Use of Black Soldier Fly Larvae for Bioconversion of Tomato Crop Residues

Abstract

This study assessed the performance of black soldier fly larvae (BSFL) fed different tomato plant residues (fruit, leaves, and stems) at doses ranging from 100 to 350 mg/larva/day over ten days. Most doses resulted in 100% survival, except for the leaf residue at the highest dose (300 mg/larva/day), which had an 88% survival rate. Growth varied by substrate, with the highest increase observed in larvae-fed tomato fruit, followed by stems and leaves. However, no doses exceeded the control diet regarding biomass accumulation, although fruit tomatoes produced the highest wet biomass (13.71 g). Larvae-fed fruit tomatoes also showed the best performance in waste reduction index (WRI) with 7.56, substrate reduction (SR) of 75%, and a feed conversion rate (FCR) of 3.29. Furthermore, the fruit tomato was the most efficient at converting organic waste into larval biomass. This study demonstrates the potential of using tomato plant residues as a sustainable substrate for BSFL, offering an effective way to manage agricultural waste and produce valuable larval biomass.

1. Introduction

Tomato production (Licopersicum sculentum) is considered one of the most important crops worldwide. According to the Food and Agriculture Organization of the United Nations, the total production worldwide in 2020 was 186.82 million tons, where China’s primary production represented 35% of the world production [1]. Mexico is one of the world’s top three exporters of tomatoes, with a 26.8% share of world exports in 2023 [2]. The total tomato production in Mexico in 2022 was 3,324,263 tons, where Sinaloa, San Luis Potosi, Michoacan, Jalisco, and Baja California Sur contributed 53% of the total production [3]. Tomatoes are essential in Mexican cuisine and many countries and regions worldwide [4]. This fruit has a fundamental role in human nutrition due to its high content of lycopene [5], vitamin C, folic acid, potassium [6], sugar, organic acid, amino acid, carotenoids, phenolic acid, flavonoids [7], and antioxidant capacity [8]; additionally, different studies have proven that tomatoes have anti-inflammatory [9] and anticancer properties [10] and counteract diabetes conditions such as inflammation, accelerated atherosclerosis, and tissue damage [11].

Agricultural land has increased due to a higher demand for food for an increasing world population. The organic residue generation requires adequate treatment and disposal to avoid contamination [12,13]. The agrarian residues, especially those from cereal cultivation, are integrated mainly by cellulose, hemicellulose, and lignin compounds. These organic residues are generally utilized for animal feed, so their management or storage is almost immediate [14]. Conversely, tomato cultivation generates other residues such as seeds, stems, leaves, tomato pomace, peels, and cull fruit. Generally, the average residue production is 30 tons/ha per crop cycle [15]. These residues have been incorporated into animal diets [16] and used as fertilizers [17]. Additionally, researchers have been developing technology (reactors) to efficiently use tomato residue composting to obtain organic fertilizer (compost) applicable to crops [18]. Some methods used for this residue treatment or disposal are pyrolysis [19], incineration [20], or composting [21]. However, most of these residues are sent to landfills [12] without the opportunity to recover compounds or to obtain subproducts through processing that could be useful in other production chains. However, the waste management and degrading processes of these organic residues imply a significant amount of time and high energy costs.

In some cases, when organic waste is added to animal feed, the results are favorable. Still, their effects are variable [22]; for example, a daily ingestion of 1.5 kg of fresh fruit tomatoes in goats causes soft feces, so its consumption is limited before causing adverse effects on their physiology [23]. Conversely, when tomato residues are composted, the emission of greenhouse gases represents a significant challenge for improving this technology [24]. The high costs of the biocompound extraction methods for these residues are another problem due to specialized equipment, solvents, and high energy consumption. There are also high risks of environmental contamination and health problems due to toxic solvents [25].

