Ooencyrtus pityocampae (Hymenoptera: Encyrtidae) is a major egg parasitoid of Thaumetopoea pityocampa, a major pest forest insect of Mediterranean ecosystems. Its life duration, host preference and seasonal plasticity are very important to be understood in order to make maximum use of it in biological control programs. This study examines the lifespan and parasitic potential of O. pityocampae under controlled conditions (26 ± 1°C, 65% RH) in two ecotypes. Our results show that O. pityocampae has a bivoltine life cycle, with two generations per year synchronized with T. pityocampa oviposition. A part of the population enters diapause, lasting a maximum of two years, to enable adaptive synchronization with host availability. Longevity tests revealed a mean life span of 24 days, with more than 70% of the population surviving for 7-18 days. Significantly, restricted host access substantially prolonged lifespan (p < 0.01), i.e., a trade-off between reproductive-activity and longevity. Observations of behaviour demonstrated that sharp discrimination against hosts and minimal superparasitism occurred. Strong preference was given by O. pityocampae to eggs of T. pityocampa but was also able to attack alternative hosts given the chance. The results have particular interest with regard to O. pityocampae's ecological specializations and for adopting it for use in biological control of T. pityocampa to help ensure sustainable forest pest control.