Stand composition and forest edge effects on insect pest dynamics in pine stands: a comparative analysis of pure and mixed forest structures in Algeria

Stand composition strongly influences forest susceptibility to insect herbivores, yet its role in shaping infestations of the pine processionary moth (Thaumetopoea pityocampa) remains poorly documented at the southern edge of the species’ distribution. This study investigated how stand structures and composition(pure vs. mixed pine forests) influences winter nest abundance and colony size in the semi-arid forests of Djelfa, Algeria, while also evaluating the influence of forest edge–interior gradients and topographic exposure. Between 2022 and 2023, winter nests were surveyed on 800 Aleppo pine trees using transects established both parallel to the forest edge (horizontal transects) and perpendicular from the forest edge toward the stand interior (vertical transects). Nest abundance, colony size, and nest characteristics were compared between stand types, edge and interior habitats, and slope orientations. Pure Aleppo pine plantations consistently supported significantly higher nest abundance and larger colonies than native mixed stands. Although nest density was greater at forest edges than in stand interiors, this pattern was observed in both stand types and therefore represented a secondary effect relative to stand composition. South-facing slopes also exhibited higher nest abundance and larger colonies, reflecting more favorable thermal conditions for larval development. These results demonstrate that forest composition is a major determinant of pine processionary moth infestation at the southern edge of its distribution, whereas edge effects mainly modify the spatial distribution of nests within stands. However, the greater density and continuity of Aleppo pine in pure plantations may also contribute to higher infestation through a resource-concentration effect. Therefore, the lower infestation observed in mixed stands may reflect both associational resistance and reduced host-resource concentration. Promoting mixed forest stands while avoiding excessive concentration of host trees instead of extensive pine monocultures may therefore improve the ecological resistance of afforestation programs such as the Algerian Green Dam.