The black soldier fly (BSF) (Hermetia illucens) is an insect of the Stratiomydae family. The larvae of this fly are considered one of the most promising organic waste treatment methods due to their metabolism. This larva treats organic waste with a high reduction efficiency and a shorter bioconversion time than other biological methods. In addition, larvae are a source of protein and lipids for feeding different animal species [26]. In this sense, new biological tools have emerged to treat agricultural residues that are difficult to treat by other processes. The black soldier fly larvae (BSFL) is an interesting organism that can feed from organic matter of animal or vegetal origin. For example, manures can be reduced to 43.85% and 42.70% (WRI; waste reduction index) and have a bioconversion rate (BR) of 1.20 and 0.92 for cow and pig manure, respectively [27]. Other examples include fruits and vegetables such as apples (WRI:0.53–0.59 and BR: 2.5–3.1%), red cabbage (WRI: 0.96–1.15 and BR: 7.6–7.9%), red onions (WRI: 1.46–1.52 and BR: 8.4–9.0%), pumpkin (WRI: 1.26–1.31 and BR: 6.9–7.7)%), and spinach (WRI:0.624–1.069 and BR:0.3–3.0%) [28]; human feces (waste reduction (WR): 39.1–48.6% and BR: 18.8–27.7% both in DM) [29]; food waste (BCR: 13.9 and waste reduction (WR): 55.3% both in dry matter) [30]; and sludge [31] from 0.2 to 2.2% of the BCR and 13.2 to 49.2% of substrate reduction [30]. In some cases, combinations of different substrates could be the best option for converting organic waste [32]. The time required to reduce and process organic matter by BSFL is less than that required by other processes, such as composting or landfilling. Also, it inhibits the growth of pathogenic microorganisms by your gut microbiota, and the secretion of antimicrobial compounds can kill bacteria, reducing the release of foul odors and gases [32,33].

Studies with BSFL to treat organic residues have reported data about the performance of different organic wastes and their bioconversion capacity. In Broeckx et al. [34], the treatment of tomato leaves was inefficient, with a 100% mortality, making it impossible to valorize the waste with this biological treatment. Additionally, the maximal mean larval weight was 3.1 mg; thus, there are no data for the FCR, the WR, and bioconversion efficiency [34]. Another study mentions that adding water to the standard diet is one of the most important elements during BSFL cultivation since it provides the necessary moisture for the larvae [35,36]. On the other hand, another study has reported that insect farming requires less land and water compared to traditional livestock production [37]. This study with tomato waste does not contemplate biological treatment before feeding the larvae. Instead, it aims to study the bioconversion of tomato plant residues, including fruits, leaves, and stems, using BSFL as an alternative method for managing and using tomato residues.

2. Materials and Methods

2.1. Black Soldier Fly Larvae Production

Eggs of the black soldier fly were obtained from one pilot-scale plant at the Bioengineering Lab of the Autonomous University of Queretaro. The eggs were hatched with a standard diet consisting of chicken feed, bran, and water (1:2:7 w/w) in containers of 34.6 × 21 × 12.4 cm in a hatching room with a temperature of 30 ± 2 °C and 80 ± 10% relative humidity. Six-day-old larvae were collected and sieved with a sieve with 1.8 mm square holes to select a uniform size; ninety larvae were weighed one by one in an analytical balance (Model Precisa LX220A, Precisa Gravimetrics AG, Dietikon, Switzerland) to have a homogeneous weight and to know the initial larval biomass weight; then, 90 larvae were placed in each of the doses of each residue and the control, all in triplicate. The initial dry weight was obtained by dehydrating the larvae at 65 °C for 24 h or until a constant weight was achieved, with triplicate measurements.

2.2. Substrate Preparation

The organic residues of the tomato plant of the variety Saladette (rootstock: Multiform, graft: Macizo) were collected from a greenhouse crop at the Autonomous University of Queretaro in the tenth month of tomato production. For leaves, the residue was collected after pruning the tomato plants; fruits were collected during pruning if they were deemed unattractive for sale due to characteristics such as color, shape, scars, or apical necrosis. Residual stems were collected at the end of crop production. Residues were stored in a −12 °C freezer until use and, after thawing at ambient temperature, were crushed in a combustion engine silage shredder (Model Azteca MO010 (7 hp), Molinos Azteca y Juper, S.A de C.V., Ocotlán, Jalisco, México) to obtain small pieces. Stem and leaf fragments measured less than 10 cm. Crushing of the fruits was necessary to break the skin and homogenize the residue.

2.3. Diets and Experimental Design

Six treatments were established for each substrate (crushed leaf, stem, and fruit) consisting of 100, 150, 200, 250, 300, and 350 mg substrate/larva/day according to [38], adding the last two doses; each treatment was carried out in triplicate in the same way as the control, which consisted of the Gainesville diet. The amount of substrate placed in each container was calculated for ten days of experimentation. Each substrate was placed in a circular plastic container (5.9 cm radius, 500 mL volume) covered with a lid with 35 holes drilled using a 3/8-inch drill bit. Ninety larvae were placed on the substrate, and then, the containers were placed in a microbiological incubator (Model Thermo Scientific Heratherm incubator IGS100, Thermo Fisher Scientific, Waltham, MA, USA) for 10 days according to Vodounnou et al. [39] at 30 °C according to Shumo et al. [40], no relative humidity control, and photoperiod of 0L:24D (light–dark) according to Liland et al. [41].

