Forests with single tree species are hypothesized to be more susceptible to pest outbreaks than mixed forests. Population densities of two insect species, Ips subelongatus Motschulsky and Lymantria dispar L., with different dietary spectra were investigated in pure and mixed stands using window and light traps in Aershan forestry bureau, northeastern China in 2009. I. subelongatus is a monophagous pest of Larix gmelinii (Rupr.) Rupr. in the study area, while L. dispar is a highly polyphagous pest species feeding on both coniferous and deciduous tree species. The individual number and dominance index of the two indicator species were investigated and analyzed in 21 forest stands in a thinning-caused gradient of larch-birch mixture ranging from pure larch stands to pure birch stands (seven types in the present study). The results showed that the number of individuals of I. subelongatus was significantly related to the proportion of larch trees, i.e. the number of individuals decreased with the rising birch ratio in the canopy layer. Similar decrease was observed in the dominance index of I. subelongatus. However, no significantly obvious effects of canopy mixture on the individual number or dominance index were observed in L. dispar.