Insect spiracle as the main penetration route of pyrethroids

The penetration route of adhered pyrethroids following direct aerosol spraying was studied in the German cockroach (Blattella germanica) by investigating the relationship between the application site of insecticide and knockdown efficacy. In direct spray, KT50 was 26.4 s and the adhered amount of pyrethroid was 0.745 μg. On the contrary, required amount of pyrethroid to obtain the same KT50 was one-eighth in topical application to the mesothoracic spiracle, while 2.6 times to the ventral mesothorax. KT50 of cockroaches with blocked mesothoracic spiracles was greater than those with unblocked spiracles by 1.8-fold. The amount of directly sprayed pyrethroid penetrating through the inner wall of the mesothoracic trachea was significantly higher than the amount penetrating through the body wall of the ventral mesothorax. Therefore, the knockdown effect of the direct spray was believed to be caused by the flow of pyrethroids into the mesothoracic spiracles and its subsequent penetration through the inner wall of the mesothoracic trachea.