The life history consequences of admixture between intentionally and unintentionally introduced populations of Trissolcus japonicus in Europe

• Released and adventive T. japonicus lines were reproductively compatible. • Release of laboratory lines could result in admixture. • Admixture could change fecundity and reproductive schedule of hybrids. • Admixture may affect biocontrol traits but has uncertain field consequences. • Field studies are needed to assess admixture and biological control impact. Introducing classical biological control agents from laboratory-reared populations into regions where adventive populations already exist could alter life history traits of the resulting admixed populations, having positive or negative consequences for pest control. In laboratory studies, we examined the life history consequences of admixture between one intentionally introduced laboratory-reared line (from China) and one pre-existing adventive line (from Switzerland) of Trissolcus japonicus (Hymenoptera: Scelionidae), an egg parasitoid of Halyomorpha halys (Hemiptera: Pentatomidae), in Europe. We tested reproductive compatibility and compared life history traits (fecundity, sex ratio, longevity, and development time) in reciprocal crosses between parental and hybrid lines of the two lines over multiple generations. The intentionally introduced and adventive lines were reproductively compatible, with no evidence of outbreeding depression or hybrid breakdown in either the first or third generations after crossing. Hybrid offspring exhibited life history traits that were either intermediate or similar to those of either parental line. Notably, hybrid lines showed higher fecundity and a more female-biased sex ratio than the adventive parental line. These findings suggest that admixture has some potential to introduce trait values thought to be positively associated with fitness and biological control efficacy. However, future studies are still needed to assess whether admixture between released laboratory-reared lines and adventive populations is actually occurring in the field, and whether it is having consequences for the efficacy of biological control by T. japonicus .