Our experience since several years in Egypt in the management of Spodoptera littoralis shows that what is often called resistance is, in fact, a multiple process in relation with the diet and environment conditions and riot with genetic factors. When vary the type of diet substratum (different plant species), the nitrogen manuring, the photoperiod and temperature, the rates to use must often be multiplied by till 5 or 6 times the rates considered as normal under standard conditions.