413Classical and augmentation biological control depend on the successful laboratory rearing of natural enemies, but rearing methods can affect the success and productivity of a laboratory colony. One process that can occur during rearing is the loss of genetic variability due to inbreeding. The aim of this study was to examine the influence of rearing methods with and without inbreeding in Mastrus ridens Horstmann, a hymenopteran parasitoid specialist of the codling moth (Cydia pomonella [L.]), on several fitness traits. Traits of females from mixed and endogamous populations resulting from different rearing methods were compared, as well as traits of females from the same population subjected to sib-mating or outbreeding. Longevity, parasitism, total fecundity, and number of sons were similar for females in both evaluations. The number of daughters was lower (1.9 ± 0.6 vs. 4.5 ± 0.5) and the offspring sex ratio was higher (0.81 ± 0.04 vs. 0.48 ± 0.05 proportion of males) in females from the endogamous population compared to females of the mixed population, as well as in sib-mating females compared to outbred females of the same population (0.5 ± 0.3 vs. 5.8 ± 1.9 daughters and 0.94 ± 0.03 vs. 0.51 ± 0.10 sex ratio). These results show that rearing methods, specifically inbreeding conditions, have an effect on M. ridens fitness; thus, successful outbred laboratory rearing should be promoted.Key words: Biological control, longevity, parasitism, rearing, sex ratio. ABSTRACTLaboratory rearing methods that promote inbreeding have a negative impact on the fitness of Mastrus ridens Horstmann (Hymenoptera: Ichneumonidae), a parasitoid used to control the codling moth