Status, Inheritance Pattern and Mechanism of Field-Evolved Resistance to Gel Bait Insecticides in The German Cockroach
German cockroach (Blattella germanica L.) is an important urban pest that poses health risks. They carry pathogenic microorganisms, and the allergens present in their feces and cast skins can trigger asthma attacks. Gel bait formulations containing insecticides (indoxacarb and fipronil) have been used for German cockroach control for more than a decade. However, historical data suggests that cockroaches can develop resistance to insecticides that are repeatedly used. Therefore, we investigated the status and inheritance patterns of fipronil and indoxacarb resistance in cockroach strains collected from the state of Indiana and Illinois. In the first objective, topical dose-response bioassays were performed to determine fipronil and indoxacarb resistance levels in adult males of three field strains (D-IL, I-IN, and S-IN) and the laboratory-susceptible Orlando strain. Comparison of LD50 (median lethal dose) values between the susceptible and field strains revealed that resistance to both insecticides in the D-IL and I-IN strains was <10-fold. However, fipronil and indoxacarb resistance levels in the S-IN strain were 20- and >10,000-fold, respectively.
In the second objective, choice feeding bioassays were performed to test the performance of the resistant S-IN strain (adult males) on commercial fipronil and indoxacarb baits. Complete (100%) mortality of the S-IN strain was observed on fipronil baits. However, average mortality on indoxacarb baits was ~20% at 14d. In the third objective, synergist bioassays were done with PBO and DEF to investigate the mechanism of indoxacarb resistance. PBO did not significantly increase mortality in the S-IN strain at LD50, but DEF did, suggesting increased hydrolase activity as a potential mechanism of indoxacarb resistance. In the last objective, reciprocal crosses were performed between the resistant S-IN strain and the susceptible Orlando strain to determine patterns of insecticide resistance. Topical bioassays and associated LD50 values for the F1 generation adult males indicated that fipronil resistance was inherited as an incompletely dominant trait with sex-linkage. In contrast, indoxacarb resistance was inherited as a codominant trait and was not sex-linked. Our results indicate that resistance can evolve independently in different field strains. High-level indoxacarb resistance observed in the S-IN strain warrants additional research on the indoxacarb target-site as a possible resistance mechanism.