The large blob on the radar measured some 80 miles wide as the beetles migrated through the area, apparently headed in the direction of the Inland Empire.
"The large echo showing up on SoCal radar this evening is not precipitation, but actually a cloud of lady bugs termed a "bloom"," the weather service tweeted.
But experts said the bugs may not appear quite that concentrated to observers on the ground, as they are spread out through the sky and at different elevations.
According to the University of California Pest Management Program, this is a common time of year for ladybugs to migrate to higher elevations in search of food. Most species of the beetle are considered beneficial to agriculture as they feed on pests.