Fireflies are such a delight that they are the official insects of two U.S. states. On the other side of the world, Japan has declared them a national treasure. But these beacons of brightness—which seem to be in decline in some locations—have yet to be systematically studied to determine when their numbers will peak each summer. Now, a group of scientists in Michigan doing a long-term survey of lady beetles has for the past 12 years also counted fireflies caught in their sticky traps. They’ve monitored these insects in 10 locations across the state, including fields and forests. And just this year, graduate students matched the data with daily temperature and rainfall for each of the summers. They found that firefly populations seem to follow a 6-year cycle, increasing for 3 years and decreasing for the next 3 years, they reported this week on bioRxiv, a preprint archive. But the conditions that determine fluctuations within that cycle are complex: Summer temperatures have to be warm enough long enough for fireflies to emerge, but peak emergence can be delayed by up to 2 weeks depending on whether conditions are too wet or too dry. Thus, the warmer springs predicted by climate change would mean an earlier firefly peak, but only if rainfall remains the same. Solving another big mystery—whether firefly numbers are declining globally—will also be a challenge, the scientists say, as captured insects vary an order of magnitude year to year, from three or four per trap to just one every two traps. So just as firefly flashes have mystified children for centuries, the insect’s fate will remain a mystery for adults for a bit longer.