Tenebrio molitor: effects of four herbs on larval composition and on their growth performance

Mealworms (MWs) can grow on bakery waste and upgrade them into high-quality biomass to produce animal feed. The dough products are sometimes prepared with small quantities of herbs that can have a negative effect on MWs growing performances. This trial investigated the effects of including four different herbs in the MWs substrate: basil, oregano, rosemary and thyme, in two concentrations, 0.125% and 2% on a dry matter basis. All treatments were performed in duplicate. Ten thousand 4-week-old larvae were placed on 2.1 kg of substrates and reared for five weeks under dark conditions at 27°C with 60% relative humidity. The data, which included growth performance, harvested prepupal biomass (wet weight), moisture content, and nutritional profile, were analyzed using a one-way ANOVA. P-values below 0.05 were considered statistically significant. Samples were taken by each tray during the trial to record growth performance parameters and resulted in no differences between groups (P>0.05). The collection of the insects was initiated upon the appearance of the first pupae (nine-week-old larvae). The chemical composition, the total polyphenolic content (TPC) and the antioxidant activity of the resulting larvae were analyzed. The dry matter content showed no differences among groups (P>0.05). The protein content was hindered by a greater herb inclusion (P=0.002) and the fat level was slightly reduced by the inclusion of oregano (P=0.024). The larval TPC has been worsened by the highest herb inclusion (P<0.001) and a negative Spearman correlation (-0.463) was observed between the substrate’s TPC and the MWs’ TPC. Conversely, the incorporation of herbs led to an augmentation in larval antioxidant activity, as evidenced by measurements of ABTS (P<0.001) and FRAP (P<0.001). The inclusion of herbs did not modify larval growth performance; however, a higher inclusion level of the tested herbs resulted in modifications to larval chemical composition, with a concomitant detriment to larval TPC and an improvement in antioxidant activity. Further researches are needed to understand the observed effects.