(CNN) - Hordes of dragonflies are showing up on weather radar and making things interesting for weather forecasters in the eastern United States.
It's not their fault -- the flying insects are just heading south to escape the coming cold weather.
The National Weather Service office in Wakefield, Virginia, tweeted radar imagery on Monday that showed strange, cloud-like masses over parts of Virginia and North Carolina.
Mike Dutter, the science and operations officer for the Wakefield office, said weather radar is sensitive enough to detect tiny droplets of rain, so it's not unusual for it to also pick up dust, flocks of birds or insects.
"It doesn't surprise me that we're seeing this," he said. "The thing that's interesting is how widespread it is. It seems like numerous radars on the eastern third of the country are seeing it."
Swarms of dragonflies were detected on radar last week over Indiana, Ohio and Pennsylvania.
Entomologist MaLisa Spring, the the state coordinator of the Ohio Dragonfly Survey, said that there are still a lot of unanswered questions about dragonfly migrations, but several species do fly south in the late summer and early fall.
"We know dragonflies go south, but as far as we are aware, there is not one specific site where they hang out, but instead they seem to just spread out southward," Spring said.
They will travel together in large groups when conditions are right.
Most of the dragonflies are Common Green Darners, she said, but several other species will also make the trip.
"This year, it seems like the numbers are much larger as a lot more people are reporting them," Spring said.
Dragonflies only live a few months as adults, she said, so it will be up to future generations of the bugs to make the return trip north.