MOUNT DIABLO STATE PARK, Calif.—Pity the male tarantula in October. He’s weak from starvation and desperate for love. The object of his desire lives underground, forcing him to wander around until he stumbles over a female, who may kill him after mating. First, he may stumble into horrified humans.

Warm weather in the San Francisco Bay Area has extended tarantula mating season—a surprisingly public affair that has led to advisories from officials to be on the lookout for thousands of giant male spiders. The spiders aren’t...