The latent effects of three plant oils; namely thyme, bitter and neem on certain biological and biochemical parameters of the 6^th instar larvae of  Spodoptera  littoralis treated as 2^nd and 4^th instars with the LC₅₀ values of these oils. The results showed that the 2^nd instars were more susceptible to all the tested oils than the 4^th instar larvae. All treatments recorded significantly difference in all durations for both 2^nd and 4^th instars. Also, highly significant prolongation in pupal duration and the most prolonged oil was occurred by thyme on the 2^nd instars, while bitter was more effective on the 4^th instar larvae. Longevity of both sexes was reduced specially for male moths. No significant reduction was noticed on female longevity as a result of pretreated 2^nd but significant reduction was recorded for pretreated 4^th instars especially with thyme. Highly significant reduction in pupal weight produced from the treated 2^nd and 4^th larval instars withthe three products. All oils caused deformations with various degrees for larvae, pupae and adults resulted from the treated 2^nd and 4^th instar larvae. The highest deformations exist in larval-pupa intermediate by thyme as pretreated 2^nd and 4^th of the larval instars. Bitter gave the highest pupal deformation as the pretreated 4^th instars followed by neem for the 2^nd and 4^th instars. Generally, all oils exhibited higher effect on the males. Total malformation during all generation recorded its maximum value with neem treatment as 2^nd instars then thyme was the most effective in both 4^th and 2^nd of the larval instars followed by bitter. Highly significant stimulation in chitinase and α- & β -esterases activity was recorded with all treatments and the most effective one caused by thyme followed by bitter. Highly significant inhibition in protease activity and acetylcholine esterase (AChE) was attained by (bitter & thyme) and (neem & thyme) oils, respectively. While high significant stimulation was recorded in protease and non significant stimulation of AChE was noticed by neem and bitter, respectively.