Leptoglossus occidentalis Heidemann (Heteroptera: Coreidae), commonly known as western conifer seed bug (WCSB), is an invasive pest in the Mediterranean, where it has been associated with the decline of Pinus pinea L. kernel production. A biological control of WCSB populations with native egg parasitoids may reduce economic losses. With the purpose to obtaining egg masses laid in situ bags with WCSB, adults were placed, for three consecutive years, in a clone bank, and for 1 year, in a pine forest in Central Spain. To estimate the impact of the parasitoid species on the WCSB population, four parasitism indexes were calculated. The WCSB activity period was estimated for two consecutive years in the clone bank through visual monitoring. Two native parasitoid species were found on the WCSB eggs: Ooencyrtus pityocampae (Mercet) (Hymenoptera: Encyrtidae) and Ooencyrtus obscurus (Mercet) (Hymenoptera: Encyrtidae). The latter parasitoid species represents a new finding for WCSB eggs. In the clone bank, discovery efficiency of the egg masses, parasitism rate of the eggs, impact on host population and exploitation efficiency of the egg masses were significantly higher for O. obscurus than for O. pityocampae. In the pine forest, parasitism index values were also higher for O. obscurus, but significant differences were not found by species. Ooencyrtus obscurus reduced the WCSB population by 6.2%, down to 19.6%, while O. pityocampae reduced it by 1.0%, down to 11.5%. Predated eggs were recorded in both experimental sites in low amounts. Both native parasitoid species could be proposed as biological control agents (BCA) in a biological control programme (BCP) in Europe after measuring their effectiveness with laboratory assessments.