remote place, like an hour from the nearest highway. No trains anywhere near, too.

Off-topic, but if you don't know Pasolini, I urge you to read his last interview which seems particularly gloomy as it appears to foreshadow his own death just a few hours after.
One memorable quote:

I listen to the politicians – all the politicians – with all their little presumptions and I turn into a mad man as they prove they do not know which country they are talking about, they are as far away as the moon. And together with them there are the men of letters, the sociologists and the experts in any kind of field.