2.4. Larval Performance

The initial dry matter content of fruits, leaves, stems, control diet, and larvae was determined in an oven at 65 °C until a constant weight was reached separately and in triplicate. The ninety larvae from each treatment were manually collected and wiped with a paper towel according to Loiotine et al. [42] every two days to remove moisture from the larvae and then weighed one by one. After weighing, the larvae were reintroduced into their respective containers and returned to the incubator. In the final trial, on day ten of experimentation, the survival rate and growth rate were calculated using Equations (1) and (2), respectively, according to Addeo et al. [37]; larvae were separated from the residual substrate; and both were placed in a freezer at −12 °C and, 24 h later, dehydrated in an oven (Model Oven Dynamica air performance AP-120, Frailabo, Paris, France) at 65 °C until a constant weight was reached.

$$Survival rate \left(\%\right)={\displaystyle \frac{Number of larvae harvested}{Initial number of larvae }}\times 100$$

(1)

$$Growth rate \left(g/d\right)={\displaystyle \frac{Biomass weight gain}{ trial duration }}$$

(2)

2.5. Waste Reduction and Conversion Efficiency

The conversion efficiency of reducing tomato plant organic residues by BSFL was calculated using the following equations: waste reduction index (WRI) (Equation (3)), feed conversion rate (FCR) according to Siddique et al. [32] (Equation (4)), bioconversion rate (BR) (Equation (5)), efficiency of conversion of digested feed (ECD) (Equation (6)) according to Lalander et al. [30], and substrate reduction (SR) (Equation (7)) according to Gold et al. [29]. All calculations were based on dry mass.

$$WRI={\displaystyle \frac{Substrate added-residual substrate/substrate added }{Trial duration }}\times 100$$

(3)

$$FCR={\displaystyle \frac{Substrate added-Residual substrate}{Final biomass }}$$

(4)

$$BR \left(\%\right)={\displaystyle \frac{Final biomass }{substrate added }}\ast 100$$

(5)

$$ECD \left(\%\right)={\displaystyle \frac{Biomass gain }{substrate added-residual substrate}} \ast 100$$

(6)

$$SR \left(\%\right)={\displaystyle \frac{distributed substrate-residual substrate}{ distributed substrate}}\ast 100$$

(7)

2.6. Bromatologial Analysis

Bromatological analysis was performed on the residual substrates and larvae of the 350 mg dose and the control, all in triplicate. The 350 mg dose was analyzed because it produced the highest total larval biomass accumulation; after dehydration, a sample was taken for each bromatological analysis. Total fats were extracted using microwave-assisted extraction according to method 3546, proposed by the Environmental Protection Agency [43]. The total protein was quantified by the Kjeldahl method with previous digestion in a Digesdahl digestion apparatus (Hach Company, Loveland, CO, USA); spectrophotometry UV-Vis (Model DR6000, Hach Company, Loveland, CO, USA) quantified total nitrogen (HACH method 8075) using different conversion factors for samples: 4.76 for larva [44], 5.84 for tomato [45], 4.23 for leaf and stem [46], and 6.25 for Gainesville diet [47]. Carbohydrate content was determined by the Antrone method according to Hewitt [48], and concentrations were calculated by absorbance at 630 nm using a microplate spectrophotometer (Model Thermo Scientific Multiskan GO, Thermo Fisher Scientific, Vantaa, Finland). To determine humidity, an oven (Model Oven Dynamica air performance AP-120, Frailabo, Paris, France) was used at 100 °C for 12 h or until constant weight according to PROY-NOM-211-SSA1-2002 [49]. For ash determination, a muffle (Model Felisa FE-360, Zapopan, Jalisco, México) was used at 550 °C for 8 h according to NMX-F-607-NORMEX-2020 [50].

2.7. Statistical Analysis

The statistical analysis section was rewritten. Statistical data analysis was conducted using Statgraphics Centurion software (Version XVI). A normality test for all measured variables was performed using the Shapiro–Wilk test (p < 0.05). Additionally, homogeneity of variances was assessed using Levene’s test (p < 0.05).

