This work aims to study the toxicity, biological effects and joint action of the insecticides with selected weeds against the 4^th larvae instar of cotton leafworm. Results obtained indicated the obvious differences in their biological parameters when the larvae fed on untreaded fresh leaves of the tested weeds resulted in 100% larval mortality in 9 out of the 16 weed plants.  Larval duration was significantly prolonged in 4 out of 16 recording. The pupation percentage was significantly reduced in 7 treatments that survived after larval feeding and pupated. Pupal duration was significantly shortened in most weed treatments. Adult emergence percentage increased significantly in 1 weed plant while decreased significantly in 2 weeds. Feeding larvae on different weeds drastically inhibited egg production. Hatchability percentage of eggs resulted in different weed treatments was significantly reduced in 2 weeds.  Feeding on weed plants resulted in significant increase in sterility in 3 weeds. The synergism factor resulted from treated weed plants with insecticides indicated that some of weed plants have synergisted effect and the rest of them have antagonism effect after 48h. Data revealed slight to moderate potentiation in toxicity when fresh leaves of 11 weeds were treated with chlorpyrifos. Antagonistic activity was exhibited when fresh leaves of 5 weeds were treated with esfenvalerate. Slight to moderate synergistic activity was achieved whenthe larvae were fed on thiodicarb-treated leaves of 9 weeds. Antagonistic was exhibited when leaves of only 3 weeds were treatd with thiodicarb.