The banana slug is common along the Pacific Coast in North America and can be seen in many parks in Northern California like Muir Woods or Big Basin Redwoods State Park. These bugs are also common in Alaska, British Columbia, Washington, and Oregon.
Banana slugs thrive in Northern California because they especially thrive and seek shelter under coastal redwoods and detritus. These bugs show like to crawl out to creek banks and tree roots during the dry summer months. For that reason, hikers commonly spot these bugs while hiking. They are easy to notice and often hang out right in the middle of the trail.
Banana slugs are very dependent on getting enough moisture. The coastal environments tend to have moisture all year, even during dry season when there is no rain. During dry season, moisture comes in from the fog and enables the unique wildlife and plant life to get enough nutrients to thrive. Banana Slugs tend to be seen more often during non-dry season when they avoid being in the sun, making it more common to spot them in non-dry season months. Banana Slugs tend to eat plants that compete with Redwood trees for resources.
Children and many hikers like to spot these bugs, pick them up and play with them. To preserve their natural environment and habitat, it is probably best to leave them alone and avoid human contact. Lets preserve their natural habitat as much as possible.