A Kruskal–Wallis test was applied for all variables with non-parametric data to determine whether differences existed between treatments. If significant differences were found, an ANOVA (p < 0.05) was conducted, followed by a Fisher’s LSD test (p < 0.05) to identify the best treatment for non-parametric data.

For data meeting the assumptions of normality and homogeneity, an ANOVA (p < 0.05) was performed, followed by Tukey’s test (p < 0.05) to examine differences between treatments. This latter test was applied exclusively to the variable of larval weight gain.

3. Results

3.1. Larval Performance

Residues from different parts of tomato plants are suitable for BSFL feeding. The first challenge for this organism is to be fed without a high mortality rate. The four doses of fruit and stem substrate (100, 150, 200, and 250 mg) and the 200 mg/larva/day dose of leaf substrate in this study resulted in 100% survival. On the other hand, a dose (300 mg/larva/day) of the tomato leaf substrate showed the lowest survival (88%), statistically different from the control, which presented a survival of 97%. The growth was different for each substrate. For fruits and stems, growth increased as the dose increased. For fruit and stem substrates, growth increased as the dose increased. For fruit substrate, only the dose of 350 mg/larva/day grew 15.24 mg/day, similar to the control (15.68 mg/day), unlike other doses with lower daily growth values than the 300 mg dose and control. The minimum and maximum values for the stem substrate were 3.65 and 12.75 mg/day for 100 and 350 mg/larva/day, respectively. In this substrate, all the doses had lower values than the control and presented the smallest values compared to the fruit and leaf substrates. All growth values were lower than the control for leaf substrates, ranging from 6.74 to 12.25 mg/day for 100 and 350 mg/larva/day, respectively. The highest values for this variable were obtained at the 350 mg/larva/day dose for the three substrates.

The accumulation of biomass in the three substrates was different. The highest value was presented in the 350 mg/larva/day dose of tomato fruit, followed by the stem and finally leaf substrate, both in the same doses. However, no doses showed higher accumulation values than the control (

Table 1. Survival, growth rate, and biomass accumulation of black soldier fly larvae reared in different substrates of tomato plants for 10 days.

Substrate	Doses (mg/Larva/Day)	Survival Rate (%)	Growth Rate (mg/Day)	Wet Total Biomass (mg)
Fruit	100	100.0 ± 0.0 a	7.3 ± 0.7 a	6609.4 ± 217.0 a
	150	100.0 ± 0.0 a	9.2 ± 0.5 b	8317.6 ± 153.5 b
	200	100.0 ± 0.0 a	10.9 ± 0.0 c	9866.7 ± 21.9 c
	250	100.0 ± 0.0 a	12.4 ± 0.5 d	11,245.4 ± 165.4 d
	300	99.0 ± 1.7 a	13.9 ± 0.3 e	12,562.3 ± 115.9 e
	350	96.6 ± 3.6 a	15.2 ± 0.2 f	13,716.8 ± 88.9 f
Control		97.0 ± 3.8 a	15.6 ± 0.2 f	14,112.9 ± 72.7 g
Stem	100	100.0 ± 0.0 a	3.6 ± 0.6 a	2191.5 ± 186.1 a
	150	100.0 ± 0.0 a	6.2 ± 1.0 b	5626.7 ± 321.6 b
	200	100.0 ± 0.0 a	9.2 ± 1.6 c	8363.4 ± 483.0 c
	250	100.0 ± 0.0 a	9.3 ± 0.1 c	8382.9 ± 55.7 c
	300	98.6 ± 4.0 a	10.1 ± 0.8 c	9110.7 ± 248.4 c
	350	100.0 ± 0.0 a	12.7 ± 0.1 d	11,475.8 ± 36.7 d
Control		97.0 ± 3.8 a	15.6 ± 0.2 e	14,112.9 ± 72.7 e
Leaf	100	98.0 ± 1.7 ab	6.7 ± 0.3 a	6067.2 ± 110.3 a
	150	94.6 ± 4.0 ab	9.6 ± 2.8 b	8696.1 ± 841.9 b
	200	100.0 ± 0.0 b	10.9 ± 0.3 bc	9890.7 ± 97.7 bc
	250	98.0 ± 1.7 ab	11.4 ± 0.9 bc	10,299.7 ± 285.9 bc
	300	88.0 ± 1.7 a	9.9 ± 1.3 bc	8992.2 ± 399.0 bc
	350	94.3 ± 5.1 b	12.2 ± 1.5 c	11,033.8 ± 453.1 c
Control	200	97.0 ± 3.8 ab	15.6 ± 0.2 d	14,112.9 ± 72.7 d

Mean values ± standard error with superscript letters indicating a significant difference at p < 0.05 according to Fisher’s LSD test (n = 3). Survival and growth rates are calculated as the averages of the triplicates for each dose. The doses correspond to milligrams of wet feed per larva per day.

