An apparatus for use in monitoring the numbers of a selected species of insect comprises one or more traps into which is put a synthesized pheromone to attract that particular insect. Each trap has a grille comprising a plurality of bars between which is applied a high voltage so that when an insect of the selected species tries to enter the trap it touches adjacent bars and is electrocuted by the electric discharge between them. This discharge is detected by a discharge detector connected to the trap and the detector passes a signal to a transmitter which transmits it to a central receiver equipped with a discriminator for ignoring signals other than those from the transmitter and for passing the signals representing discharges to a pulse counter whereby a total of the number of insects trapped in a given time can be obtained at a central station without visiting each trap.