Mealworm Frass as a Sustainable Organic Fertilizer for Greenhouse Tomato Cultivation

Abstract

Due to the environmental impact and increasing cost of inorganic fertilizers, farmers are exploring alternative fertilization strategies. Tenebrio molitor, otherwise known as the mealworm, is one of the most widely reared insect species for the production of high-quality protein for animals and humans. Mealworm frass (MF), a nutrient-rich byproduct of Tenebrio molitor cultivation, presents a viable option for organic fertilization. To investigate the fertilizer potential of frass, a greenhouse pot experiment was conducted, comparing three levels of MF (MF1, MF2, and MF3 at 20, 40, and 80 g/L soil, respectively), organic compost (ORG), and inorganic fertilizer (FERT). MF gave comparable results to FERT in terms of the measured parameters of vegetation, flowering, and production. ORG also gave comparable results to FERT as far as flowering and production but had significantly lower height compared to it. The MF3 treatment significantly improved the average fruit weight and total yield by 19.56% and 30.81%, respectively, compared to the ORG treatment. The two highest doses of MF outperformed FERT in terms of leaf and soil nutrient status, while MF1 and ORG did not differ from it. Furthermore, MF3 yielded 20% greater fruit weight than MF1. However, MF1 was comparable to FERT in fruit weight, resulting in superior fruit color. These results support reduced-input agriculture by providing data for optimizing soil fertility and nutrient management in crops. The findings of this experiment suggest that MF is a viable alternative to inorganic fertilizers and organic compost for greenhouse cultivation of tomatoes. These results highlight the potential of MF as a circular, bio-based fertilizer capable of maintaining tomato productivity while improving soil fertility under protected cultivation systems.

1. Introduction

The Green Revolution, which began in the early 1960s, served as a catalyst to meet the ever-increasing demand for agricultural goods and to feed the rapidly growing population. However, reliance on the application of chemical fertilizers, pesticides, and other agrochemicals to increase agricultural production and meet the world’s food needs has had a detrimental effect on both the environment (contamination of groundwater, release of greenhouse gases, loss of crop genetic diversity, and eutrophication of rivers and coastal marine ecosystems) and human health.

Furthermore, global warming is perhaps the greatest challenge of our time. Concurrently, livestock farming is considered to contribute significantly to this problem, as this sector alone is responsible for approximately 14.5% of total greenhouse gas (GHG) emissions worldwide [1].

Predictions suggest that the world population will reach 9.7 billion by 2050, resulting in an even greater demand for resources such as water, food, and energy. To meet this demand, food production will need to increase by 70% by 2050 and double or even triple by 2100 [2,3]. Alternative solutions, such as the development and application of new organic products that promote plant growth, could provide a means of addressing the challenges of increasing food production while protecting the environment [4,5,6].

Organic fertilizers, such as manure or compost, serve as an alternative, providing nutrients to plants in smaller quantities, but they act over a long period of time, unlike chemical fertilizers, which are immediately available to the plants. They present important advantages, such as soil improvement (microbiological, physico-chemical, and biochemical), organic matter supply, an increase in available nutrients in the soil, or less environmental damage [1,7].

Insect production has been proposed as an approach that could help solve the above problem owing to its high nutritional value. This is a new, rapidly growing industry that is attracting increasing interest as a sustainable alternative to meat and animal feed. Insects can convert feed into protein much more efficiently than many conventional animals, such as cattle or pigs, due to their ectothermic nature [8]. Insect production consumes very little water [9], and greenhouse gas emissions from this industry are estimated to be extremely low compared to other types of meat production [1,8].

Frass (insect excreta) is a valuable byproduct resulting from the mass rearing of insects and is produced in significant quantities in insect farms. Many studies have considered frass to be a promising resource for plant nutrition and plant health [10,11] that could play a pivotal role in the ongoing transition towards more sustainable, efficient, and environmentally friendly crop production systems [12]. Moreover, mealworm frass (MF) has recently been proposed as a novel food-based insect attractant [13]. Due to its favorable properties in terms of macronutrients and micronutrients, high water retention, beneficial microorganisms, and unique structure, the use of frass as a fertilizer could help reduce the use of agrochemicals. Studies have shown that using insect frass as fertilizer in fields or in pots can have beneficial effects on plant growth, photosynthetic rate, disease severity, tolerance to abiotic stresses, and the activation of plant defenses [3,14]. In addition, frass from the mealworm (Tenebrio molitor L.) has shown great potential to be used as a partial or complete substitute for mineral NPK fertilizer for the growth of barley [10], as well as stimulating earthworm activity [15]. However, Beesigamukama et al. [16] highlighted that application rates could influence the fertilizer potential of frass, and Chavez and Uchanski [11] noted that the optimal rate of frass application requires further clarification, as it may differ significantly from one study to another.

