Switch on the small, gray metal box and listen: A sharp, pulsating, high-pitched tone burrows into the ear like a power drill bit, prompting an agitated, please-shut-that-blasted-thing-off grimace.

Making your own insect repellent sounds so simple: Just whip up a mixture of apple cider vinegar or witch hazel plus a few drops of some herby essential oils—no deet required. But there aren’t a lot ...