Abstract: Apart from being secondary storage insect pests, mealworms, particularly the yellow mealworm, Tenebrio molitor L., and the lesser mealworm, Alphitobius diaperinus (Panzer) (Coleoptera: Tenebrionidae), have recently also been launched as highly promising nutrient sources. Therefore, the optimization of their rearing for both stored product and edible insect applications is highly important. Most of the studies though focus on the impact of the dry feed, whereas the wet feed that is provided to mealworms as moisture source is usually overlooked. In a series of bioassays we determined the effect of several wet feeds on the larval growth of T. molitor and A. diaperinus. Our results clearly show that larval growth, as well as other larval fitness-related traits can be significantly affected by wet feed, emphasizing its importance for the lab- and mass-rearing of T. molitor and A. diaperinus larvae.