The dry biomass of four cyanobacterial microalgal strains Anabaena flos aquae, Anabanea laxa, Anabanea fertilissima and Nostoc muscorum was evaluated in invitro assay as a biopesticides against 2nd and 4th instars of Agrotis ipsilon. Larvae were fed on leaf discs painted with the algal solution for 24h. The 2ndinstar larvae were more susceptible than the 4th ones to the four tested algal treatments. N. muscorum treatment was the most toxic one against the 2nd and 4th larval instars. Where, its LC50values were 45 and 150 mg/ml, respectively while, A. fertilissima was the least one, its LC50 values were 250 and 660 mg/ ml, respectively. All the treated larvae were susceptible to four tested algal treatments and its resistance varied according to the larval instars and tested alga. Marked decrease in pupation for both 2nd and /or 4thinstar by the four algal treatments, whereas, the 2ndinstar treated with N. muscorum alga exhibited the most suppressive decrease to 44%, as compared to control (97%). Significant reduction for pupal weight, where, 4thinstar treated with A. flos aquae alga recorded the highest pupal weight decrease to average 290mg, as compared to 484mg of control. Besides prolonging pupal duration where, treatment for both instar with A. flos aquae and A. laxa algae induced the longest period and increased the pupal malformation percent where, 4thinstar treated with N. muscorum and A. flos aquae algae induced noticeable percent reached to21.3 and20% and increased adult malformation to30 and20% respectively, as respect to control. However, the larval treatment of 2nd instar with N. muscorum and A. fertilissima algae had the strongest effect in larval duration increase to average25 and 25.8day as compared to20.4day of control. In addition, the larval treatment of 4thinstar with the four algae decreased the adult fecundity and fertility. Moreover, N. muscorum and A. flos aquae algae inhibited the adult fecundity and eggs fertility to zero, as compared to505eggs/female of control. Marked declining of adult longevity with N. muscorum, A. flos aquae and A.laxa to average 7day, as compared to11day of control. Also, the treatment with the three algae disorder the sex ratio of males and females, as compared to that of control. While the larval treatment with A. fertilissima alga didn't affect the pupal malformation, eggs fertility, adult longevity and sex ratio.