).

3.2. Biomass Generated by Larvae Fed on Tomato Plant Substrates

3.2.1. Tomato Fruit

The maximum larvae weight in the tomato fruit substrate was recorded at 350 mg/larva/day and the minimum at 100 mg/larva/day. This pattern may be related to the amount of food available in the container; specifically, lower doses led to decreased larvae weight by the end of the experiment. In contrast, as food doses increase, larvae weight increased over time.

Weight consistently rose from day zero across all doses, including the control group. Notably, the control group showed a significant weight gain on the second day, distinguishing it from the other doses. By the fourth day, the weight at 100 mg/larva/day stabilized and remained constant until the end of the experiment, with no significant difference among doses. Weight stabilization began on the sixth day for the 350 mg/larva/day dose, with minimal accumulation until the experiment concluded (remaining above 0.15 g). This weight was not different from the control group, although both differed from the other doses (Figure 1).

3.2.2. Tomato Leaves

The maximum weight achieved for larvae fed with tomato plant leaves was 350 mg. From day zero to day two, there were no differences in larvae weight among the various doses. However, the control group exhibited exponential weight gain on the third and fourth days. On the fourth day, the weights of all the larvae from all the doses of tomato leaves were the same, and there was no statistically significant difference. As observed with the tomato fruit substrate, a consistent relationship exists between the amount of food and the larval weight gain. The latter was evident as the weight of the larvae increased in proportion to the food offered during the experiment. The maximum average weight of larvae from this substrate was below 0.15 g, which is lower than the weight recorded from those fed with tomato fruit. No dose achieved the larval weight of the control diet (Figure 2).

3.2.3. Tomato Stems

The weight of larvae fed with tomato stems was similar to that of those fed with tomato fruit. All doses gradually gained weight from the second day. The 100 mg/larva/day dose was the first to stop gaining weight, recording the lowest value of all doses. The final weights of the other doses were in ascending order: 150, 200, 250, 300, and 350 mg/larva/day and the control. All doses statistically differ from the control (Figure 3).

3.3. Performance of Black Soldier Fly Larvae in Plant Tomato Waste

Table 2. The waste reduction index (WRI), substrate reduction (SR), the feed conversion rate (FCR), the efficiency of conversion of digested feed (ECD), and the bioconversion rate (BR) from the rearing of BSFL on various tomato plant substrates.

