The flies invaded a beach in Torrance, and also turned up in large numbers at Redondo Beach, Manhattan Beach and Hermosa Beach, the Los Angeles Times reported Saturday.
"We've seen blooms in the past," said Garth Canning, a section chief with the Los Angeles County Fire Department's Lifeguard Division. "(The flies) are very obviously greater, and they're extending over a larger area than they usually do."
The international marine conservation organization Oceana said the flies are drawn to dead kelp that washes up on beaches.
Brian Brown, curator of entomology at the Natural History Museum of Los Angeles County, said the flies lay eggs on the kelp and the hatchlings feed on the decaying matter, breaking it down.
Brown said he hasn't seen the latest outbreak, but several factors might have contributed to the increase.
"Sometimes there's large amounts of kelp on the beach, or it could be because it's spring and the temperatures are starting to warm up," Brown said. "Especially the warmer temperatures -- that's what really drives the flies."
The flies have a life span of about 11 days.
Officials said they don't plan to spray insecticides on beaches.