Dear Straight Dope:
Growing up in the early 1950s in the urban area of Indianapolis, I recall swarms of fireflies – they just seemed to be everywhere. Now when I go back to Indianapolis or other places in that part of the country I see very few. I've even traveled out into the countryside to see if it was just civilization that pushed them out. But even there I saw very few. Where have they gone?
Short version: I don’t really know.
Long version: This is a hard one to answer simply, because there are many different things going on that might be playing a part, and any explanation is going to be pretty much purely theoretical – it’s not as if there are teams of scientists from around the world working on this problem and collecting data we can look at. That said, let’s see how many factors I can list off the top of my head that might result in your seeing fewer fireflies on recent visits to the midwest:
(1) Weather. The first thing that comes to mind is that fireflies eat snails and slugs, so anything that negatively affects snails or slugs will mean fewer fireflies. No predator can live where there isn’t any prey. Given overall temperature and rainfall trends, I’ll bet that Indianapolis in the last three years or so has been a lot warmer than it was back in the 50s, and very possibly drier. That could make a difference in how many snails and slugs (and thus fireflies) are around.
(2) Habitat alteration. I’ll bet a fair bit of the moist land that used to support snails and slugs has been converted to developed property with better drainage and completely different vegetation. That almost certainly would make a difference.
(3) Timing. Fireflies tend to be fairly synchronous in emergence, so if your visit happens to be just a week or two early or late, you might not see any in a spot where they’re actually quite abundant.
(4) Pesticides. It could be that more of the area is affected by direct or indirect application of pesticides than in the 50s, though I don’t actually expect this would be the case – I think people tended to use more pesticides, and more dangerous ones, back in that era. (That’s one thing we’ve actually managed to improve on the whole – thank you, Rachel Carson.)
(5) Pollution. There may be substances besides pesticides that are contaminating the area where the fireflies used to be, and snails and slugs can be quite sensitive to pollutants.
(6) Species turnover. It may be that the local balance among various firefly species has changed – i.e., the particular species that were most common in the 50s are no longer the major ones in that area, and the ones that have taken over are just less noticeable. Firefly displays differ from species to species, after all, and some are a lot subtler than others. Alternatively, if different snail or slug species have become prevalent, maybe the fireflies don’t like to eat them.
(7) Introduced species. Or, similarly, it could be that the native fireflies and/or snails and slugs have been attacked by (or simply displaced due to competition from) some exotic species that never used to occur there – maybe parasites, maybe predators, maybe other slugs and snails – which has led to a reduction in their numbers.
(8) Disease. It’s possible, though I’m not aware of it, that there’s been some disease that’s affected fireflies and/or snails and slugs in that region in the intervening time. Something like that could happen without anyone’s noticing for a long while.
I could probably think of more, but you get the idea. It could be any of these things, it could be a combination of some of them, or it could involve other factors entirely – and that assumes your observation is correct in the first place. As it happens, I think that’s a safe assumption, but in science one tries hard not to make assumptions. At least now you see how scientists go about saying “I don’t really know” – we qualify it with a long, cautious list of possibilities.
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