This manuscript reports the preparation and toxicological efficacy testing of nine oxoimidazolidine and cyanoguanidine compounds (3a, 3b, 3c, 3d, 3e, 3f, 3g 4a, and 4b) against cowpea aphid, Aphis craccivora Koch. Bioefficacy data revealed that the tested compounds exhibited a range of toxicological activities against these insects, with compound 3b being the most toxic and compound 4a being the least effective. The LC50 value of compound 3b was 1.72 ppm for adults and 0.02 ppm against nymphs of the cowpea aphid, while the LC₅₀ value of compound 4a was 72.51 ppm for adults and 18.02 ppm against nymphs. The manuscript also presents the structure-activity relationships of these compounds. These results provide valuable insights into the development of effective pest control agents for the management of such insects.