Fluxametamide, a novel isoxazoline insecticide, is newly registered for the control of various lepidopteran, coleopteran and thysanopteran insect pests on lethal and sublethal levels. In the present study, the toxicity and sublethal effects of fluxametamide on diamondback moth Plutella xylostella (L.), an invasive lepidopteran foliage feeder of cruciferous vegetables, were assessed to explore its bio-ecological impact on pest populations. The toxicity of fluxametamide to the third instar larvae of P. xylostella was 0.18 mg L⁻¹ (LC₅₀) at 72 h bioassay. After treatment with LC₁₀ and LC₃₀ concentrations of fluxametamide, the fourth instar larval duration, the rate of deformed pupa and adults, and the adult pre-oviposition period were significantly increased, whereas the pupation rate and pupal weight were significantly decreased in the F₀ generation. In the F₁ generation, sublethal effects of fluxametamide were indicated by a reduced fecundity, rate of pupation and adult emergence, pupal weight, and adult longevity, however a significant increase in eggs and total larval duration, deformed adults rate, and total longevity and pre-oviposition period was observed in the offspring. The intrinsic rate of increase (r), finite rate of increase (λ) and net reproductive rate (R₀) of sublethal treatments were significantly lower than the control. The relative fitness of F₁ was 0.68 and 0.64 in LC₁₀ and LC₃₀ treatments, respectively. The LC₃₀ fluxametamide treatment exhibited increased glutathione S-transferase activities (elevated 1.433-fold) in P. xylostella. Our results suggest that in addition to its high lethal toxicity, the sublethal concentrations of fluxametamide might suppress the reproduction, development and survival of the P. xylostella population and its progeny, which can help to optimize integrated pest management program.

The fall armyworm (FAW), Spodoptera frugiperda, is one of the most devastating invasive polyphagous pests, which has attracted recent global attention by developing resistance to various insecticidal active ingredients with independent mode of action. Fluxametamide, a newly commercialized isoxazoline insecticide, is exceptionally selective towards several lepidopteran pests. The present study aimed to evaluate resistance risk in FAW to fluxametamide and the fitness costs associated with fluxametamide resistance. A field-collected and genetically mixed population of FAW was artificially selected through continuous exposure to fluxametamide. After successive selection of 10 generations, there was no obvious increase in the LC₅₀ (RF: 2.63-fold). The realized heritability (h²) of fluxametamide resistance was estimated as h² = 0.084 using a quantitative genetic approach. Compared with the susceptible F0 strain, the Flux-SEL (F10) strain of FAW displayed no significant cross-resistance to broflanilide, chlorantraniliprole, fipronil, indoxacarb, lambda cyhalothrin, spinetoram, and tetraniliprole, except emamectin benzoate (RF: 2.08-fold). Increased activity of glutathione S-transferase (ratio 1.94) was observed in the Flux-SEL (F10) strain of FAW, while the cytochrome P450 and carboxylesterase activities were not altered. The fluxametamide-selection significantly affected the development and reproductive traits of FAW with a lower R₀, T and relative fitness (R_f = 0.353). The results alluded that the risk of fluxametamide resistance evolution in FAW is relatively lower; however, proactive implementation of resistance management approaches should be done to maintain the field efficacy of fluxametamide against FAW.