When this is not possible, the treatments can be timed according to the actual pest population levels, as is now done with many field-crop pests. With those crops that do not yet have fixed chemical ccontrol schedules, every effort should be made to plan programs depending upon pest population levels and to avoid dependence upon insurance and prophylactic treeatments

Vernon M Stern, Ray F Smith, Robert van den Bosch, Kenneth S Hagen. 1959. The integrated control concept. Hilgardia 29 (2): 81-101.