I'll put in my two cents for another non-pesticide option.
I had lice for 6 months when I was a teenager and *nothing* worked to get rid of them. Not nightly combing, not pesticide treatments. Finally we slathered mayonnaise into my hair and then wrapped my head with saran wrap (ok, wouldn't do that part now...but an old towel would work) and slept on it. Apparently the mayo smothered the lice and my hair was super conditioned