Five insect growth regulators (IGR's) were tested for increasing the susceptibility of the cotton leaf worm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae) to its homologous nucleopolyhedrovirus (SpliMNPV). S. littoralis MNPV was tested alone or in combination with IGR's at LC₁₀ level against the 2^nd instar larvae of the pest. An increased viral infection rate was detected in the mixture treatment in the case of using Chlorfluazuron, Flufenoxuron, Triflumuron, Hexaflumuron or Teflubenzuron at 10%.  The LC₅₀ value of the virus alone treatment  1x10⁷ PIB's was reduced to 4.3x10⁶, 9.9x10⁴, 4.9x10⁴, 3.1x10⁵ and 1.69x10⁶ PIB's, with the tested five IGR's, respectively. It was observed that Flufenoxuron and Triflumuron mixtures slightly prolongated larval duration compared either untreated control or IGR's alone treatments. The highest rate of decrease in the pupation percentage (47%) was recorded in case of Triflumuron mixed with SpliMNPV followed by Flufenoxuron and Chlrofluazuron mixes. On the other hand there is no significance difference in pupal weight for all treatments. Changes Adult longevity was increased for all treatments compared to untreated control. Addition Flufenxuron (LC₁₀) to SpliMNPV (LC₅₀) showed histopathological effects to mid gut e.g. loss of the compact appearance of the muscularis layer, vaculation and exfoliation of the columnar cells.