There is a need for alternatives or complements to synthetic fertilizers to enhance agricultural
sustainability. Applying organic amendments can play a significant role in this. Insect droppings show
high potential, though studies evaluating their agronomic value have only recently begun to emerge. This
study compared black soldier fly (Hermetia illucens L.) and mealworm (Tenebrio molitor L.) larvae frass with
another organic amendment (Nutrimais) derived from composting forestry, agro-industrial, and domestic
waste. The experiment also included ammoniumnitrate at two rates [the same as the organic amendments,
50 kg ha−1 nitrogen (N) (FullR), and half that rate (HalfR)] and an unfertilized control. The study spanned
two growth cycles of lettuce (Lactuca sativa L.) grown in pots, followed by unfertilized oats (Avena sativa L.)
to assess the residual effects of the fertilizing treatments. Mealworm larvae frass mineralized rapidly, with
an apparent N recovery of 37.4% over the two lettuce growth cycles, indicating its high availability to soil
heterotrophic microorganisms. The average dry matter yield (DMY) of lettuce was the highest among
all treatments (12.8 and 9.8 g plant−1 in the first and second lettuce cycles), even compared to the FullR
treatment (12.2 and 7.8 g plant−1), though without significant differences. Although mealworm larvae
frass exhibited a highmineralization rate, the DMY cannot be attributed solely to N supply, as plants in the
FullR treatment showed better N nutritional status. Mealworm larvae frass provided strong evidence of a
plant biostimulant effect, not explained by the variablesmeasured in this study. Black soldier fly larvae
frass exhibited typical behavior of amoderately reactive organic amendment, while Nutrimais showed
low reactivity, with a near-neutral mineralization/immobilization balance. The results suggest mealworm
larvae frass is recommended for earlymaturing vegetable crops, whereas Nutrimais appearsmore suitable
for perennial crops with low short-termnutrient requirements.
