Is age just a number? Sex-specific age effects on the reproduction of yellow mealworm

Tenebrio molitor is one of the most commercially significant insect species reared across Europe for human food and animal feed. For the optimisation of its rearing, the attention of research has mainly focused on the larval stage, particularly on the enhancement of its nutritional value and its ability to bioconvert agro-industrial by-products. Comparatively, although rearing strategies at the adult stage are also necessary tool for any insect farm, this area of research remains largely unexplored. Thus, this study investigates the effect of age of female and male adults on their fecundity and fertility. A total of 960 pupae were collected, sexed and divided into four age categories (1-4 weeks old (WO)), creating 16 treatments, each composed of five males and five females belonging to a specific age category. Adults were placed in plastic vials containing wheat flour, which served as oviposition substrate, while agar was provided 3 times per week as a moisture source. Six replicates were conducted per treatment. Egg production and hatching rate were recorded every 7 days for a 4-week period. Based on the results, treatments containing younger females produced a significantly higher number of eggs compared to pairs with older females. This outcome was consistent regardless of the age of the males. In particular, the pairs with 1 WO females produced more eggs than the pairs with 4 WO females (P < 0.001). However, the number of hatched larvae showed an opposite trend: a higher number of larvae were produced by older females. Particularly, the pairs with 3 WO females produced significantly more larvae than the pairs with 1 WO females (P < 0.01). The results of this experiment reveal that the age of females matters more than that of males. Moreover, results highlight how egg quantity does not equate to egg fertility, a crucial consideration for rearing efficiency. Finally, the fact that females reach their peak reproductive potential at week 3 provides a time-specific window to optimise mating protocols in commercial rearing.