These experiments were conducted inside gastight steel bins, each of about 0.5 m³ volume and filled with approximately 450 kg wheat grains. The bins were situated on the ground of the agricultural experimental farm of the Faculty of Agric., Moshtohor, Tukh, Qalyubia. Grain temperature was 26±2 ºC. Modified atmospheres (MAs) tested were 99% N₂ and 30±5% CO₂. Tested dusts used were katel-sous dust and powdered black pepper seeds. Tested insects were the adults of Sitophilus oryzae L., Rhizopertha  dominica F., Tribolium castaneum Herbst., and active and diapausing larvae of Trogoderma granarium Everts. Obtained results indicated that katel-sous was more active than black pepper to all tested insects. Diapausing larvae of T. granarium were the least sensitive species to all the test dusts and MAs, followed by T. castaneum adults. The sensitivity of the tested insects to dusts or MAs varied according to insect species. The efficacy of the tested MA of CO₂ against the tested insects was greater than the MA of N₂. Percentage mortality of all tested insects was dependent on concentration and exposure period. The results also showed that the efficacy and combined action of tested dusts under MA of N₂ or CO₂ were pronounced additive, potentiation and antagonistic effects according to dusts concentration, exposure period and insect species.