Although frass from T. molitor has been used in a few crops, such as chard (Beta vulgaris var. cicla), bean (Phaseolus vulgaris), and barley (Hordeum vulgare L.), further research is needed to assess its effects on other economically important crops [10,15]. Despite growing evidence of the agronomic potential of insect frass, limited information is available on optimal application rates in fruiting vegetable crops under controlled greenhouse conditions, particularly with respect to yield–quality trade-offs. This study evaluated the potential use of frass from mealworms (T. molitor) as an organic fertilizer in greenhouse tomato cultivation by investigating the optimal dosage and its impact on nutrient availability, plant growth, productivity, and fruit quality. The objectives of this study were to: (i) evaluate the dose-dependent effects of MF on tomato growth, yield, and fruit quality under greenhouse conditions; (ii) compare MF with conventional inorganic and organic fertilization; and (iii) assess changes in soil chemical properties and leaf nutrient status associated with frass application.

2. Materials and Methods

2.1. Pot Experiment

A greenhouse pot experiment was conducted at the Institute of Olive tree, Subtropical plants and Viticulture (HAO-DEMETER) in Chania, Crete, Greece (35°29′38.3″ N, 24°02′39.6″ E). The study was conducted in 2018, starting from 9 August 2018 and continuing until 28 December 2018. During the experimental period, the mean air temperature was 22.4 °C, and relative humidity ranged between 65% and 80%. The greenhouse was shaded with lime and maintained under natural photoperiod conditions. The experiment was conducted to evaluate the effect of different types of fertilizers (organic and inorganic) on plant growth (height, number of internodes and flowers), productivity (number of fruits and fruit weight), and fruit quality (TSS and color). Moreover, leaf and soil samplings were conducted to assess the nutrient availability for plants. The experiment included five treatments: mealworm frass at 20 g L⁻¹ soil (MF1), 40 g L⁻¹ soil (MF2), and 80 g L⁻¹ soil (MF3); organic compost at 15 g L⁻¹ (ORG); and inorganic fertilizer at 6.25 g L⁻¹ soil (FERT).

Table 1. The different fertilization treatments of the pot experiment.

Treatments	Contents
MF1	20 g mealworm frass L−1 soil
MF2	40 g mealworm frass L−1 soil
MF3	80 g mealworm frass L−1 soil
ORG	15 g compost L−1 soil
FERT	6.25 g of N:P:K L−1 soil as inorganic fertilizer

shows the different fertilization treatments. The MF1 treatment (20 g MF L⁻¹ soil) was designed to provide an equivalent nitrogen input to the inorganic fertilizer treatment (6.25 g L⁻¹ soil of 11–15–15) and the organic compost treatment (15 g L⁻¹ soil), while higher doses (MF2 and MF3) were included as exploratory treatments to evaluate the dose–response relationship but remain within the range of realistic field-scale application rates when converted to area-based units [17]. All treatments were mixed into the soil one week prior to the transplanting of tomato seedlings in the pots. Black plastic pots (of 23 cm diameter and 21 cm height) were filled with 8 L of soil, and one seedling of tomato was transplanted in the pots on 9 August 2018. Pots were irrigated to near field capacity and spaced approximately 50 cm apart to avoid shading and ensure uniform growth conditions. The treatments were arranged in a randomized complete block design with three replicates and 5 tomato plants per replication.

In addition to the basal fertilization treatments, plants received fertigation throughout the experimental period. Nitrogen was supplied as ammonium nitrate (NH₄NO₃; 105 ppm N), while potassium was applied as potassium nitrate (KNO₃), corresponding to 335 ppm K₂O in the nutrient solution. During the greenhouse cultivation period, nutrient solution composition was adjusted according to plant developmental stage. Consequently, the N:K₂O ratio progressively changed from 1:3 during early vegetative growth to 1:2 at initial fruit set and finally to 1:1.5 during advanced reproductive development. These adjustments coincided with key phenological stages of tomato growth, namely immediately after transplanting, at the fruit set of the first two inflorescences, and at the fifth inflorescence fruit set, reflecting the changing nutritional requirements of the crop [18].

