The seedcorn maggot (Delia platura) is a pest that eats seeds and seedlings, and is prevalent in the spring after a wet period. It is in the same genus as the cabbage maggot, and looks the same. It is also known as the larva of the bean seed fly. It is abundant in California, primarily in spring.
Google images of seedcorn maggots
Please see “Delia Species” for information on this pest.