LOS ANGELES -- A huge blob that appeared on the National Weather Service's radar wasn't a rain cloud, but a massive swarm of ladybugs over Southern California.

Meteorologist Joe Dandrea says the array of bugs appeared to be about 129 kilometres wide as it flew over San Diego Tuesday.

But Dandrea tells the Los Angeles Times that the ladybugs are actually spread throughout the sky, flying at between 1,525 and 2,745 metres, with the most concentrated group about 16 kilometres wide.

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

The Times says one species, adult convergent lady beetles, mate and migrate from the Sierra Nevada to valley areas where they eat aphids and lay eggs.