In Terai region of West Bengal the Diamond back moth (Plutella xylostella) appeared on cabbage during February and the infestation continued up to May in an overlapping manner, within and over the generations. During February-March the prevailing climatic conditions did favour not only the multiplication of Plutella population but also activated its parasitoids. Higher level of potential fecundity (76.5), net reproductive rate (14.145) and innate capacity for increase in number (0.101) of adult of February-March generation and faster development (life cycle of 32 days) irrespective of shorter doubling time (6.887 days) and higher weekly multiplication rate (2.027), more number of hypothetical F2 (200.08) females and higher survival value (0.46) during February-March resulted in higher level of infestation in March-April. The key mortality factor for different generations were recorded as physical character of leaves for 1st and 2nd instar larvae, diseases and some unknown causes for 3rd and 4th instar larvae, parasitoid for pupae/larvae-pupae and low hatchibility for eggs. All of these had a direct bearing upon variation in climatic conditions as well as with crop phenology in all the generations. After April, adverse climatic conditions and late stage of crop growth reduced the infestation level under field conditions.