Land use changes have generally been considered a local matter, but they are becoming an issue of much wider concern as changes in the use of non-agricultural lands occasionally generate nationwide agricultural pest problems. Plautia stali Scott is a serious pest in orchards, and the widespread distribution of this pest is likely associated with the expansion of planted coniferous forests in Japan. An experiment was conducted on 29 study sites in diverse areas of planted coniferous forest in central Japan, and the abundance of P. stali was monitored over four seasons for 3 years. Results revealed that the optimum radius required to cause an effect on the area surrounding the conifer planted forest was up to 4 km-specifically, 2,000, 400, 1,500, and 200 m in April, June, August, and October, respectively. Positive correlations were observed between conifer planted forests and the collected numbers of P. stali in April, June, and August. These temporal scales provide a direct evaluation of the landscape effects of a conifer planted forest on population dynamics, and this information will contribute to the development and improvement of area-wide P. stali management.