The actions of neonicotinoid insecticides on nicotinic acetylcholine subunits Ldα1 and Ldα8 of Leptinotarsa decemlineata and assembled receptors

Expression and Functional Analysis of Two Cytochrome P450 Monooxygenase Genes and a UDP-Glycosyltransferase Gene Linked with Thiamethoxam Resistance in the Colorado Potato Beetle

Cytochrome P450 monooxygenases (P450s) and UDP-glycosyltransferases (UGTs) are involved in the evolution of insecticide resistance. Leptinotarsa decemlineata (Say), the Colorado potato beetle (CPB), is a notorious insect that has developed resistance to various insecticides including neonicotinoids. This study investigated whether the differentially expressed P450 genes CYP9Z140 and CYP9AY1 and UGT gene UGT321AP1, found in our transcriptome results, conferred resistance to thiamethoxam in L. decemlineata. Resistance monitoring showed that the sampled field populations of L. decemlineata adults collected from Urumqi City and Qapqal, Jimsar, and Mulei Counties of Xinjiang in 2021-2023 developed low levels of resistance to thiamethoxam with resistance ratios ranging from 6.66- to 9.52-fold. Expression analyses indicated that CYP9Z140, CYP9AY1, and UGT321AP1 were significantly upregulated in thiamethoxam-resistant populations compared with susceptible populations. The expression of all three genes also increased significantly after thiamethoxam treatment compared with the control. Spatiotemporal expression patterns showed that the highest expression of CYP9Z140 and CYP9AY1 occurred in pupae and the midgut, whereas UGT321AP1 was highly expressed in adults and Malpighian tubules. Knocking down all three genes individually or simultaneously using RNA interference increased the sensitivity of adult L. decemlineata to thiamethoxam. These results suggest that overexpression of CYP9Z140, CYP9AY1, and UGT321AP1 contributes to the development of thiamethoxam resistance in L. decemlineata and provides a scientific basis for improving new resistance management of CPB.

Mutation V65I in the β1 Subunit of the Nicotinic Acetylcholine Receptor Confers Neonicotinoid and Sulfoxaflor Resistance in Insects

Neonicotinoids have been widely used to control pests with remarkable effectiveness. Excessive insecticides have led to serious insect resistance. Mutations of the nicotinic acetylcholine receptor (nAChR) are one of the reasons for neonicotinoid resistance conferred in various agricultural pests. Two mutations, V65I and V104I, were found in the nAChR β1 subunit of two neonicotinoid-resistant aphid populations. However, the specific functions of the two mutations remain unclear. In this study, we cloned and identified four nAChR subunits (α1, α2, α8, and β1) of thrips and found them to be highly homologous to the nAChR subunits of other insects. Subsequently, we successfully expressed two subtypes nAChR (α1/α2/α8/β1 and α1/α8/β1) by coinjecting three cofactors for the first time in thrips, and α1/α8/β1 showed abundant current rapidly. Acetylcholine, neonicotinoids, and sulfoxaflor exhibited different activation capacities for the two subtypes of nAChRs. Finally, V65I was found to significantly reduce the binding ability of nAChR to neonicotinoids and sulfoxaflor through electrophysiology and computer simulations. V104I caused a decrease in agonist affinity (pEC50) but an increase in the efficacy (Imax) of nAChR against neonicotinoids and reduced the binding ability of nAChR to sulfoxaflor. This study provides theoretical and technical support for studying the molecular mechanisms of neonicotinoid resistance in pests.

The interaction of nicotinic acetylcholine receptor subunits Ldα3, Ldα8 and Ldβ1 with neonicotinoids in Colorado potato beetle, Leptinotarsa decemlineata

The Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), is an extremely destructive notifiable quarantine pest. Over the last two decades, neonicotinoid insecticides, particularly thiamethoxam and imidacloprid, have been used to control it in Xinjiang, and local field populations have developed different levels of resistance in consequence. However, the contributions of nicotinic acetylcholine receptors (nAChRs) to neonicotinoid resistance are currently poorly understood in CPB. Previous studies have shown that nAChRα1, α3, α8 and β1 are major target subunits for neonicotinoids in some model and important agricultural insects including nAChRα1 subunit of L. decemlineata (Ldα1). In this study, the expression levels of Ldα3, Ldα8 and Ldβ1 following 72 h of treatments with median lethal doses of thiamethoxam and imidacloprid were compared using real-time quantitative PCR. These genes were then individually and simultaneously knocked down with Ldα1 by RNA interference (RNAi) using a double-stranded RNA (dsRNA) feeding method for six days to explore their roles in CPB susceptibility to imidacloprid and thiamethoxam. The results showed that the expressions of Ldα3, Ldα8 and Ldβ1 were significantly decreased by 36.99-74.89% after thiamethoxam and imidacloprid treatments, compared with the control. The significant downregulation of the target genes resulting from RNAi significantly reduced the mortality of adults exposed to thiamethoxam and imidacloprid by 34.53% -56.44% and 28.78%-43.93%, respectively. Furthermore, the adult survival rates were not affected by every dsRNA-feeding treatment, while the body weight of the test adults significantly deceased after four and six days of individual gene RNAi. This study showed that Ldα3, Ldα8 and Ldβ1 are down-regulated by thiamethoxam and imidacloprid and play important roles in the tolerance of CPB to neonicotinoids.

Dinotefuran exposure induces autophagy and apoptosis through oxidative stress in Bombyx mori

As a third-generation neonicotinoid insecticide, dinotefuran is extensively used in agriculture, and its residue in the environment has potential effects on nontarget organisms. However, the toxic effects of dinotefuran exposure on nontarget organism remain largely unknown. This study explored the toxic effects of sublethal dose of dinotefuran on Bombyx mori. Dinotefuran upregulated reactive oxygen species (ROS) and malondialdehyde (MDA) levels in the midgut and fat body of B. mori. Transcriptional analysis revealed that the expression levels of many autophagy and apoptosis-associated genes were significantly altered after dinotefuran exposure, consistent with ultrastructural changes. Moreover, the expression levels of autophagy-related proteins (ATG8-PE and ATG6) and apoptosis-related proteins (BmDredd and BmICE) were increased, whereas the expression level of an autophagic key protein (sequestosome 1) was decreased in the dinotefuran-exposed group. These results indicate that dinotefuran exposure leads to oxidative stress, autophagy, and apoptosis in B. mori. In addition, its effect on the fat body was apparently greater than that on the midgut. In contrast, pretreatment with an autophagy inhibitor effectively downregulated the expression levels of ATG6 and BmDredd, but induced the expression of sequestosome 1, suggesting that dinotefuran-induced autophagy may promote apoptosis. This study reveals that ROS generation regulates the impact of dinotefuran on the crosstalk between autophagy and apoptosis, laying the foundation for studying cell death processes such as autophagy and apoptosis induced by pesticides. Furthermore, this study provides a comprehensive insight into the toxicity of dinotefuran on silkworm and contributes to the ecological risk assessment of dinotefuran in nontarget organisms.