Earthworms can leave small dirt mounds in the yard. It is common for earthworms to tunnel through soil to break down organic matter, ingesting the soil and leaving behind tiny mounds.
While some homeowners see the worm mounds as a nuisance, they are actually very beneficial, as they produce potassium and other micronutrients. This worm-produced fertilizer helps keep the soil healthy and can even out pH balances. This is especially important where gardens and other types of vegetation are in place. As the worms tunnel deeper, it allows other types of organisms to penetrate the ground, which also helps improves soil quality. It is more common for homeowners to notice the mounds when the nights begin turning warmer as rising temperatures increase earthworm activity.