Fleas are bothersome pests for pets and humans, and homes can become infested. Scientific studies that the scent of thyme, mint, yarrow, and myrtle are effective flea repellents. Thyme: The herbal oils extracted from common thyme and lemon thyme have scents that fleas avoid. Mint: Any cultivar from the Mentha plant family—peppermint, spearmint, pineapple mint, and more—contains oils that produce smells that fleas hate. Yarrow: Yarrow plants, while lovely with feathery foliage and colorful blooms, do not have a particularly pleasant scent to humans or fleas. Myrtle: Often called sweet myrtle, the oil has a sweet, fresh, green, and slightly camphorous scent fleas tend to avoid. Eucalyptus: Similar to myrtle oil, eucalyptus oil has a fresh, minty, woodsy, slightly sweet note that fleas hate. Basil: The herbal scent of basil oil is not pleasant to fleas. Lemongrass: Lemongrass oil contains citral and geraniol notes that repel both fleas and other insects. Rosemary: The pungent, piney smell of rosemary is a natural insect repellent. Pine: Pine oil is a good choice for cleaning products because fleas hate the smell.