Fitness Costs of Chlorantraniliprole Resistance Related to the SeNPF Overexpression in the Spodoptera exigua (Lepidoptera: Noctuidae).

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.

Effects of bistrifluron resistance on the biological traits of Spodoptera litura (Fab.) (Noctuidae: Lepidoptera).

Spodoptera litura is one of the major insect pests of vegetables in Sichuan, China, and it has developed serious resistance to many traditional chemical insecticides. In this present study, S. litura individuals collected from five regions in Sichuan in 2017, which were detected the resistance to bistrifluron. We found that the resistance ratios (RRs) bistrifluron were in the ranges of 15.9- to 77.7-fold, respectively, compared with a laboratory susceptible (Lab-HN) strain. To elucidate the effect of bistrifluron on the life history parameters of S. litura, we established the bistrifluron-susceptible (Bis-UNSEL) and bistrifluron-resistant (Bis-SEL) strains based on the DY field population with the highest RR of 77.7-fold among the all field populations to bistrifluron compared with a laboratory Lab-HN strain. The results show that the Bis-SEL strain developed a 113.8-fold RR while that of the Bis-UNSEL strain was almost equal to that of the Lab-HN strain (with overlapping confidence intervals (CI) of their LC50 values, P < 0.05). Meanwhile, the life span of the egg and pupa stage of the Bis-UNSEL strain was prolonged, and the strain also showed shorter adult duration and lower fecundity. Demographic characteristics, such as the intrinsic rate of increase (r), finite rate of increase (λ) and net reproductive rate (R0) in the Bis-SEL strain were significantly higher than the Bis-UNSEL strain, whereas the mean generation time (T) and gross reproduction rate (GRR) were lower. The results clearly showed that the developing resistance of bistriflueon on S. litura promoted its population growth. Therefore, the present study could provide useful informations for determining potential management methods to prevent the development of bistrifluron resistance in S. litura.

Fitness costs associated with chlorantraniliprole resistance in Spodoptera exigua (Lepidoptera: Noctuidae).

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.

Molecular identification of four novel cytochrome P450 genes related to the development of resistance of Spodoptera exigua (Lepidoptera: Noctuidae) to chlorantraniliprole.

BACKGROUND: The beet armyworm, Spodoptera exigua, is an omnivorous insect that damages a variety of crops worldwide. Chlorantraniliprole is a new diamide insecticide that acts on the ryanodine receptors in insects. The aim of this study was to explore key genes related to the development of chlorantraniliprole resistance in S. exigua. RESULTS: Transcriptomes were compared between beet armyworms from a susceptible laboratory strain (Sus-Lab) and Sus-Lab screened with LC25 sublethal doses of chlorantraniliprole for six generations (SE-Sel). Ten of 11 cytochrome P450 genes with upregulated expression verified by quantitative reverse transcription polymerase chain reaction (qRT-PCR) in SE-Sel strains were detected in SE-PZ16 and SE-ZY16, two extremely resistant field populations. In addition, expression of four new cytochrome P450 genes, CYP9A21v1, CYP9A21v2, CYP9A21v3 and CYP9A21v4, was shown in the two field populations and was significantly higher in the SE-Sel strain than in the Sus-Lab strain (P < 0.05). Their full-length and protein tertiary structures were also cloned and predicted. The function of CYP9A21v3 was analysed by RNA interference, and the relative expression of CYP9A21v3 in the SE-ZY16 population after feeding on dsRNA was lower than in the control group. Moreover, mortality rates in insects treated at the LC50 of chlorantraniliprole after dsRNA feeding were significantly higher than in the control group 24 h after treatment (P < 0.05). CONCLUSION: Overexpression of CYP9A21v3 may be a primary factor in the development of chlorantraniliprole resistance in beet armyworms. © 2018 Society of Chemical Industry.