Regarding plant protection, prior to the initiation of the experiment, plants were sprayed with spinosad to control key insect pests (e.g., Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) and Liriomyza spp. (Diptera: Agromyzidae)). Moreover, on 7 September 2018, an acaricide (abamectin) was applied against Tetranychus urticae Koch (Acari: Tetranychidae). A second application of spinosad was performed on 12 September 2018, followed by a Bacillus thuringiensis treatment on 20 September 2018. Fungicides were applied to manage diseases, such as powdery and downy mildew. On 12 October 2018, plants were sprayed with difenoconazole to control powdery mildew. On 26 October 2018, copper hydroxide was applied to control downy mildew.

2.2. Plant Material

The Frisco F1 (H.M. Clause Inc., Portes-lès-Valence, France), an early determinate tomato hybrid, was used in the experiment. It is characterized by high productivity and good post-harvest preservation. Moreover, it is resistant to Fusarium, Verticillium, and Nematodes. Plants were supported using strings, and axillary shoots (suckers) were removed periodically. It was primarily selected as a productive cultivar, resistant to pests and diseases, and widely cultivated by local farmers. Moreover, it can be grown in a greenhouse and is more suitable for short greenhouse cultivation.

2.3. Mealworm Frass and Organic Compost

Mealworm frass from Tenebrio molitor L. (Coleoptera: Tenebrionidae) was provided as a powder by Terra Insecta (Crete, Greece), a local company farming this insect on a large scale. The mealworms were fed exclusively on local agricultural raw materials (wheat bran and carrots) authorized by Greek and European regulations for farm animal feeds. ORG was supplied by DEDISA SA (Enterprise of Integrated Solid Waste management of Chania Regional unit, Chania, Greece).

Table 2. Chemical characteristics of mealworm frass (MF) and organic compost (ORG). Macronutrients are expressed as % dry weight; micronutrients as mg 100 g⁻¹ dry weight.

Amendment	EC	pH	C/N	C	N	P	K	Ca	Mg	Fe	Zn	Mn	Cu	B
	(dS m−1)			(%)						(mg 100 g−1)
MF	5.3	5.8	10	29.8	2.98	1.84	2.37	0.42	0.96	266.0	150.3	272.0	21.9	10.2
ORG	2.0	6.5–7.5	15	60.0	4.0	0.45	0.90	1.20	0.21	65	26	–	–	–

Values are expressed on a dry weight basis. Dashes (–) indicate values not determined.

shows selected chemical properties of mealworm frass (MF) and organic compost (ORG). Moisture content of frass was 11.0% (103 °C; 4 h). Frass was analyzed for pH and electrical conductivity (EC) in a 1:5 frass:water suspension (w/v), and for concentrations of organic C, total nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), iron (Fe), manganese (Mn), copper (Cu), boron (B) and zinc (Zn) were used.

2.4. Experimental Soil

The experimental soil, used as potting soil, was sampled (0–30 cm depth) from a cultivated land (35°29′56.0″ N 23°54′13.9″ E) in Chania, Crete (Southern Greece).

Table 3. Chemical characteristics of the 0–30 cm deep soil as measured before the beginning of the experiment.

Soil Properties	Unit	Value
Clay	%	11.4
Silt	%	23.0
Sand	%	65.6
Texture		Sandy loam
CaCO3	%	4.93
CEC	cmolc kg−1	12.5
pH		7.29
EC	dSm−1	0.159
Organic carbon	%	1.54
Total nitrogen	%	0.18
Available –P	mg/kg	72
Available –K	mg/kg	292
Available –Ca	mg/kg	3869
Available –Mg	mg/kg	101

shows selected chemical properties of the studied soil.

Soil samples were initially air-dried, crushed, and then sieved through a 10 mm and a 2 mm mesh. Soil pH and EC were determined by preparing a 1:2 and a 1:1 soil/distilled water (w/v) suspension, respectively, and the respective measurements were made with a multi-meter (Mettler-Toledo AutoChem Inc., Columbia, MD, USA) using the relevant electrodes. Soil particle size analysis was determined by the Bouyoucos hydrometer method [19]. The modified Walkley–Black wet combustion method [20] was used to determine the soil organic matter (SOM) content. Available P was measured using the Olsen method [21], while the carbonate content (CaCO₃%) was analyzed by the Bernard calcimeter method [22]. Exchangeable cations in soil (Ca, Mg, and K) were extracted using 1 N ammonium acetate at a 1:20 dilution, while the bioavailable fraction of micronutrients (Fe, Mn, Zn, and Cu) was determined by extraction with 0.005 M DTPA (pH 7.3) and quantified by ICP-OES (Optima 8300, PerkinElmer, Waltham, MA, USA). Nitrate nitrogen in the soil was measured colorimetrically using the Cd reduction method [23] and Nitraver reagent (Hach Company, Loveland, CO, USA) after extraction with 1 M KCl for 1 h.

