A chemical that affects the behaviour of an organism. Such chemicals include pheromones, which are used for communication between members of the same species, and allelochemicals, which act as chemical signals between members of different species. Terrestrial organisms may respond to chemical signals both in the air and on the ground, using their senses of smell or taste; whereas aquatic organisms are influenced very largely by chemicals dissolved or dispersed in their watery medium. Synthetic semiochemicals, especially artificial sex pheromones, are now widely used to control populations of agricultural and horticultural pests, notably insect pests. For example, they can be used to lure male insects to field traps, or to raise the general level of sex attractants in the area and so confuse males and thereby disrupt mating.