Where, tests were conducted inside fiberglass bins during summer time, the temperature of grain was 32 ± 2°C.The plant extracts tested were the acetone and petroleum ether extracts of Poinciana seeds (Delonix regia); Cloves flowering buds (Syzgium aromaticum); Cinnamon (Cinnamonum zeylancicum); Radish seeds (Raphanus sativus); and Mustard seeds (Brassica alba). Test insects were Sitophilus orzae(L); Rhizopertha dominica (F.) and Tribolium Castaneum. The various plant extracts were tested alone at 0.4% and 0.8% (w/w) concentrations and under modified atmosphere (MA) of 20 ± 5% Co2. Results concerning Co-toxicity values resulted from addition of 20 ± 5% Co2 to the acetone and petroleum ether extracts of various plants revealed that the extracts at 0.4% and 0.8% produced additive effects for all tested insect species at all exposure periods. Comparing the toxicity of these materials to the insect species, the rice weevil Sitophilus oryzae (L.) was the most sensitive, followed by the lesser grain borer Rhizopertha dominica (F) and the red flour beetle Tribolium castaneum (Herbst) adults. Meanwhile, treatment of various insect species with the plant extracts under modified atmosphere of Co2 produced obviously higher mortality values than each treatment alone and proved to be effective against the three insect species under study.