Competition is a driving force regulating communities often considered an intermittent phenomenon, difficult to verify and potentially driven by environmental disturbances. Insecticides are agents of environmental disturbance that can potentially change ecological relationships and competitive outcomes, but this subject has seldom been examined. As the co-existing cereal grain beetle species Sitophilus zeamais Motschulsky and Rhyzopertha dominica F. share a common realized niche, directly competing for the same resources, they were used as models in our study. Intraspecific competition experiments were performed with increasing insect densities and insecticide doses in additive and replacement series using various density combinations of both beetle species maintained on insecticide-free or -sprayed grains. Insecticide-mediated release from competitive stress was not observed in our study of intraspecific competition in grain beetles. The insecticide enhanced the effect of insect density, particularly for the maize weevil S. zeamais, further impairing population growth at high densities. Therefore, insecticide susceptibility increased with intraspecific competition favoring insecticide efficacy. However, the effect of insecticide exposure on competitive interaction extends beyond intraspecific competition, affecting interspecific competition as well. Sitophilus zeamais was the dominant species when in interspecific competition prevailing in natural conditions (without insecticide exposure), but the dominance and species prevalence shifted from S. zeamais to R. dominica under insecticide exposure. Therefore, high conspecific densities favored insecticide efficacy, but the strength of the relationship differs with the species. In addition, the insecticide mediated a shift in species dominance and competition outcome indicating that insecticides are relevant mediators of species interaction, potentially influencing community composition and raising management concerns as potential cause of secondary pest outbreaks.