Substrate	Doses	WRI	SR (%)	FCR	ECD (%)	BR (%)
Fruit	100	7.5 ± 0.4 b	75.2 ± 4.8 b	3.2 ± 0.4 a	27.4 ± 4.1 b	23.0 ± 3.0 d
	150	7.6 ± 0.4 b	76.8 ± 4.0 b	3.3 ± 0.1 ab	24.7 ± 1.0 b	20.4 ± 0.3 c
	200	7.3 ± 0.5 b	73.8 ± 5.0 b	3.9 ± 0.4 ab	23.9 ± 3.0 b	18.8 ± 1.0 bc
	250	7.3 ± 0.4 b	73.9 ± 1.7 b	4.0 ± 0.2 ab	23.4 ± 1.5 b	18.2 ± 0.8 bc
	300	7.4 ± 0.3 b	74.0 ± 3.0 b	4.1 ± 0.4 b	23.3 ± 2.4 b	18.0 ± 1.2 b
	350	7.0 ± 0.8 b	70.9 ± 8.2 b	3.9 ± 0.5 ab	24.7 ± 3.3 b	17.7 ± 0.4 b
Control	200	5.4 ± 0.1 a	54.5 ± 1.2 a	6.3 ± 0.5 c	15.6 ± 1.3 a	8.6 ± 0.6 a
Stem	100	2.7 ± 0.2 a	27.7 ± 2.1 a	6.8 ± 1.2 ab	12.7 ± 2.6 ab	4.0 ± 0.3 a
	150	3.3 ± 0.6 abc	33.0 ± 6.6 abc	7.5 ± 2.5 ab	12.8 ± 3.8 ab	4.5 ± 0.7 a
	200	3.3 ± 0.3 abc	33.5 ± 3.3 abc	5.9 ± 1.0 a	16.2 ± 2.7 b	5.7 ± 0.7 ab
	250	3.0 ± 0.2 ab	30.3 ± 2.5 ab	6.7 ± 0.5 ab	14.1 ± 1.2 ab	4.5 ± 0.0 a
	300	3.5 ± 0.0 bc	35.0 ± 0.2 bc	8.2 ± 0.4 b	11.6 ± 0.5 a	4.2 ± 0.2 a
	350	3.6 ± 0.3 c	36.7 ± 3.0 c	7.6 ± 0.3 ab	12.6 ± 0.5 ab	4.8 ± 0.3 a
Control	200	5.4 ± 0.1 d	54.5 ± 1.2 d	6.3 ± 0.5 ab	15.6 ± 1.3 b	8.6 ± 0.6 c
Leaf	100	3.7 ± 0.4 abc	37.3 ± 4.4 abc	4.0 ± 0.9 a	24.2 ± 6.8 c	9.4 ± 1.3 c
	150	4.0 ± 0.3 de	40.4 ± 3.8 de	4.6 ± 1.4 a	21.8 ± 7.1 bc	9.0 ± 2.0 bc
	200	4.6 ± 0.2 d	46.3 ± 2.4 d	5.6 ± 0.8 ab	17.4 ± 2.9 abc	8.3 ± 0.9 bc
	250	4.0 ± 0.1 bcd	40.2 ± 1.0 bcd	5.6 ± 0.6 ab	17.3 ± 1.7 abc	7.2 ± 0.8 ab
	300	3.4 ± 0.5 ab	34.4 ± 5.5 ab	6.6 ± 0.7 b	14.4 ± 1.8 a	5.2 ± 1.2 a
	350	3.3 ± 0.3 a	33.9 ± 3.0 a	6.3 ± 0.8 b	15.3 ± 2.0 ab	5.4 ± 0.5 a
Control	200	5.4 ± 0.1 e	54.5 ± 1.2 e	6.3 ± 0.5 b	15.6 ± 1.3 ab	8.6 ± 0.6 bc

The data are presented with standard deviation (n = 3). A super letter indicates a significant difference according to Fisher’s LSD test, with p < 0.05.

presents the performance of BSFL rearing with different doses and substrates of tomato plants. High values of the waste reduction index (WRI) indicate greater efficiency in matter reduction, with organic matter being converted into larval biomass. Larvae exhibited different behaviors in this variable; the highest WRI values were found in tomato fruit, from 7.09 to 7.68. Leaf substrate followed, with WRI values ranging from 3.39 to 4.63, where the lowest value was at the 350 mg dose and the highest at 200 mg. The lowest WRI was observed in the stem substrate, specifically at the 100 mg dose, which had lower values than the other doses. Only the treatments with tomato fruit substrates presented higher WRI values than the control; however, for stem and leaf substrates, the values were lower than the control.

The highest substrate reduction values were observed in tomato fruit (ranging from 70 to 75%), followed by leaf (from 33 to 46%), and finally stem (from 27 to 36%) substrates. The feed conversion rate (FCR) is calculated by dividing the feed intake weight by the weight the larvae gained. A lower FCR value indicates higher efficiency. In this study, tomato fruit substrates exhibited the best FCR (from 3.29 to 4.13), followed by leaves (from 4.04 to 6.97). The lowest FCR values were recorded for tomato stems (5.90–7.65). The fruit and leaf substrates of the tomato plant were better than the control. The efficiency of converting digested food (ECD) into body tissue measures the larvae’s ability to utilize food for growth. Higher values indicate that BSFL can convert organic waste into biomass. Fruit residue had the best values, followed by leaves, while stems were the least effective for biomass conversion.

The bioconversion rate refers to the dry larval biomass produced per waste unit. Fruit substrate exhibited the highest values, making it the most effective for producing dry biomass per unit of waste. Leaves rank second in biomass production, while stems are the least effective for generating BSFL biomass (Table 2).

