Global meat consumption is projected to reach about 377 million tons by 2030, a 15% increase compared with 2022, driven by population growth and economic expansion [1]. This trend will intensify the demand for feed ingredients, especially protein sources, while expansion of grain production is increasingly constrained by environmental factors. Consequently, the supply of high-quality feed proteins is expected to fall short of future demand [2]. Fish meal (FM), one of the most widely used animal-derived protein sources, faces supply risks due to ocean warming and overfishing, raising concerns about long-term sustainability [3]. Insects have recently received attention as alternative feed ingredients that can help address both nutritional and environmental challenges. Among them, black soldier fly (BSF, Hermetia illucens) larvae are particularly promising due to their rapid growth, minimal land requirements, high feed conversion efficiency, and ability to upcycle low-value organic materials into high-quality protein [4–6]. BSF cultivation also generates useful byproducts such as frass, which can be applied as organic fertilizer [7]. Compared with other insect species such as houseflies or mealworms, BSF is advantageous for large-scale production because adults do not feed, reducing risks of pathogen transmission [2]. Defatted BSF larvae meal typically contains 40%–50% crude protein and 7%–12% fat, with an amino acid profile comparable to FM, particularly in essential amino acids [8]. Its chitin content may also exert antimicrobial and immunomodulatory effects [9]. Recent studies have reported encouraging results when BSF larvae partially replaced conventional protein sources such as soybean meal, FM, and plasma protein in pig diets [10–12]. Beyond nutritional adequacy, BSF farming supports circular economy principles by recycling food waste into animal protein, thus reducing environmental impacts and potentially lowering production costs [13,14]. Despite these advantages, most studies have evaluated single inclusion levels or limited outcome parameters, and systematic investigations into graded replacement levels of FM with BSF in weaning pigs are still scarce. As the weaning phase is a critical period characterized by stress, digestive challenges, and immune development, it provides a suitable model to assess the feasibility of BSF inclusion. Therefore, the present study was conducted to evaluate the effects of replacing FM with defatted BSF larvae at different levels on growth performance, blood metabolites, immune response, and diarrhea incidence in weaning pigs.

