The ones on radar weren't packed quite so tightly, but there were a lot more of them.
The ones on radar weren’t packed quite so tightly, but there were a lot more of them. Elaine with Grey Cats/CC by-SA 2.0

Something strange was happening. On Tuesday, June 4, 2019, the evening sky over San Diego was fairly free of clouds, but meteorologists saw a thick swirl on their radar.

The culprit was a bloom of ladybird beetles, more commonly known as ladybugs. The little beetles were soaring more than a mile above the ground in a diffuse cluster many miles wide, a meteorologist from the National Weather Service’s San Diego office explained to the Los Angeles Times.

In this case, meteorologists were able to check in with people on the ground, who saw some of the little speckled beetles up close and helped clear up confusion about the blob. Without this ground truthing, though, it’s really hard to tell what’s making its mark on radar. The technology measures the shape, reflectivity, and altitude of things in the sky, and can gauge speed of movement. That allows meteorologists to assess, for instance, the size of the hail being dropped by a storm or where plumes of smoke are blowing—but it definitely doesn’t provide enough detail to be able to tell if the sky is awash with particularly charismatic insects. “The radar does not explicitly say ‘bugs or ladybugs,’” explains Alex Tardy, a meteorologist at the NWS San Diego office, via email. “It can tell us by the shape and reflectivity returns if there are birds or bats for example, but not the bug type or amount of bugs.

To drill down to the species level, you need more than just a blob on a screen. “There needs to be an expert on insect migration to evaluate the science,” Tardy says. California is home to scores of ladybug species, and Cornell University entomologist John Losey told NPR that these were likely convergent lady beetles, which are known to migrate in early summer. Still, it’s not totally clear why so many ladybugs are clustered this way. Tardy suggests that it might have to do with a spell of wet, cool weather, and Losey wondered if they were responding to some consequence of recent wildfires, or a change in the population of their prey, which is mostly aphids.

It’s not so unusual for swarms of small insects to turn up on radar: mayflies over Wisconsin, butterflies above Colorado, and midges moving across Ohio, to name just a few. Radar can detect clusters of any creature—birds, bats, insects—as long as there are enough of them, relatively close together, and flying at the right altitude, Tardy says.

The ladybugs were surprise guest stars—the meteorologists hadn’t set out to find them—but scientists sometimes intentionally tap into existing weather-tracking infrastructure to keep tabs on animals. During the 2017 total solar eclipse, for instance, some researchers from the Cornell Lab of Ornithology compiled data from 143 Doppler stations across the United States to see how flying animals reacted to the midday darkness—such as whether hordes of birds or insects took to the sky en masse at that moment. In this case, “the weather watchers lost sight of the cloud overnight, and the ladybugs’ current location isn’t clear,” NPR reported. They were a fleeting wonder that’s since flitted away.