Particle size analysis revealed that the soil was a sandy loam with 65.6% sand, 23% silt, and 11.4% clay. Chemical characterization of the soil revealed that organic C was 1.54%, total N was 0.18%, the cation exchange capacity (CEC) was 12.5 cmol_c kg⁻¹, and pH was 7.29. Available concentrations were Ca 3869 mg kg⁻¹, Mg 101 mg kg⁻¹, K 292 mg kg⁻¹, and P 72 mg kg⁻¹.

2.5. Measured Parameters

Plant height, number of internodes, and flowers were counted at 7-day intervals. Harvesting commenced 49 days after transplanting; the collected fruits were transferred to the laboratory to determine the yield and quality characteristics.

A sample of leaves consisting of the fourth or fifth fully expanded leaf from the top was collected on 18 October 2018 and 22 December 2018 from each replicate and treatment. The first sampling (18 October 2018) corresponded to the early fruiting stage, when the first clusters had set and fruit development had begun. The second sampling (22 December 2018) was performed during the full fruiting stage, when plants were actively producing and harvesting fruits. For sample preparation, the leaf analysis procedures described by Chapman and Pratt [24] were followed. Leaves were washed with distilled water and dried at 65 °C to a constant weight prior to fine grinding using a Polymix PX-MFC 90 D mill (KINEMATICA AG, Luzern, Switzerland). Ground samples (1 g) were dry-ashed (520 °C for 5 h) and then dissolved with 20% HCl with mild heating (50–60 °C) before filtering and diluting to 25 mL. Concentrations of K and Ca in plant tissue were then measured by ICP-OES (Optima 8300, PerkinElmer, Waltham, MA, USA), while P concentration was determined according to the vanadate–molybdate yellow method.

Nitrogen was analyzed using Kjeldahl mineralization [25]. A ground sample (0.05 g) was digested with sulfuric acid (H₂SO₄) at 320 °C (Kjeldahl method), and ammonium N was determined colorimetrically [26] using a Photolab 6100 VIS spectrophotometer (Wissenschaftlich-Technische Werkstätten GmbH, Weilheim, Germany).

Harvesting commenced on 27/9/2018 and finished on 28/12/2018. A total of 12 harvests were conducted during this period. The harvest dates were 27/9, 1/10, 4/10, 11/10, 18/10, 25/10, 8/11, 15/11, 6/12, 13/12, 20/12, and 28/12 of 2018. Fruit sampling for the analysis of quality characteristics was collected on 20 December 2018.

Six tomato fruits per pot were selected randomly to evaluate the fruit juice quality attributes, including total soluble solid content (TSS) (Brix). The parameters of juice quality were determined following standard methods (AOAC 2000) [27]. The selected fruits were washed with tap water and then with distilled water. The tomato juice was extracted using a juice extractor to measure TSS. Fruit color was measured simultaneously using a Muse colorimeter (Variable, Inc., Chattanooga, TN, USA).

2.6. Statistical Analysis

All statistical analyses were performed using GraphPad Prism version 8.0 (GraphPad Software, San Diego, CA, USA). Before proceeding to analysis, all datasets were examined for normality using the Shapiro–Wilk test and for homogeneity of variances using Levene’s test. When the assumptions of normality and homoscedasticity were met, differences among treatments (MF1, MF2, MF3, ORG, and FERT) were evaluated using one-way analysis of variance (ANOVA). When significant treatment effects were detected (p ≤ 0.05), means were separated using Tukey’s multiple comparison test.

Leaf nutrient concentrations (N, P, K, Ca) and soil chemical properties (nitrate-N, available P, exchangeable cations, EC, and pH) were analyzed using one-way ANOVA followed by Tukey’s post hoc test. Yield components (number of fruits per plant, average fruit weight, and total yield per pot) and fruit quality parameters (TSS and color indices) were also subjected to one-way ANOVA.

All results are presented as means ± standard error (SE). Differences were considered statistically significant at p ≤ 0.05. Graphs and summary statistics were created in GraphPad Prism 8.