The defatted BSF larvae meal used in this study contained 593.4 g/kg of crude protein (CP) and 48.3 g/kg of crude fat (CF), values comparable with FM (634.8 g/kg CP and 67.5 g/kg CF). These results are in line with previous reports, which showed that defatted BSF larvae typically contain 40.8% to 60.7% CP and 7.9% to 12.3% ether extract (EE) [18]. Although the amino acid profile of BSF larvae was generally lower than that of FM, several essential amino acids such as isoleucine, phenylalanine, proline, and serine were present in comparable amounts. In addition, BSF larvae contain chitin, a fibrous polysaccharide that can only be degraded by endogenous chitinase into chitooligosaccharides, which may serve as prebiotics to improve gut health and intestinal barrier function [19]. The present study demonstrated that replacing 4% FM with defatted BSF larvae had no detrimental effects on BW and ADG throughout the experimental phases. This finding is consistent with Driemeyer [20], who reported that partial FM replacement with BSF larvae (3.5%) did not alter BW or ADG in piglets. Similarly, Chang et al [21] confirmed that defatted and hydrolyzed BSF larvae could substitute FM at 3% of the weaning diet without negative impacts on growth. In our trial, the BW and ADG of nursery pigs up to day 21 were unaffected even when BSF larvae contributed up to 50% of animal protein. However, some studies have shown improved growth performance with insect-based diets. Jin et al [22] observed that dietary inclusion of dried mealworms (up to 6%) enhanced BW and ADG, while Lee et al [23] reported greater BW and ADG at 6 weeks when FM was completely replaced by defatted BSF larvae compared with a control containing 5% FM. These discrepancies may stem from variations in nutrient composition among insect species, differences in rearing substrate and larval age at harvest, or processing methods that affect protein digestibility and bioactive compounds [18,19]. Notably, although no statistical differences in BW or ADG were detected at the final phase, the BSF50 group exhibited numerically higher values, supporting the feasibility of FM replacement up to 50% in practical diets. In general, newly weaned pigs experience multiple concurrent stressors—including abrupt maternal separation, environmental changes, and social hierarchy establishment—which reduce feed intake, predispose to post-weaning diarrhea, and restrict growth [24]. Consequently, highly palatable and digestible ingredients are recommended in nursery diets. In line with previous reports, our data showed a tendency for increased ADFI as BSF inclusion rose, although the G:F ratio tended to decrease. Jin et al [22] reported linear improvements in both ADFI and G:F with mealworm supplementation, while Biasato et al [12] attributed greater feed intake in BSF-fed piglets to enhanced diet palatability. In our trial, the reduced G:F ratio may reflect the progressive increase in dietary chitin (0.08% in BSF25, 0.17% in BSF50, and 0.34% in BSF100). Recent evidence also suggests that high chitin intake may reduce feed efficiency by interfering with nutrient absorption and increasing endogenous losses [19]. Thus, while BSF larvae appear to support feed intake, optimizing processing methods to reduce chitin could further enhance feed efficiency in swine nutrition. In Phase II, the linear increases in ALBand total protein indicate efficient utilization of dietary protein from BSF. ALB, which represents 50%–60% of serum protein, is synthesized in the liver and serves as a reliable marker of protein supply and nutritional status [25]. The increase observed here is more likely attributable to the high protein content and digestibility of BSF rather than dehydration, since diarrhea incidence was unaffected. Consistent with our findings, previous studies also reported increased ALB and total protein in pigs receiving BSF or other insect-based diets [26,27]. The decline in CRE observed in Phase I remained within the normal physiological range for pigs (0.6–1.6 mg/dL) [28,29] and may reflect improved nitrogen utilization and reduced muscle catabolism. Recent reports further showed that insect proteins lowered serum CRE and blood urea nitrogen compared with FM-based diets, supporting this interpretation [30]. In addition, the linear rise in glucose in Phase II could reflect altered nutrient partitioning associated with the supply of highly digestible amino acids from BSF [21]. Collectively, these results suggest that BSF inclusion modulates protein and energy metabolism in a manner consistent with efficient nutrient use, without adverse effects on systemic health.

IgA is the primary immunoglobulin at mucosal surfaces, while IgG is the major antibody in serum and interstitial fluids, both crucial for host defense [25,31]. Previous studies have shown that BSF larvae can enhance immune function through gut microbiota modulation and increased mucosal IgA [27,32], largely attributed to chitin and antimicrobial peptides [33,34]. However, in this study, serum IgA and IgG were not significantly affected, consistent with reports that mealworm or defatted BSF diets did not alter systemic immunoglobulins [26]. This discrepancy may be due to the defatting process reducing bioactive components and to limited digestibility of chitin, which depends on endogenous chitinase activity [19,35]. Therefore, while BSF larvae contain immunomodulatory factors, their systemic effects may be modest under normal health conditions, and future work should focus on mucosal immunity and local gut responses. Jin et al [26] demonstrated that full-fat BSF larvae powder rich in antibacterial peptides and chitosan enhanced gut health and reduced postweaning diarrhea in piglets, and Boontiam et al [32] similarly reported decreased diarrhea when BSF larvae meal was included up to 12%. In contrast, other studies observed no effect when defatted BSF larvae replaced conventional protein sources [21,36], likely due to lower fat and bioactive components after defatting. Moreover, the limited secretion of chitin-degrading enzymes in piglets may restrict the prebiotic benefits of chitin [19], explaining why diarrhea incidence was unaffected in the present study.

In conclusion, defatted BSF larvae can effectively replace up to 50% of FM in weaning pig diets without detrimental effects on growth performance or health parameters. These findings support the viability of BSF as a sustainable alternative protein source. Future studies should explore the long-term effects of BSF inclusion on nutrient digestibility, economic feasibility, and environmental impact across different stages of pig production.