1- The risk of pest damage in forest monocultures is a growing concern as the area of plantation forest expands world‐wide and sustainable management is becoming a general goal. Numerous studies have demonstrated that pest insects are less abundant in more diverse agroecosystems, but limited experimental evidence exists for a similar relationship in forestry. Because plantation forests are usually mosaics of single‐species stands, the aim of this study was to test the effect of forest diversity at a landscape level on pest insect infestations. 2- Within a stand of maritime pine, trees attacked by the stem borer Dioryctria sylvestrella showed no spatial aggregation and clusters of 66 trees proved to be an optimal sampling unit for determining overall percentage infestation. 3- The spatial distribution of stem borer infestation levels was examined in three maritime pine stands bordered by mixed‐species stands of broad‐leaved trees, and overall infestation levels compared between four such maritime pine stands and four other maritime pine stands of the same structure but far from any broad‐leaved stands. 4- The percentage of trees infested was significantly lower in the pine stands bordered by broad‐leaved stands than in the pine stands surrounded by other pine stands, and in the former, stem borer infestation showed a significant logistic increase with distance from the stand edge. This could be due to effects of resource concentration or natural enemy attack, both of which are discussed. 5- These results suggest that the conservation or restoration of non‐productive mixed‐species stands adjacent to intensively managed plantations is a useful preventive method for pest management in forest monocultures.