Earthworms are not insects, but annelids (segmented worms). They are beneficial organisms because they break down organic matter, their tunneling aerates the soil and reduces compaction.
However, on very rare occasions, earthworms can become pests in home lawns. For example in an area just north of Houston, Texas they can become so numerous they disrupt turfgrass areas, leaving them spongy and littered with earthworm castings (see image at left).
There is no registered pesticide for their control on the rare occasions they do become pests. Most people are more concerned about preserving them rather than using pesticides that may be toxic to these creatures.