Geographic variation of the sex pheromone of <i>Mythimna separata</i> (Lepidoptera: Noctuidae) associated with overwintering sites

The sex pheromone of the oriental armyworm, Mythimna separata (Walker), in China has always been confusing. It was initially considered as cis-11-hexadecenal (Z11-16:Ald) or cis-11-hexadecenyl acetate (Z11-16:Ac) and cis-11-hexadecenol (Z11-16:OH), as well as a mixture of these compounds. However, inconsistent results have frequently occurred using these compounds or mixtures in field trapping. Through gland analysis from individual females and nationwide field experiments, we identified two distinct types: Z11-16:Ald (aldehyde type or A type) and a mixture of Z11-16:Ac and Z11-16:OH (ester type or E type). Electroantennography (EAG) results indicated that both strains of males co-existed in the field. A strain of males had a strong EAG response to Z11-16:Ald but weak responses to Z11-16:Ac and Z11-16:OH, whereas E strain males had strong responses to Z11-16:Ac and Z11-16:OH but a weak response to Z11-16:Ald. Z11-16:OH identified in female E strain inhibited the attraction of A strain males to A type pheromones. Both A and E types of sex pheromone were respectively formulated and field evaluation (2014-2021) across > 20 provinces from southern to northern China showed geographic and generational variations. During overwintering, E strain males were predominantly caught in Hainan, Fujian, and Guangdong provinces, while A strain males were mainly trapped in Guangxi, Anhui, Sichuan, Jiangsu, and Jiangxi provinces. Subsequent generations exhibited mixed populations in most regions. These findings suggest regional adaptation and migration-driven hybridization. The study highlights the necessity of strain-specific pheromone blends for effective monitoring and control. This work resolves long-standing ambiguities in M. separata sex pheromone ecology.