Detection of Chitin Synthase Mutations in Lufenuron-Resistant Spodoptera frugiperda in China

SIMPLE SUMMARY: Lufenuron is one of the main insecticides for controlling lepidopteran pests, and is especially suitable for controlling pests that are resistant to pyrethroids and organophosphorus pesticides. Resistance to lufenuron is a serious obstacle to effective pest control. A major mechanism of lufenuron resistance is due to mutations in chitin synthase, the target of the toxic effects of benzoylurea (BPUs), according to a reported mutational analysis, I1040 (I1042 in Plutella xylostella) in the SfCHSA gene. The mutation of the site can significantly improve the resistance of insect larvae to BPU insecticides. Secondly, the expression level of insecticide targets in insects is also an important factor affecting insect resistance. In this study, we firstly identified the number of chitin synthase genes and their corresponding names in Spodoptera frugiperda, and detected the mutation sites of the SfCHSA gene. The expression level of chitin synthase gene was then analyzed. Our results showed that no mutation was found in the SfCHSA gene in the currently low-resistant field populations in China, and the expression of chitin synthase was affected by lufenuron. The information obtained from this study is valuable for the use of lufenuron in the field. ABSTRACT: Spodoptera frugiperda (J. E. Smith), is commonly known as fall armyworm, native to tropical and subtropical regions of America, is an important migratory agricultural pest. It is important to understand the resistance and internal mechanism of action of S. frugiperda against lufenuron in China. Lufenuron is one of the main insecticides recommended for field use in China and has a broad prospect in the future. We conducted a bioassay using the diet-overlay method and found that the current S. frugiperda in China are still at a low level of resistance to lufenuron. Secondly, we examined whether the mutation I1040M (I1042M in Plutella xylostella), associated with lufenuron resistance, was produced in the field. And then we tested the expression of chitin synthase SfCHSA and SfCHSB in different tissues, and the changes of these two genes after lufenuron induction. The results showed that there is still no mutation generation in China and there is a significant change in the expression of SfCHSA under the effect of lufenuron. In conclusion, our study suggests that field S. frugiperda populations in 2019 and 2020 were less resistant to lufenuron. In fall armyworm, chitin synthases included SfCHSA and SfCHSB genes, and after induction treatment with lufenuron, the expression of the SfCHSA gene was significantly increased. In SfCHSA, no mutation has been detected in the site associated with lufenuron resistance. Secondly, in S. frugiperda larvae, the SfCHSA gene was the highest in the head of the larvae, followed by the integument; while the SfCHSB gene was mainly concentrated in the midgut. Therefore, we believe that the SfCHSA gene plays a greater role in the resistance of S. frugiperda to lufenuron than the SfCHSB gene. It is worth noting that understanding the level of resistance to lufenuron in China, the main mechanism of action of lufenuron on larvae, and the mechanism of resistance to lufenuron in S. frugiperda will help in crop protection as well as in extending the life span of this insecticide.