Increase in insecticide susceptibility after sublethal exposure to deltamethrin in the German cockroach (Blattodea: Ectobiidae).

The German cockroach, Blattella germanica (L.), is a major urban insect pest primarily controlled by insecticides, including pyrethroid sprays, fipronil baits, and indoxacarb baits. However, widespread pyrethroid resistance increases the probability that cockroaches will survive treatments in the field. Many insecticides are applied concurrently or repetitively on a periodic basis, meaning survivors have the chance to be re-treated, but any lasting physiological effects from initial survival may impact the performance of subsequent treatments. We investigated the effects of sublethal deltamethrin exposure on the susceptibility of susceptible (UCR) and resistant (Ryan) strains of German cockroaches. Ineffective pyrethroid treatment was simulated by treating cockroaches with the LD20 of deltamethrin. Survivors were treated again with the same dose of deltamethrin, a food source containing the LC50 of fipronil, or a food source containing the LC50 of indoxacarb. Both strains of cockroaches experienced greater mortality when treated with deltamethrin 3 d (49.8% to 67.6%), after the sublethal exposure. No impact on fipronil or indoxacarb susceptibility was observed after pre-treatment with deltamethrin. Because surviving a deltamethrin treatment did not induce tolerance to deltamethrin or other insecticides, this suggests no within-generation drawback of current conventional management practices of applying multiple treatments in the same site. Furthermore, sequential exposure to deltamethrin caused greater mortality than expected, indicating that this insecticide may remain useful even if initial efficacy is low. Comprehensive examinations of additional resistant strains treated with formulated products are warranted to understand whether these effects may impact field control.