Indoxacarb resistance-associated mutation of Liriomyza trifolii in Hainan, China

V1848I Mutation in the Voltage-Gated Sodium Channel Confers High-Level Resistance to Indoxacarb and Metaflumizone in Spodoptera exigua

Spodoptera exigua is one of the most serious lepidopteran pests of global importance. With the intensive use of insecticides, S. exigua has evolved resistance to many insecticides, including the sodium channel blocker insecticides (SCBIs) indoxacarb and metaflumizone. In this study, we investigated the role of the V1848I mutation in the voltage-gated sodium channel (VGSC) in SCBI resistance and its inheritance patterns in S. exigua through the development and characterization of a near-isogenic resistant strain. The AQ-23 strain of S. exigua, collected in 2023 from Anqing, Anhui province of China, shows 165-fold resistance to indoxacarb compared with the susceptible WH-S strain. A frequency of 44.6% for the V1848I mutation was detected in the SeVGSC of the AQ-23 strain, while no F1845Y mutation was found. Through repeated backcrossing and marker-assisted selection, the V1848I mutation in the AQ-23 strain was introgressed into the susceptible WH-S strain, creating a near-isogenic strain named WH-1848I. This WH-1848I strain exhibits high levels of resistance to indoxacarb (146-fold) and metaflumizone (431-fold) but remains susceptible to broflanilide and spinosad compared with the WH-S strain. Inheritance analysis revealed that SCBI resistance in the WH-1848I strain is autosomal, nonrecessive, and genetically linked to the V1848I mutation. These findings establish a clear link between the V1848I mutation and SCBI resistance in S. exigua, offering valuable insights for developing molecular detection tools and resistance management strategies.

Impact of biochar on the degradation rates of three pesticides by vegetables and its effects on soil bacterial communities under greenhouse conditions

A 28 days pesticide degradation experiment was conducted for broccoli (Brassica oleracea L. var. italica Planch) and pakchoi (Brassica chinensis L.) with three pesticides (chlorantraniliprole (CAP), haloxyfop-etotyl (HPM), and indoxacarb (IXB)) to explore the effects of biochar on pesticide environmental fate and rhizosphere soil diversity. Rice straw biochar (RB) was applied to soil at a 25.00 t ha-1 dosage under greenhouse conditions, and its effects on the degradation of three pesticides in vegetables and in soil were investigated individually. Overall, RB application effectively facilitated CAP and HPM degradation in broccoli by 13.51-39.42% and in broccoli soil by 23.80-74.10%, respectively. RB application slowed the degradation of CAP, HPM and IXB in pakchoi by 0.00-57.17% and slowed the degradation of CAP in pakchoi by 37.32-43.40%. The results showed that the effect of RB application on pesticide degradation in crops and soil was related to biochar properties, pesticide solubility, plant growth status, and soil characteristics. Rhizosphere soil microorganisms were also investigated, and the results showed that biochar application may be valuable for altering bacterial richness and diversity. The effect of biochar application on pesticide residues in crops and soil was influenced by the vegetable variety first, and the second was pesticide characteristics. RB applied to soil at a 25.00 t ha-1 dosage under greenhouse conditions is recommended for broccoli production to ensure food safety. Our results suggested that biochar application in soil could reduce pesticide non-point source pollution, especially for highly soluble pesticides, and could affect soil microorganisms.

Identification and detection of the V1848I indoxacarb resistance mutation in the beet armyworm, Spodoptera exigua

Indoxacarb is a pivotal insecticide used worldwide to manage Spodoptera exigua, a devastating agricultural pest. This active compound plays a crucial role in resistance management strategies due to its distinctive mode of action. A field population of S. exigua (SH23) from Shanghai, China, exhibited significantly reduced susceptibility to indoxacarb, with a resistance ratio of 113.84-fold in biological assays. Following two rounds of laboratory screening with indoxacarb, the resistance of the new strain (SH23-S2) escalated steeply to 876.15-fold. Genetic analyses of both the SH23 and SH23-S2 strains demonstrated autosomal inheritance and incompletely dominant resistance patterns. Synergist assays indicated a minor role of detoxification enzymes (glutathione s-transferases and cytochrome P450) of SH23-S2 strain in this resistance, implicating target-site resistance as the primary mechanism. To explore the impact of target-site resistance, segment 1-6 of domain IV (IVS1-6) of the sodium channel in S. exigua was cloned, and the sequences from susceptible and indoxacarb-resistant S. exigua were compared. The V1848I mutation, linked to indoxacarb resistance in Plutella xylostella, Tuta absoluta and Liriomyza trifolii, was identified and strongly associated with the indoxacarb-resistant phenotype in the S. exigua SH23-S2 strain, whereas the F1845Y mutation was not detected. Furthermore, a molecular test for the V1848I mutation in field populations was created using an allele-specific PCR (AS-PCR). The discovery of indoxacarb resistance mutation and the creation of diagnostic tool will enable the early detection of indoxacarb resistance, which will facilitate the implementation of targeted resistance management strategies, ultimately delaying the proliferation of resistance.

Functional Characterization of Double Mutations T929I/K1774N in the Voltage-Gated Sodium Channel of Megalurothrips usitatus (Bagnall) Related to Pyrethroid Resistance

Megalurothrips usitatus (Bagnall), the main pest on legume vegetables, is controlled by pyrethroids in the field. Field strains of M. usitatus resistant to pyrethroids were collected from three areas in Hainan Province (Haikou, Ledong, and Sanya City), and two mutations, T929I and K1774N, were detected in the voltage-gated sodium channel. In this study, the sodium channel in M. usitatus was first subcloned and successfully expressed in Xenopus oocytes. The single mutation (T929I or K1774N) and double mutation (T929I/K1774N) shifted the voltage dependence of activation in the hyperpolarization direction. The three mutants all reduced the amplitude of tail currents induced by type I (permethrin and bifenthrin) and type II (deltamethrin and λ-cyhalothrin) pyrethroids. Homology modeling analysis of these two mutations shows that they may change the local hydrophobicity and positive charge of the sodium channel. Our data can be used to reveal the causes of the resistance of M. usitatus to pyrethroids and provide guidance for the comprehensive control of M. usitatus in the future.

Insect hormones affect the toxicity of the insecticidal growth regulator cyromazine in Liriomyza trifolii (Diptera: Agromyzidae)

The leafminer Liriomyza trifolii is an important insect pest of ornamental and vegetable crops worldwide. Cyromazine is an effective, commonly-used insecticide that functions as a growth regulator, but its effect on L. trifolii has not been previously reported. In this study, transcriptome analysis was undertaken in L. trifolii exposed to cyromazine. Clusters of orthologous groups analysis indicated that a large number of differentially expressed genes responding to cyromazine were categorized as "lipid transport and metabolism", "post-translational modification, protein turnover, chaperones", and "cell wall/membrane/envelope biogenesis". Gene ontology analysis indicated that pathways associated with insect hormones, growth and development, and cuticle synthesis were significantly enriched. In general, the transcriptome results showed that the genes related to insect hormones were significantly expressed after treatment with cyromazine. Furthermore, the combined exposure of L. trifolii to cyromazine and the hormone analogues 20-hydroxyecdysone (20E) or juvenile hormone (JH) indicated that hormone analogues can change the expression pattern of hormone-related genes (20EP and JHEH) and pupal length. The combined application of cyromazine with 20E improved the survival rate of L. trifolii, whereas the combination of JH and cyromazine reduced survival. The results of this study help elucidate the mechanistic basis for cyromazine toxicity and provide a foundation for understanding cyromazine resistance.