3. Results and Discussion

3.1. Production, Plant Growth, and Development

Tomato growth and yield components responded differently to fertilization treatments, reflecting both nutrient availability and potential biostimulatory effects associated with mealworm frass application. Significant responses (p < 0.05) in terms of total production per plant and mean fruit weight were observed between treatments MF3 and ORG (Figure 1a,d), with MF3 increasing tomato fruit production and mean fruit weight by 30.81% and 19.56%, respectively, compared to the ORG treatment. No significant differences were observed between FERT and the other treatments (ORG and MF1–3) in terms of total yield and mean fruit weight. Furthermore, fruit weight increased by 20% in the higher frass dose (MF3) compared to the lower one (MF1).

Bilalis et al. [28], comparing organic (compost) with inorganic fertilization in tomatoes, found greater fruit weight, greater yield, and a greater number of fruits with inorganic fertilization. It appears that organic fertilization with T. molitor frass is a different case, since according to Amorim et al. [29], the use of T. molitor frass can produce yields that are comparable to or even better than those of other organic fertilizers. For example, T. molitor frass compared to poultry manure showed 93% more total carbon, 60% more total nitrogen, and an intermediate carbon/nitrogen ratio, suggesting its potential as an effective soil conditioner [30]. In fact, T. molitor frass increased zucchini yield by 62% [30,31]. In a recent meta-analysis study [32], it was also revealed that organic fertilizers can enhance tomato yield by 42.18%.

Regarding plant height and number of internodes, there were significant differences between treatments. ORG had significantly lower height compared to FERT and MF1–3. MF treatments were not significantly different compared to FERT in terms of plant height. However, MF treatments resulted in greater height from 51 days after treatment application onwards compared to FERT, while ORG had the shortest plants.

Hénault-Ethier et al. [33] observed an increase in the height of dwarf sunflower (Helianthus gracilentus) plants to which T. molitor frass was applied. Poveda et al. [3] report that MF was able to promote the growth of chard plants, stem length and width, and fresh weight of the aerial part. MF resulted in greater plant height, leaf area, and fresh and dry weight of the aerial part in zucchini [31].

The number of flowers per plant was significantly higher in treatments MF1 and MF2 compared with ORG, while the number of fruits per plant did not significantly differ. Likewise, Hénault-Ethier et al. [33] found that treatment with T. molitor frass led to a 32-fold increase in the flowering of nasturtium and doubled the flowering of dwarf sunflower (Helianthus gracilentus) and zinnia [30,34].

Generally, treatments with MF yielded results comparable to FERT, and some attributes, such as production, fruit weight, plant height, and number of flowers, were higher than ORG. This may be attributed to the rapid mineralization of MF [10], which is not common in most animal manures. Additionally, MF contained higher concentrations of micronutrients such as Fe, Zn, and Mn, which may further benefit crops, unlike conventional mineral fertilizers.

The enhanced yield observed under the MF3 treatment may be associated not only with improved nitrogen availability and rapid nutrient mineralization but also with additional biological mechanisms inherent to insect frass. MF contains chitin residues and chitin-derived compounds originating from insect exoskeletons, which have been reported to act as elicitors of plant defense responses and growth stimulation. Chitin and its derivatives may influence root architecture, enhance nutrient uptake efficiency, and stimulate plant physiological activity. Furthermore, frass application may indirectly promote plant growth through modulation of the soil microbial community, enhancing microbial activity and nutrient cycling processes. Previous studies have demonstrated that frass amendments can increase microbial biomass and stimulate beneficial soil microorganisms, potentially contributing to improved nutrient availability and plant performance. Therefore, the yield-promoting effect of MF3 likely reflects a combination of nutritional and biostimulatory mechanisms [30].

3.2. Macronutrient Concentrations in Tomato Leaves

The concentrations of N, P, and K in the leaves of tomato, as affected by the studied fertilization treatments, are illustrated in Figure 2. Throughout the experiment, significant responses to fertilization treatments were noted for nitrogen (p < 0.001), while the effects of treatments on phosphorus and potassium were not significant. Specifically, on the last sampling of the experiment, nitrogen concentrations in tomato leaves under the MF2 and MF3 treatments were significantly higher than those of FERT, ORG, and MF1, which could be attributed to root growth enhancement and an increase in nitrogen availability in the soil solution [35,36]. In general, frass treatments were as effective or better than the FERT treatment at improving leaf macronutrient concentration (Figure 2), indicating a high effectiveness of N, P, and K contained in frass. According to Foscari et al. [34], there are also significant differences in the macronutrient concentration (N, P, and K) in the different tissues (leaves, stem, and seeds) of sunflower. Interestingly, in leaves and stem tissue, N content appears higher in mixed MF and inorganic fertilization treatment, followed by MF and inorganic treatment. They also noted that both N and P leaf and stem contents were higher for plants grown in the MF treatment compared to the inorganic treatment. Moreover, Houben et al. [10] also reported that frass treatment was as effective as NPK fertilizer at improving nutrient uptake in barley and hypothesized that this may be explained by the rapid frass mineralization and the presence of N, P, and K in a readily available form that gradually supplied nutrients to plants throughout the growing period [37].