3.4. Proximal Substrates and Diet Composition

The carbohydrate concentration in organic waste was lower than in the diet control, which consisted of 55% carbohydrates. Tomato fruit stems and leaves followed in carbohydrate content. Protein levels in the tomato fruit, leaves, and stems were also lower than in the diet control. Lipid concentrations were similar in diet control and tomato waste, followed by stems, with the leaves showing the lowest levels. Ash content was the highest among the leaves with 37.21%, while the diet control had the lowest with 10.48%. After composting tomato substrates with BSFL, variations in component concentrations were noted. Carbohydrates decreased in all substrates, with a significant decrease in tomato waste from 15.97% to 2.33%. Protein levels decreased in the stem and fruit substrate, while diet control and leaf residue increased. Lipid content increased in the tomato fruit and leaf waste, while other residues decreased. All the ash content values were higher at final composting by larvae for tomato waste and diet control, except in stem tomato waste (

Table 3. Initial and final proximal analysis of tomato substrates and Gainesville diet used to feed black soldier fly larvae per ten days.

Component	Substrate	Initial	Final
Carbohydrates (%)	Gainesville	55.85 ± 5.34 c	9.43 ± 2.89 b
	Leaf	4.18 ± 1.93 a	2.06 ± 0.00 a
	Stem	14.39 ± 5.75 ab	13.02 ± 5.55 b
	Fruit	15.97 ± 2.53 b	2.33 ± 0.20 a
Protein (mg/g)	Gainesville	16.26 ± 1.22 a	19.78 ± 2.84 c
	Leaf	10.09 ± 0.08 b	12.45 ± 0.55 b
	Stem	4.61 ± 0.84 c	2.35 ± 0.26 a
	Fruit	12.18 ± 0.97 d	10.85 ± 0.01 b
Lipids (%)	Gainesville	4.10 ± 0.20 c	1.37 ± 0.30 b
	Leaf	2.49 ± 0.16 b	3.13 ± 0.24 c
	Stem	1.48 ± 0.25 a	0.48 ± 0.06 a
	Fruit	4.71 ± 0.24 d	6.98 ± 0.29 d
Ashes (%)	Gainesville	10.48 ± 0.07 a	30.56 ± 4.81 c
	Leaf	37.21 ± 2.86 c	40.71 ± 1.64 d
	Stem	12.20 ± 1.23 ab	11.37 ± 1.52 a
	Fruit	13.92 ± 3.73 b	19.17 ± 2.47 b

Mean values (n = 3) with superscript letters indicating significant difference at p < 0.05 according to Fisher’s LSD test.

).

3.5. Proximal Larval Composition

The composition of larvae varied significantly among fruit, stem, and leaf substrates compared to the control. The carbohydrate content in the larvae was highest in the stem substrate, while the leaf and fruit substrates showed no significant difference with the control. Regarding protein, leaf and fruit substrates exhibited higher concentrations than the control, and the lowest protein levels were present in the stem substrate. The lipid concentration was higher in the fruit substrate, with no statistical significance compared to the control, and the lipid concentration in leaf and stem substrates was lower than in the control. Finally, the ash content was highest in the larvae obtained from stem feeding, followed by leaf, control, and tomato fruit feeding (Figure 4).

4. Discussion

4.1. Larval Performance

The survival rate obtained from larval rearing in different tomato plant substrates differed from those reported by other authors. The survival in the leaf substrate was higher in all doses than in other studies. In this investigation, the survival was from 88% to 100% compared to other research, which had a 0% survival rate in leaf tomato substrate and a maximum larval weight of 3.1 mg [34]. Significant survival rates were obtained in the stem, followed by fruit and leaf substrate, similar to other substrates such as vegetables and fruit mix (88%), fruit (80%), brewery by-product waste (90%) [51], pumpkin (98%) [28], and values higher than larvae reared in restaurant and fruit waste with 76.86% survival [52] and red onion with 76% [28]. The leaf substrate, which exhibited the highest number of doses with mortality, recorded the lowest survival rate at 88%. These findings are consistent with a study in which plant-derived fibers (cellulose) were used as a substrate for BSFL cultivation, showing survival rates ranging from 85% to 94% while also enhancing the performance of the BSFL [53]. On the other hand, plant waste, particularly leaves, results in a higher mortality rate compared to waste mixtures or other food sources. For instance, larvae fed with spinach have been reported to exhibit survival rates as low as 2.8% [28].

