In a process for pest control using pyrethrin and/or pyrethroids, pref. with added synergist, the pyrethrin and/or pyrethroids and any synergists are at least partly microencapsulated, either together or separately, and then applied. The process is esp. useful for the control of sucking and biting insects on crop plants. By appropriate choice of for mulation, the process can be controlled to give an initial high rate of kill followed by an extended period of repellency, in which the normal-balance between the insect pests and the useful predatory insects is restored.