The effect of late-season spraying by the conventional insecticide (chlorpyrifos) and the mineral oil (KZ oil), before and after harvest of seed cotton, on the population of pink bollworm larvae entering diapause was studied during 2004 and 2005 cotton seasons at the farm of Sakha Agricultural Research Station, Kafr El-Sheikh Governorate. The data revealed that the highest average reduction percentage in pink bollworm diapausing larvae during the two experimental seasons was 64.09% for the treatment of one insecticidal spray one week before harvest followed by another spray one week after harvest. The rest treatments could be arranged descendingly as follows: one spray of the insecticide before harvest followed by one spray of the oil after harvest (57.53%), one spray of the insecticide after harvest (47.32%). One spray of the insecticide before harvest (43.72%), and one spray of the oil after harvest (28.7%)   average reduction on diapausing pink bollworm larval population.