A mysterious “blob” that appeared on the US National Weather Service’s radar wasn’t a rain cloud or a storm, but a massive swarm of ladybugs.

Meteorologist Joe Dandrea said the bug mass appeared to be about 130 kilometers wide as it flew over San Diego County in Southern California yesterday.

Mr. Dandrea told The Los Angeles Times the ladybugs were actually spread throughout the sky, flying at an altitude of between 1500m and 2700m, with the most concentrated group about 16km wide.

After seeing it on the radar, Mr. Dandrea said he called a spotter near Wrightwood in the San Bernardino Mountains, northeast of Los Angeles, to ask what they were really seeing.

“I don’t think they’re dense like a cloud,” he said. “The observer there said you could see little specks flying by.”

It wasn’t immediately known what type of ladybug was causing the phenomenon.

However, The Times reported that one type, the convergent lady beetle, migrates to higher elevations in the early summer to find food, according to the University of California Integrated Pest Management Program.

“The large echo showing up on SoCal radar this evening is not precipitation, but actually a cloud of ladybugs termed a ‘bloom,’” the National Weather Service office in San Diego wrote, in a tweet that has had more than 1000 likes and retweets.

This story originally appeared in news.com.au.