The uncertainty surrounding the part played by an invasive North American seed bug, Leptoglossus occidentalis, in the observed decrease in seed production of the Mediterranean pine, Pinus pinea, is a limiting factor for its management. Furthermore, the possibility of increasing cone production through irrigation and fertilization regimes is gaining interest among landowners, but its effects on insect pests are still unknown. Using bagged branches in the field, we aimed at evaluating the impact of L. occidentalis on young and mature cones of stone pine trees submitted to fertirrigation (FR) compared to trees with no treatment (C). For two consecutive years (2015 and 2016), we carried out both an insect-exclusion trial and an insect-bagged trial. In the first one, polyester-covered branches, excluding insects, were compared to branches exposed to natural insect infestation. In the second trial, bags included either two adult bugs or 3-4 third-instar nymphs, placed there for one month during mid-summer, or were kept without insects. Branch protection resulted in a significant decrease in the mortality of second-year conelets which dropped to 6% compared to 30% in exposed branches. Seed damage also decreased from 60% on exposed branches to 10% on protected ones. The presence of nymphs in the bags resulted in a mortality of second-year conelets 63% higher than in exclusion bags, whereas adults had no effect. In contrast, bags with adults presented the highest seed damage. The partial damage of the kernel can be a signature of L. occidentalis feeding since such damage was not observed in exclusion bags. Additionally, another type of seed damage, showing a wholly shrunken and dry embryo without remaining endosperm, and a reduction in the number of extractable seeds may also be attributed to this seed bug. Overall, seed damage per mature cone reached up to 12% in bags with two adult bugs enclosed for a month, i.e., twice the seed damage in protected cones. Overall, FR trees were more susceptible to both conelet mortality and seed damage. In the particular case of L. occidentalis, FR regime influenced the consumption positively by the nymphs but not by adults.