Background and Aim: As a mechanical vector of some communicable diseases and a self-adaptive species to human environments, the German cockroach can transmit pathogens, such as bacteria, viruses, and fungi, to human beings. This study was conducted to determine the toxicity of imidacloprid and chlorpyrifos against German cockroaches.

Materials and Methods: In this experimental study, the last instar German cockroach nymphs were used to test their sensitivity to imidacloprid and chlorpyrifos insecticides by the contact and bait methods. In bioassay (jar test), the nymphs were isolated from the main colony and were exposed to the insecticides for 30 min. The mortality rate was recorded 24 h after the recovery time. In bioassay (bait), 24-96 h after exposure to the poisonous bait, the mortality rate was recorded and regression analysis was run to analyze the data in the SPSS software (IBM, Chicago, USA).

Results: The lethal doses (LD) of imidacloprid and chlorpyrifos were 9.5 mg/m² and 39.78 mg/m², respectively. The LD₅₀ for imidacloprid and chlorpyrifos were 2.66 and 9.92 mg/m², respectively. Results revealed that the highest concentration of imidacloprid (45%) could cause the highest mortality rate (95%) 24 h after exposure. There is a significant difference in the mortality of the samples during the follow-up period. Moreover, the chlorpyrifos smeared bait, with a concentration of 16% after 24 h, had the highest mortality rate (95%).

Conclusion: The highest mortality rate occurred in the ingestion of imidacloprid smeared bait within the first 48 h after being exposed to the insecticide. Therefore, the results show that imidacloprid can prove significantly effective in controlling cockroaches.

Keywords: Blattella germanica, chlorpyrifos, German cockroach, imidacloprid.