Capsaicin - the compound that makes hot peppers hot - is used as an animal and insect repellent. It works by activating pain receptors (TRPV1) in mammals and irritating insect sensory organs. Used to repel deer, rabbits, squirrels, dogs, cats, birds (birds cannot taste capsaicin), and some insects.
Deer (proven effective), rabbits, squirrels, chipmunks, dogs, cats, raccoons, voles. Also repels some insects: aphids, spider mites, whiteflies, lace bugs. Birds cannot detect capsaicin and are unaffected - this makes capsaicin-treated bird seed effective for keeping squirrels off bird feeders while not deterring birds.
Critter Ridder (Havahart), Liquid Fence Deer and Rabbit Repellent, Bonide Hot Pepper Wax, Cole Hot Meats bird seed, Repels-All Animal Repellent, various hot pepper wax insect sprays. Pure hot pepper flakes/spray (DIY).
Safe for humans at normal use concentrations though it causes significant eye and skin irritation. Wash hands thoroughly after handling. Avoid touching face, especially eyes, after application.
Non-toxic if ingested by wildlife (they simply avoid treated areas). Will not harm birds - their TRPV1 receptors do not respond to capsaicin, which is why birds can eat the hottest peppers without discomfort.
For deer: Apply to ornamental plants, garden borders, and young trees. Most effective when alternated with other repellent types (egg-based, putrescent) to prevent habituation. Reapply after rain. Deer can habituate to any single repellent over time.
For squirrels on bird feeders: Capsaicin-treated bird seed (Cole Hot Meats) is one of the most effective and humane squirrel deterrents. Birds eat it happily; squirrels learn to avoid it after one taste. Apply hot pepper flakes to existing seed as a DIY alternative.
Insect spray (DIY): Blend 2-3 hot peppers with 1 quart water, strain, add 1 tsp dish soap. Spray on plants for aphid and mite repellent. Reapply every 3-5 days and after rain. Works as contact irritant and feeding deterrent.
Limitations: Short residual (days, not weeks). Must be reapplied frequently. Heavy rain washes it off completely. Determined animals may push through the discomfort if hungry enough. Best as one component of an integrated deterrent strategy.