Really? Are there more fireflies this summer? Or can we really tell?

By Dale Bowman

File photo of a firefly in hand. Credit: Dale Bowman — A firefly, hand-caught on July 5, but the question remains unanswered whether there are more than usual this summer.
Credit: Dale Bowman

I’m not alone in thinking there are more fireflies. Whether I’m right is debatable.

‘‘I usually have a large population of fireflies, but I would say it is approximately twice the usual number this year!’’ Dave Froede emailed.

‘‘My wife and I were watching fireworks last night in Lansing,’’ Danny Stamper tweeted. ‘‘We both noticed more fireflies compared to previous years.’’

‘‘There are certainly more fireflies this summer,’’ Christian Howe posted on Facebook. ‘‘Even my kids have noticed. Maybe something to do with the mild winter, good spring rains, maybe a natural life cycle, like cicadas or even cottontail rabbits.’’

That’s my guess, too.

Even Doug Taron, the chief curator of the Chicago Academy of Sciences, responded on Facebook: ‘‘I don’t have anything quantitative, but my impression from where I live in Elgin is that it’s a very good year for fireflies.’’

When asked if there was a reason, he responded: ‘‘Insect numbers jump around so much from year to year that it can be hard to ascribe a reason why any given year is particularly good or bad. I think the reasonably wet spring that we had probably helped keep populations of prey for their larvae [earthworms, tiny snails, and other similar critters] fairly high.’’

In late June, I contacted Derek Rosenberger, a professor in biological sciences reviving the insect collection at Olivet Nazarene University.

‘‘Funny you should ask,’’ he emailed back. ‘‘To be honest, I was looking for them this year because there has been so much in the press about there seemingly being fewer now than there have been.’’

(I greatly condensed his extended response.)

He noted the only real way to consider this is to look at time-series data, such as surveys over many years. There isn’t much.

‘‘Compounded with that is the fact that so many insects have cyclic population trends,’’ he emailed. ‘‘They go up due to good climate/conditions/lack of predation or disease, then they come down as those things catch up with them. Fireflies also peak and decline during the summer. So if you happened to be outside a lot in the evening as a kid, you likely saw the peaks, whereas if you are staying inside as an adult, you might not catch that peak. So you have to be careful about reporting declines because it might be a natural cycle.’’

He noted there’s lots on luciferase (enzymes that produce bioluminescence) but little on the ecology or population dynamics of fireflies. There are standardized surveys for agricultural pests and species of concern (butterflies and bees), ‘‘but who is going to invest time into anything else?’’

Citizen-science efforts are starting, but accurate sampling techniques must be developed.

He found one paper from Michigan State on data collected accidentally from sticky traps set for pests.

‘‘What [author Sara Hermann] and her colleagues found is that fireflies seem to prefer less disturbed fields . . . and that they seem to be on a six- to seven-year population cycle, with us just now starting to come out of a low,’’ he emailed.

‘‘This cycle seems similar to what has been observed in a longitudinal study in Asia. So that is some evidence for anecdotal reports of a seeming increase this year. I don’t think we really know what factors (predation, disease, etc.) cause highs and lows, so regarding what factors may be leading to an upswing, I think that still needs to be investigated.’’

Follow me on Twitter @BowmanOutside.