We assessed the functional and numerical responses of Trichogramma brassicae Bezdenko under long-term rearing (45 generations, G5-G45) on eggs of a common factitious host, Ephestia kuehniella Zeller (Lepidoptera: Pyralidae). Under such long-term mass rearing, the function response of this parasitoid shifted from Type III (from G5 to G20) to Type II (from G25 to G45). The maximum attack rate (T/Th) (33.62 hosts/day) and the shortest handling time (0.7138 ± 0.0272 h) were recorded in G5. Also, G45 had the lowest attack rate (21.67 hosts/day) and longest handling time (1.1076 ± 0.0689 h). The handling time increased gradually over the generations (P reg <0.01, R2 = 0.903). The number of eggs laid by the parasitoid increased significantly with increased host density, but reached a plateau at very high densities. While no significant differences in daily parasitism were observed over 45 generations at densities of 2, 4, 8, and 16 available host eggs per wasp, there were significant differences in parasitism at host densities of 32, 64, and 128 eggs per wasp. T. brassicae females were more active and foraged more frequently in earlier generations (G5 to G20), which allowed wasps to parasitize more E. kuehniella eggs compared to later generations. Also, continuous mass rearing of T. brassicae negatively affected foraging behavior and the quality of wasps after G20. However, rejuvenation of the colony by adding field-collected parasitoids or rearing wasps on more suitable factitious hosts is strongly recommended to lower the negative effects of long-term mass rearing on quality of this parasitoid.

Trichogramma brassicae (Bezdenko) is an important biological control agent that has been used widely against many lepidopteran pests. Commonly, colonies of Trichogramma are mass reared on factitious hosts such as Sitotroga cerealella (Olivier) over many generations. In this study, we evaluated the quality and performance of a colony of T. brassicae that had been reared for over 45 generations (G) using two-sex life table parameters and parasitism capacity. We found that female adult longevity was significantly different among sequential generations, ranging from 5.58 ± 2.5 d (at G5) to 3.75 ± 1.42 d (at G45). However, no significant difference was found in male adult longevity among different generations. Although female wasps survived longer until the 15th generation, they allocated more days for egg laying at G5 and G10. The highest values of gross reproductive rate (GRR), net reproductive rate (R 0), intrinsic rate of increase (r), finite rate of increase (λ) and mean generation time (T) were found in G5 and G10, which also showed significantly higher c 0. No significant difference in the finite parasitism rate (ω) was found among generations up to G15. These results suggest that T. brassicae wasps held under continuous laboratory rearing can be used successfully in biological control programs until the 15th generation without any loss of quality or performance. However, laboratory mass rearing colonies declined in quality after 15 generations and we suggest that they should be rejuvenated regularly by adding field-collected parasitoids periodically.

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Background Domestication usually has negative effects on insect performance, especially when they are reared continuously for many generations. Rejuvenation can reduce the negative effects of domestication in the parasitoid, Trichogramma brassicae Bezdenko (Hymenoptera: Trichogrammatidae) reared for 30 generations (G) on Sitotroga cerealella (Olivier) (Lepidoptera: Gelechiidae). Life table and parasitism parameters in the primary colony (G31–G45) were compared with the respective colony rejuvenated with field-collected individuals. Results There were significant differences in life history traits between domesticated and rejuvenated populations. The highest (38.4 eggs/female) and lowest (13.3 eggs/female) fecundity was observed in rejuvenated G31 and domesticated G45, respectively. The highest values of the life table and parasitism parameters were observed in rejuvenated G31 without significant difference with G32, G33 and G34. The relationship between finite parasitism rate (ω) and generation number in both rejuvenated and domesticated populations were fitted to the cubic regression model, indicating the parasitism potential of T. brassicae increased significantly after adding feral individuals but decreased remarkably over the generations. Conclusions Therefore, it seems that adding 10% feral individuals re-established the reproductive performance of the wasps at least for four generations, and it needs to be repeated routinely or it needs to be made by adding a higher rate of feral individuals. However, this issue should be investigated by more studies in which different rates of feral individuals are added to the primary colony.