Pyrethroids are synthetic insecticides that have a repellent action. This effect has been associated with an increase in the locomotor activity, which causes the avoidance of the insecticide-treated area (excito-repellency). In this work, we studied with behavior and electrophysiological recordings the occurrence of olfactory-mediated repellency caused by pyrethroids of different volatility in the German cockroach Blattella germanica (Linnaeus, 1767). Male cockroaches were spatially repelled when they were exposed to D-allethrin vapors and vapothrin vapors in a dose-dependent manner. No repellency was observed when insects were exposed to permethrin, a non-volatile pyrethroid. To confirm the role of olfaction in this phenomenon, we measured the electrical activity of the cockroaches' antennae in response to these insecticides. There was a significant increase in the electrical activity in response to D-allethrin and vapothrin, but no increase was observed in insects exposed to permethrin. Locomotor activity of cockroaches exposed to pyrethroids was measured in order to discard excito-repellency. No changes in locomotor activity were observed for any of the insecticides. Finally, we found that volatile pyrethroids in the vapor phase cause spatial repellency in cockroaches, being the first report of an olfactory-mediated repellency phenomenon caused by pyrethroids in cockroaches.

The use of chemical insecticides is the main control method for Blattella germanica worldwide. The prolonged and frequent use of insecticides produced the selection of insecticide-resistant individuals. The German cockroach is one of the most widespread urban pests in Argentina. In the last decades, resistance monitoring studies in this country demonstrated that there is a high prevalence of pyrethroid-resistant populations of B. germanica in the field. In this work, we studied the resistance mechanisms of a field-collected strain of B. germanica at toxicological, enzymatic, and molecular levels. A resistance ratio of 100 was obtained for the resistant strain when it was exposed to β-cypermethrin. The pretreatment with specific synergists (piperonyl butoxide and triphenyl phosphate) led to a significant increase in the toxicity of the pyrethroid, suggesting an involvement of oxidases and esterases in the detoxification of this insecticide. Moreover, esterase and oxidase activities in the resistant strain were 1.5-fold and 2-fold higher respectively, compared to the susceptible individuals. On the other hand, the voltage-gated sodium channel gene of the resistant cockroaches did not show nucleotidic substitutions in the domain II which are associated to knockdown resistance in this species. These results suggest that the main mechanism of resistance of the studied cockroaches' strain is metabolic, mainly due to an increase in the activity of oxidase and esterase enzymes. The results of this work in addition to other reports found in literature show that the extended use of a single active principle for cockroach control promotes the development of resistance leading to control failure in the field. In contrast, integrated pest management strategies include the use of different control tools in addition to chemical insecticides, which delay the appearance of resistance increasing the efficacy of pest control.

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