3.3. Impact of Fertilization on the Potting Soil

At the start and conclusion of the study, various soil properties such as EC, pH, and the availability of N, P, K, and Ca were carefully evaluated (Figure 3). The results revealed that the highest dose of mealworm frass (MF3 treatment) resulted in a significant increase in EC compared to the other treatments. This finding is supported by the research of Nogalska et al. [38], who similarly discovered that higher nitrogen rates corresponded with a notable rise in EC values.

Antoniadis et al. [39] also reported increased electrical conductivity (396 μS cm⁻¹) of the soil with the higher dose of frass (1%).

Cultivated tomato is moderately sensitive to salinity and tolerates soils with an EC of 2.5 dS m⁻¹ (~25 mM NaCl). Above this threshold, crop productivity declines by approximately 10% per unit increase of EC (~10 mM NaCl increase). High salinity levels (3–5 dS m⁻¹) may also interfere with the uptake of N, K, Ca, and other nutrients due to ion antagonism and reduced water uptake [40]. In the present study, the highest dose of MF increased soil EC to 0.466 dS/m, which remains well below the critical threshold of 2.5 dS/m for tomato. However, in closed irrigation systems (especially hydroponics or drip irrigation), salts from irrigation water or fertilizers may accumulate in the root zone, leading to increased EC and osmotic stress. This may lead to reduced water uptake by plants, nutrient imbalances, leaf burn, and reduced growth [41].

pH levels were significantly higher under the MF3 treatment compared to the ORG treatment. However, there were no other significant differences among the other treatments. Antoniadis et al. [39] found a slight but significant increase in soil pH in the 0.25% treatment (pH = 8.23) compared to the negative control (pH = 7.56; significant at p < 0.001); however, further frass addition resulted in the pH value returning to the control (pH at 1% frass = 7.44). Moreover, Praeg et al. [42] reported a slight increase in soil pH with MF (8.18 ± 0.1) and Jamaican field cricket (Gryllus assimilis) frass (8.59 ± 0.08) and a slight decrease with black soldier fly (Hermetia illucens) frass (7.62 ± 0.03) compared to the control soil (8.06 ± 0.05).

SOM was significantly higher under the MF3 treatment compared to other treatments, and inorganic fertilization (FERT) had the lowest organic matter content. These findings are supported by the research of Antoniadis et al. [39], who similarly discovered that SOM showed a remarkable upward trend with the addition of frass at higher doses. At 1% frass (where SOM was 3.42%), SOM increased significantly compared to the negative control (no treatment) (2.67%) and the positive control (inorganic fertilization). Organic matter improves physico-chemical and biological properties and processes within the soil and constitutes a key indicator of soil health. SOM addition leads to enhanced soil productivity and nitrogen status. Thus, organic matter amendments offer a balanced supply of nutrients, improving crop productivity, facilitating microbial activity, diversity, and growth, and thus making unavailable nutrients in the soil more accessible to plants [43]. Furthermore, available N, P, K, and Ca data showed significant differences among treatments (Figure 3). The MF2 and MF3 treatments had significantly higher nitrate nitrogen compared to FERT and ORG. Sarhan et al. [35] and Youssef and Eissa [36] suggested that MF can be beneficial for crops, as it increases root growth and nitrogen availability in the soil. In general, it is characterized by high SOM and available nutrient content (N, P, K, and Ca), with a narrow ratio of carbon to nitrogen [44].

