Small black bugs with orange and white spots are showing up all over San Diego County in increasing numbers, but county officials say the shield-shaped insects have a taste for plants, not people.

College student Sandra Drago said she noticed the bugs at Carlsbad and Oceanside beaches last week. She said they appeared every time she put her towel down and she became worried they might bite her.

"It just seems like they're everywhere," Drago said. "I lay my towel out and after 10 minutes, I have a good two or three of them on there."

San Diego County entomologist David Kellum examined a cell-phone shot of one of the bugs on Tuesday and identified it as Bagrada cruciferarum, otherwise known as the Bagrada bug or "painted bug."

Kellum said sightings of the small plant-loving insect have spiked in recent weeks because of a dry climate reducing available food such as wild mustard in Southern California's canyons.

"It's a seed-type bug. When it gets dry, it moves out of the canyons and into backyards and gardens, and then we start getting questions," Kellum said.

He said the bug prefers to eat seeds and succulent plants and is not known to bite humans or carry any sort of disease. The species, Kellum added, is considered exotic because it originated in Africa.

The bug was first found in Los Angeles County in June 2008 and has spread widely in other Southern California regions since then, according to the Center for Invasive Species Research at UC Riverside.

Bagrada bug is a major pest of crop plants such as cabbage, kale, turnip, cauliflower, mustard, broccoli and radish, the center says on its Web site.

Normal pesticides can be used to ward off a Bagrada bug infestation, Kellum said.

He said the population in San Diego County, especially in more rural portions of North and East County, seems to be larger this year.

"It seems to have settled in and it's established in the area now and we're seeing more of it," Kellum said.

Drago said she is even more surprised that she saw the bugs near the ocean, especially south of Tamarack Beach in Carlsbad where there is very little vegetation.

"You don't see them in the sand, but then they're crawling on your towel. It's very strange," she said.

Kellum said the plants that grow along the edges of ocean bluffs, many of them succulents, can beckon Bagrada bugs. He said they're also more easily spotted on the beach because there is little other vegetation around and sunbathers hang around in one spot for hours.

He said natural predators, from birds to spiders, will probably get the population under control.

"Every now and then we get an exotic species like that, that comes through and just explodes for a short time," he said.

Call staff writer Paul Sisson at 760-901-4087.