That thin line of dirt on your wall could mean termites are already inside

Mud tubes are termite highways from soil to wood

A slim line of dirt on walls or fences may actually be mud tubes made by subterranean termites. These tunnels help termites move from soil to wood while staying protected from predators and from drying out. Since termites need both moisture from soil and food from wood, their presence can signal likely damage to wooden structures—so it should be checked quickly.