This study investigates the role of UDP-glycosyltransferases (UGTs) in determining the host plant range of *Spodoptera* species, a genus of phytophagous insects. Comparative genomics and CRISPR-Cas9 genome editing were used to analyze the UGT gene family in *Spodoptera* species with varying host plant breadths. The results show that UGT genes play a crucial role in detoxifying plant defense compounds, such as the benzoxazinoid DIMBOA, which is produced by poaceous plants like maize, wheat, and rice. In generalist species like *S. frugiperda*, specific UGT genes, particularly *SfUGT33F32*, are essential for detoxifying DIMBOA and utilizing these plants. However, in the specialist species *S. picta*, which specializes on *Cinnam* plants, these genes are pseudogenized, rendering it unable to detoxify DIMBOA. The study also reveals that the UGT33 and UGT40 families have expanded in *S. frugiperda* through tandem duplication events, and that these genes are highly conserved across different *Spodoptera* species. The findings provide insights into the molecular mechanisms underlying ecological adaptation and the evolutionary transitions between generalism and specialism in insects. Additionally, the study suggests that targeting UGT genes, particularly *SfUGT33F32*, could be a potential strategy for controlling *Spodoptera* pests.This study investigates the role of UDP-glycosyltransferases (UGTs) in determining the host plant range of *Spodoptera* species, a genus of phytophagous insects. Comparative genomics and CRISPR-Cas9 genome editing were used to analyze the UGT gene family in *Spodoptera* species with varying host plant breadths. The results show that UGT genes play a crucial role in detoxifying plant defense compounds, such as the benzoxazinoid DIMBOA, which is produced by poaceous plants like maize, wheat, and rice. In generalist species like *S. frugiperda*, specific UGT genes, particularly *SfUGT33F32*, are essential for detoxifying DIMBOA and utilizing these plants. However, in the specialist species *S. picta*, which specializes on *Cinnam* plants, these genes are pseudogenized, rendering it unable to detoxify DIMBOA. The study also reveals that the UGT33 and UGT40 families have expanded in *S. frugiperda* through tandem duplication events, and that these genes are highly conserved across different *Spodoptera* species. The findings provide insights into the molecular mechanisms underlying ecological adaptation and the evolutionary transitions between generalism and specialism in insects. Additionally, the study suggests that targeting UGT genes, particularly *SfUGT33F32*, could be a potential strategy for controlling *Spodoptera* pests.