MF3 treatment resulted in significantly higher P-Olsen content compared to MF1 and MF2 treatments, while organic (ORG) and inorganic (FERT) fertilization yielded the lowest P-Olsen content. The ORG treatment had the highest soluble calcium, but it was significantly higher only compared to the MF1 treatment. MF3 showed a significant increase in potassium compared to all other treatments. MF3 resulted in the greatest increase in nitrate, phosphorus, and potassium levels. These findings are consistent with previous studies demonstrating the fertilizing potential of frass-based amendments. According to Gondim et al. [14], the levels of P, K, Ca, Mg, and Na in the soil increased proportionally with increasing rates of MF applied to the soil. Ashworth et al. [45] reported that even low frass application (3400 kg ha⁻¹) significantly increased soil P, K, and Mg by 37, 31, and 32%, respectively, compared to inorganic fertilization (ammonium nitrate). In the same study, soil total N increased by 1 Mg ha⁻¹ in high-frass treatments, approximately three times higher than the increase observed with ammonium nitrate (0.38 Mg ha⁻¹; p  =  0.04). Moreover, after two years, soil available K increased by 55 kg ha⁻¹ in high-frass-amended plots. Similarly, Karkanis et al. [46] reported that the application of MF increased soil P concentration by up to 43.3% compared to the unfertilized control treatment and significantly exceeded levels obtained with inorganic nitrogen fertilizer. Likewise, Nogalska et al. [38] found frass to be an effective organic fertilizer, promoting growth of microorganisms and increasing N, P, Mg, K, and Na content in peat.

3.4. Fruit Quality

In terms of fruit quality characteristics, no treatment differed significantly from FERT in terms of total soluble solids (TSS) (Figure 4).

However, the MF1 treatment yielded the highest TSS value of 5.44, which was significantly higher than the corresponding values of 4.61 and 4.60 for the MF3 and ORG treatments. Consistent with the production data, the MF3 treatment yielded significantly higher fruit weight than the MF1 treatment. In other words, larger fruit yielded lower TSS. This suggests an inverse relationship between fruit weight and sugar content, as TSS consists mainly of sugars.

Furthermore, regarding the color components [47], it appears that the MF1 treatment was at a more mature stage than inorganic fertilization; the color had a higher value for component “a” (although not statistically significant), resulting in a redder color, a significantly lower value for “L” (i.e., a darker red color), and a significantly lower value for “b” (it had passed the yellowing stage).

The finding that MF1 achieved yields comparable to FERT while enhancing certain fruit quality attributes suggests a potential trade-off between productivity and compositional quality. The higher TSS and improved color indices observed under MF1 indicate enhanced sugar accumulation and ripening characteristics. However, fruit quality is a multifactorial trait that also depends on parameters such as titratable acidity, sugar–acid ratio, and volatile compounds associated with flavor perception. Although these variables were not assessed in the present study, future investigations should evaluate the effects of MF on broader biochemical quality indicators to better characterize its influence on sensory and nutritional attributes of tomato fruits.

According to Murmu et al. [48], the color of tomatoes is judged by the Hunter a/b ratio, which measures the relative amounts of red (a) and yellow (b) and is used as an indicator of ripeness [49]. A higher value of this ratio indicates a redder color in tomato fruits. The a*/b* ratio increases with a higher percentage of red color and produces a good linear regression with the stages of tomato ripening [50]. Thus, considering the a/b ratio, the MF1 treatment resulted in a significantly higher value than the FERT. Similarly, Bilalis et al. [28] reported significantly higher TSS (total soluble solids) and significantly higher value for the “a” component of color in organic treatment, while the a/b value was also higher, although the difference was not significant.

Kakabouki et al. [51] also supported that the best results in quality traits were obtained with the organic fertilization treatments (lycopene content, L*, a*, b*) but did not report statistically significant differences among treatments for TSS. In a meta-analysis study [32], it was found that tomato quality was significantly improved with the application of organic fertilizers, and specifically, TSS increased by 11.86% compared to the control.

3.5. Cost and Economic Perspectives of Mealworm Frass

Regarding the economic cost of each treatment, FERT (11–15–15) costs about 0.7 € kg⁻¹, ORG costs 0.45 € kg⁻¹, and MF costs approximately 10 € kg⁻¹. Therefore, the direct input cost of MF was significantly higher than that of the other treatments. However, the market price of MF varies considerably among producers and in the market, and it is expected to decrease in the future as insect farming expands and economies of scale are achieved. To date, no economic assessments have been published specifically for MF. Nevertheless, Foolen-Torgerson et al. [52] emphasized that the economic benefits of frass application should be assessed over multiple years rather than within a growing season. Their analysis suggests that frass use may become economically justifiable, and potentially attractive, for farmers over time, particularly as it may reduce reliance on chemical pesticides while contributing to improved soil life and quality.

Similarly, research conducted in Africa by Beesigamukama et al. [16] demonstrated promising financial outcomes for insect-derived fertilizers. Specifically, the application of black soldier fly (BSF) frass resulted in substantial input cost savings, increased crop profitability by 10–154%, and improved farmers’ gross margins by approximately 35% [16,30].

