Abstract Mealybugs are highly aggressive pests that infest various plants and cause substantial economic losses. Histone lysine methyltransferases (KMT) are evolutionarily conserved and proposed to be essential in early embryo development in animals. However, few KMTs have been reported in mealybugs. Here, we identified a novel KMT gene, PsASH2 , in the cotton mealybug, Phenacoccus solenopsis Tinsley. This gene was highly expressed in the ovary of female adults. Through RNA interference (RNAi) of PsASH2 by dsRNA microinjection, we found a reduction in the number of male embryos and total embryos in the ovaries of pregnant females. Continuous downregulation of PsASH2 in mated females until their death resulted in few changes in sex ratio but significant decreases in the number of both male and female offspring. Therefore, we believe that PsASH2 plays essential roles in embryo survival for both sexes of the cotton mealybug which may provide a potential target gene for the management of cotton mealybug by disrupting embryo development.