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The chemical senses (olfaction, gustation, and chemical irritation or pain) were likely the first to evolve. Their functions are among the most basic-to attract and to repel. Attracting compounds often signal food or sex; repelling compounds presumably signal danger. Among the chemical senses, only olfaction appears to have several functional roles, two of which are modulation of social behaviors and identification of food. Whether an odor attracts or repels often depends to a large degree on learning. Consequently, dissociated olfactory stimuli may be relatively poor candidates for repellents since, after repeated exposure, pest animals are likely to ignore them. Taste, in contrast, is primarily related to the identification of food and the avoidance of poisons. Learning, here, appears to be less important although still a significant factor. Therefore, dissociated bitter tastes are good candidates for repellents. The development of effective, specific, bitter repellents, however, is complicated by the fact that herbivores tend to be relatively insensitive to many bitter compounds. Chemical irritants warn of danger; painful stimuli, almost by definition, are to be avoided. Although irritants may be the most potent of repellents, the extent to which a compound is irritating varies for many different species, pest and non-pest alike, which makes them difficult to use in certain situations. Finally, it is likely that the most effective strategy in the formulation of effective repellents will employ stimuli that activate a combination of the chemical (and other) senses. Research should focus in this area.