Willow oyster scale, Lepidosaphes salicina Borchsenius (Homoptera: Diaspididae) is a serious, univoltine, pest of willows and poplars in northern China. Its main natural enemy was Hemisarcoptes sp. (Acari: Astigmata: Hemisarcoptidae), which attacks eggs, nymphs, immature males and females of the scale. Mite prevalence was greatest during June, on 2nd-instar female nymphs of the pest. The numerical responses and density dependencies of the mite on L. salicina are discussed in relation to field data. More scales were parasitized on the eastern and northern sides of the trees, which had more scales, than on their souther and western aspects. Another diaspidid, Quadraspidiotus gigas (Thiem and Gerneck), was also attacked on poplars by Hemisarcoptes sp.