Overexpression of CYP9A21v3 may be a primary factor in the development of chlorantraniliprole resistance in beet armyworms. © 2018 Society of Chemical Industry.

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BACKGROUND The field population of Spodoptera exigua, an intermittently occurring polyphagous pest, has developed resistance to chlorantraniliprole, while whether or not such resistance carries fitness costs remains poorly understood. Here we selected six generations of the Leshan population (LS‐P) by two‐way selecting method, and obtained a highly resistant strain (CH‐RE) and resistant degeneration strain (CH‐SE) sharing a similar genetic background. After that fitness costs were evaluated by comparing the life history characteristics of CH‐RE, CH‐SE and the laboratory susceptible strain (SE‐Lab) via the age‐stage two‐sex life table method. RESULTS The resistance ratio of CH‐RE and CH‐SE were 226.69‐fold and 3.72‐fold, respectively, and the estimated realized heritability (h2) of CH‐RE was 0.058. Compared with CH‐SE, the duration of pre‐adult, the longevity of adult, adult preoviposition period (APOP) and average generation time (T) of CH‐RE had significantly increased, but the oviposition days, average fecundity, intrinsic growth rate (r), weekly growth rate (λ) and reproductive rate (R0) decreased significantly. Moreover, the relative fitness of CH‐RE was 0.25, and showed fitness costs. Concurrently, the fecundity of CH‐SE was slightly lower than SE‐Lab, but there was no significant difference in r, λ and R0, and the fitness (1) of CH‐SE was similar to SE‐Lab (1.02), which was no fitness cost. CONCLUSION These findings represent that chlorantraniliprole resistance in S. exigua has a fitness cost, and the fitness cost will disappear with the recovery of sensitivity when the insecticide is stopped for field populations, supporting that such resistance would be managed by switching off the selection pressure with rotation with alternate insecticides. © 2020 Society of Chemical Industry

Spodopteraexigua, a multifeeding insect pest, has developed a high level of resistance to chlorantraniliprole, which is a benzoylurea insecticide that targets the ryanodine receptors (RyRs). Herein, the resistant strain (SE-Sel) and sensitive strain (SE-Sus) were obtained by bidirectional screening for six generations. The potential oviposited eggs and oviposition rate of the SE-Sel strain were dramatically lower than those of the SE-Sus strain; on the contrary, the weights of prepupae and preadult were significantly increased. As a post-mating response, the higher number of non-oviposited eggs in the SE-Sel strain was caused by a lower mating rate. In addition, the expression levels of vitellogenin (SeVg) and its receptor (SeVgR) in the SE-Sel strain were consistently lower than those in the SE-Sus strain. An RyRI4743M mutation, contributing to the resistance to chlorantraniliprole, was located in the S3 transmembrane segments and might have affected the release of calcium ions; it led to the upregulated expression of the neuropeptide SeNPF and its receptor SeNPFR, and the mating and oviposition rate were significantly recovered when the SeNPF was knocked down though RNA interference (RNAi) in the male adult of the SE-Sel strain. Moreover, the expression of the juvenile hormone-binding proteins SeJHBWDS3 and SeJHBAN in the male adult of the SE-Sel strain was significantly decreased, which proved the existence of a fitness cost from another angle. Therefore, these results indicate that the fitness cost accompanied by chlorantraniliprole resistance in S. exigua may be related to the decrease in mating desire due to SeNPF overexpression.