DEET is a repellent which is meant to keep them away, but if they land then they land. Permethrin is a pesticide so it kills. When I'm in the woods I wear outdoor clothing that's been sprayed with permethrin, but I also put a "heavy-duty" DEET repellent on my clothes and use OLE (oil of lemon and eucalyptus) on exposed skin, even though the last one might just be placebo.