Populations of natural enemies in classical biological control programs often undergo recurring bottlenecks during importation and rearing, which can result in lower genetic variability and fitness. Enriching colonies with wild individuals, multiple importations or combining populations from different origins may help reduce this problem. Mastrus ridens (Hymenoptera: Ichneumonidae), a gregarious idiobiont ectoparasitoid of codling moth (Cydia pomonella) larvae was introduced to New Zealand from laboratory colonies dating to the 1990s. To improve populations for release, individuals from a most recent collection were imported from Chile (CL). Tests were conducted to measure genetic diversity of the parental populations (F0) and fitness traits of pure and mixed combinations of NZ and CL populations for F0 and the F1 generations. Results showed several effects depending on the population origin. CL females and males in the F0 generation lived longer than NZ females and males. Offspring from a CL mother lived longer than those from a NZ mother, and F0 and F1 females from a CL mother parasitized more larvae than females from a NZ mother. Genetic diversity estimates of F0 CL and NZ populations were 24% and 17% heterozygosity, respectively, and the NZ population had a larger proportion of diploid males than the CL population. Thus, no improvement in reproductive output resulted by mixing NZ and CL populations, as hypothesized, but a maternal effect was found in this species. This study shows the importance of understanding and tracking imported populations' genetic diversity and fitness traits in classical biological control programs.