Effects of itol A on the larval growth and development of Spodoptera frugiperda (Lepidoptera: Noctuidae)

Characterization and transcriptomic analyses of the toxicity induced by toosendanin in Spodoptera frugipreda

The fall armyworm, Spodoptera frugiperda, is a destructive agricultural pest that seriously threatens global food security. Insecticide resistance of this pest has gradually formed in recent years due to improper usage, and alternative methods are badly needed. Toosendanin (TSN) is a botanical compound with broad-spectrum insecticidal activities against many pests. However, the effects of TSN on S. frugiperda are still unclear. In this study, the growth inhibition phenomenon, including weight loss and prolonged developmental duration, in the larvae with TSN exposure was clearly observed. Compared to the control group, a total of 450 and 3314 differentially expressed genes (DEGs) were identified by RNA-Seq in the larvae groups treated with 10 and 20 mg/kg TSN, respectively. Furthermore, the DEGs involved in the juvenile hormone and ecdysone signal pathways and downstream processes, including detoxifying enzyme genes, chitin synthesis and metabolism genes, and cuticular protein genes, were found. Our findings suggest that TSN regulates the expression of key genes in juvenile hormone and ecdysone signal pathways and a series of downstream processes to alter the hormone balance and cuticle formation and eventually inhibit larval growth, which laid the foundation for the molecular toxicological mechanism research of TSN on S. frugiperda larvae.

Discovery of Novel Isoxazoline Derivatives Containing Diaryl Ether against Fall Armyworms

With the continuous evolution of insect resistance, it is a tremendous challenge to control the fall armyworm (Spodoptera frugiperda) with traditional insecticides. To solve this pending issue, a series of novel isoxazoline derivatives containing diaryl ether structures were designed and synthesized, and most of the target compounds exhibited excellent insecticidal activity. Based on the three-dimensional quantitative structure-activity relationship (3D-QSAR) model analysis, we further optimized the molecular structure with compound L35 obtained and tested for its activity. Compound L35 (LC₅₀ = 1.69 mg/L) exhibited excellent insecticidal activity against S. frugiperda, which was better than those of commercial fipronil (LC₅₀ = 70.78 mg/L) and indoxacarb (LC₅₀ = 5.37 mg/L). The enzyme-linked immunosorbent assay showed that L35 could upregulate the levels of GABA in insects. In addition, molecular docking and transcriptomic results also indicated that compound L35 may affect the nervous system of S. frugiperda by acting on GABA receptors. Notably, through high-performance liquid chromatography (HPLC), we were able to obtain the two enantiomers of compound L35, and the insecticidal activity test revealed that S-(+)-L35 was 44 times more active than R-(-)-L35 against S. frugiperda. This study established the chemistry basis and mechanistic foundations for the future development of pesticide candidates against fall armyworms.

Itol A May Affect the Growth and Development of Spodoptera frugiperda through Hijacking JHBP and Impeding JH Transport

Isoryanodane and ryanodane diterpenes have a carbon skeleton correlation in structures, and their natural product-oxidized diterpenes show antifeedant and insecticidal activities against Hemiptera and Lepidoptera. While ryanodine mainly acts on the ryanodine receptor (RyR), isoryanodane does not. In this study, we demonstrated that itol A, an isoryanodane diterpenoid, could significantly downregulate the expression level of juvenile hormone-binding protein (JHBP), which plays a vital role in JH transport. RNAi bioassay indicated that silencing the Spodoptera frugipreda JHBP (SfJHBP) gene decreased itol A activity, which confirmed the developmental phenotypic observation. Parallel reaction monitoring (PRM) further confirmed that itol A affected JHBP's expression abundance. Although JHBP is not proven as the direct or only target of itol A, we confirmed that itol A's action effect depends largely on JHBP and that JHBP is a potential target of itol A. We present foundational evidence that itol A inhibits the growth and development of Spodoptera frugiperda mainly through hijacking JHBP.