4. Conclusions

Insects are increasingly recognized as valuable components of circular bioeconomy systems due to their ability to convert diverse organic waste streams into high-quality proteins and beneficial lipids while requiring minimal water and generating fewer greenhouse gas emissions compared to conventional livestock [1,8]. As industrial insect farming expands, Tenebrio molitor has gained particular attention following its EU approval for aquaculture and its potential integration into poultry, pig, and human diets [53]. Among the major byproducts of this rapidly growing industry is MF, a nutrient-rich organic amendment that offers a sustainable pathway for recycling nitrogen, phosphorus, and potassium in line with the principles of the European Green Deal [38].

This study demonstrates that all MF doses gave comparable results to inorganic fertilization in tomato cultivation regarding vegetation (plant height, internode number), flowering (total flower number), and production (total production, mean fruit weight, number of fruits per plant). However, EC, SOM, leaf N concentration, and soil nutrient status (N-NO₃, P-Olsen, exchangeable K) were dose-dependent, indicating superior performance in higher doses. Specifically, higher MF doses (MF2 and MF3) enhanced leaf N, P, and K concentrations and improved soil nutrient status and organic matter beyond levels achieved with inorganic fertilization, while MF1 performed similarly to FERT and ORG. MF3 produced significantly greater fruit weight than MF1, and MF1, although comparable to FERT in yield, resulted in tomatoes with superior color. These observations align with previous findings that frass can act as a partial or complete substitute for inorganic fertilizers due to its rapid mineralization and readily available nutrient content [10].

The broader literature supports frass as a promising soil amendment capable of enriching soils with organic carbon and nitrogen [39] and enhancing plant growth and nutritional quality in crops such as barley and leafy vegetables [10,15]. Although Ashworth et al. [45] observed increased soil N, P, K, and SOC stocks in frass-amended systems, yield benefits were constrained by nutrient immobilization and site-specific soil histories, highlighting the need for context-dependent nutrient management. The improved tomato color observed in MF treatments is consistent with reports of higher redness, soluble solids, and acidity under organic fertilization [54].

Despite the higher initial cost, additional factors should be considered when evaluating the overall economic performance of MF. Mealworm frass has the potential to improve soil structure, enhance microbial activity, and increase long-term soil fertility, which may reduce the need for additional inputs. Furthermore, its gradual nutrient release may reduce runoff, thereby minimizing environmental impacts and reducing the economic losses associated with inefficient use. As a natural product permitted in organic agriculture, MF may also provide added value within sustainable organic production systems, with associated environmental and human health benefits.

Overall, MF emerges as a viable and environmentally beneficial alternative to inorganic fertilizers in tomato production. Its use can contribute to nutrient recycling, reduce reliance on synthetic fertilizers, and improve certain aspects of fruit quality. The results presented here can support the development of precision agriculture strategies that optimize fertilizer inputs and reduce nutrient losses and environmental impacts.

Future research should prioritize: (i) optimizing MF application rates across crops, especially in high-value organic production systems; (ii) improving frass nutrient profiles by modifying insect species or feed substrates; (iii) establishing nutrient-based (N and P concentration) rather than mass-based application guidelines to prevent over-application; and (iv) evaluating nutrient release dynamics of frass in different physical forms (loose, pelletized, composted, heat-treated) [45]. Collectively, these insights will help advance the sustainable integration of frass into modern agricultural systems and support its adoption as a reliable and circular nutrient source.

Although this experiment was conducted under controlled greenhouse conditions, the agronomic performance of MF may differ under open-field environments. Field conditions are characterized by greater variability in temperature, rainfall patterns, evapotranspiration rates, and soil heterogeneity, which can significantly influence nutrient mineralization dynamics and plant nutrient uptake [10]. Fluctuations in soil moisture and temperature may alter the rate of frass decomposition and nitrogen release, while increased soil microbial diversity and biological activity in field systems could enhance or modify nutrient transformation processes [34].

Future field studies should, therefore, evaluate MF under realistic agronomic conditions using adequately sized experimental plots arranged in a randomized complete block design. Trials should include comparisons across different soil types and climatic regions, as well as assessment of fertilization frequency (single basal application versus split applications) and dose optimization based on crop nutrient demand. Long-term monitoring of soil organic matter dynamics, nutrient release patterns, and potential accumulation effects will also be essential to determine the sustainability and scalability of frass-based fertilization strategies.