Repellents generally rely on fear, conditioned avoidance, pain, or taste...or a combination of several.
Fear-inducing repellents contain compounds that emit sulfurous odors such as predator urine or faeces, animal proteins, or garlic. The avoidance of these odours can be considered a fear response, suggesting herbivores perceive sulfurous odours as indicators of predator activity.
Conditioned avoidance occurs when ingestion of a food is paired with nausea or gastrointestinal distress. Animals generally don’t eat as much of a food if it is associated with illness.
Active ingredients, such as capsaicin, allyl isothiocyanate, and ammonia, cause pain or irritation when they contact receptors in the mucous membranes of the mouth, eyes, nose, and gut. An inherent problem of using pain repellents is that they are universally aversive to all mammals, in other words they are non-selective.
Bad taste can also induce avoidance so bittering agents are often used to induce a bad taste. Unfortunately, while omnivores normally avoid bitter tastes, herbivores are generally indifferent, at least at the concentrations used in most repellents.
Of the above, fear-inducing repellents are preferred as they repel BEFORE the browser has had a chew and taste of the plant because by then it's often too late and damage to the plant is done. These repellents can be either delivered to the plant topically (via spray or swipe) or scattered around the area. Both FATBANG and BANGBANG are topically applied fear-inducing repellents and are designed to repel the pest before they chew or attack the plant.