Biological control agents such as entomophagus insects (e.g., Trichogramma sp.) are important components of integrated pest management (IPM) programs. Because pesticides are also an important component of IPM programs, it is essential to understand how biological control agents are affected by pesticides and how these effects may affect the effectiveness of the biological control agents. Contrary to most insect species that use volatile pheromones for mate location, T. brassicae is one of the few species using a substrate‐borne sex pheromone for mating. In this work we determined the effects of an organophosphorus insecticide, chlorpyrifos, on sex pheromone reception by males and emission by virgin females. The insects used in the pheromone tests were survivors from an acute toxicity test in which individuals were exposed to a dose of chlorpyrifos equivalent to a 20% lethal dose. Males that survived the insecticide exposure spent much less time on the area marked with the female pheromone than control males (mean choice indexes, 0.46 [0.34 SD] vs 0.70 [0.25 SD], p < 0.001). The kinetics of the response of males to marking by control and treated females indicated that chlorpyrifos decreased the emission of sex pheromone by females surviving the insecticide. Therefore, chlorpyrifos inhibited pheromone reception by males and decreased pheromone emission by virgin females. These results are interpreted in the context of sex pheromone regulation by the nervous system of insects and according to the mode of action of chlorpyrifos. Extensions of these results on actual work on sex pheromone, the use of pheromone traps, and the population biology of Trichogramma sp. are discussed.