The growth rate presented in this study is directly proportional in all substrates to the dose. Higher values were obtained in fruit from 7.34 to 15.24 mg/day and leaves from 6.74 to 12.25 mg/day, while in the stem substrate, the value was the lowest with 3.65 to 12.75 mg/day. Conversely, according to the literature, larval performance can be significantly modified by the nutritional content of the substrates, such as protein and fat content, and the C/N ratio [54]. This study obtained high larval yield in 350 mg/larva/day in fruit, stem, and leaf substrates with a biomass production of 13.716 g, 11.475 g, and 11.033 g, respectively. The differences in biomass production may be due to the composition of the substrate from the tomato plant. In the case of the stem (lignocellulosic compounds), the lowest biomass production data were obtained; on the other hand, the survival of the larvae in this type of substrate is also affected, as in the case of larvae fed with leaves of the tomato plant, where the highest mortality values were obtained. A similar case was obtained by Wang et al. [55] in larvae fed with corn straw (88%) and alkaline hydrogen peroxide-pretreated corn straw (81%), and tomato fruit presented higher values in biomass accumulation, similar to BSFL reared in locust waste [56] and a mix of vegetable and butchery waste [37].

4.2. Generation of Biomass by Larvae Fed on Tomato Plant Substrates

Black soldier fly larvae are organisms used to convert organic matter into biomolecules such as lipids, carbohydrates, and proteins [32]; the composition of the organic food matter in larval rearing influences their metabolism as well as temperature [40], moisture [57], oxygen, density [58], among others, which allow or do not allow for the generation of biomolecules. The total biomass obtained in this study differed in the three tomato substrates. The highest weight was presented in tomato fruit waste, with an average weight of the larva of 0.16 g; this result is similar to the one reported by Jucker et al. [56] at 10 days in larvae reared in cricket waste.

Another interesting finding of this study is the ability of BSFL to feed on substrates high in lignocellulosic compounds, such as stems and leaves, or high in water content, such as tomato fruit. It is noteworthy that the larvae cultured on tomato fruit substrates spent most of the time feeding on a substrate with high water content, easily exceeding 80% moisture in the food without hurting mortality or biomass accumulation; this is in contrast to other studies that have found humidity levels close to 80%, which is considered to be optimum or maximum for larval growth [57,59]. In contrast, this study found that the substrate with the highest moisture content was the tomato fruit (93%); the leaf and stem substrate had the lowest moisture content, 82 and 72%, respectively. However, moisture levels did not influence the accumulated biomass across all three substrates. This aligns with the findings of Bekker et al. [57], who reported no adverse effects on black soldier fly larvae’s metabolic performance within a 45% to 75% moisture range.

On the other hand, larvae exhibit slower growth and accumulate less biomass when feeding on plant-derived materials, such as lignocellulosic substrates, compared to those with lower lignocellulose content and higher levels of carbohydrates, lipids, and proteins. In this study, growth and biomass accumulation were more pronounced in tomato fruit substrates rich in carbohydrates, lipids, and proteins. However, recent research has demonstrated that the gut microbiota of BSFLs can degrade lignocellulosic components found in specific substrates [60,61]. In contrast, waste from kitchens, supermarkets, or restaurants, which is higher in carbohydrates, lipids, and proteins, provides more easily accessible nutrients, promoting better BSFL growth [62].

4.3. Bioconversion of Organic Waste of Tomato Plant

The bioconversion variables of BSFL are influenced by environmental factors, rearing conditions, and substrate composition [36]. The waste reduction index (WRI) showed distinct patterns in this study. Higher substrate doses resulted in a higher WRI, indicating that the larvae were more effective at converting the substrate into larval biomass. The highest WRI values were observed in fruit substrate, with 7.68 for doses of 150 mg. This is related to the amount of water in the substrate since the higher the amount in the doses, the lower the WRI value, resulting in a negative effect on larval feeding, as demonstrated in another study by Ribeiro et al. [28]. The highest WRI values for stems were 3.67 for 350 mg/larva/day and 4.63 for 200 mg/larva/day. Leaves are comparable to those of other studies with different substrates, such as cricket waste, where values ranged from 1.32 to 4.50 [56]. Pig manure mixed with roadside weeds presented lower values than the doses of the three substrates in this experiment; however, fast food residues presented similar results to fruit tomato substrate but medium-dose values of leaf and stem substrates of tomato plants [63,64].

Tomato fruit substrates showed the highest substrate reduction, followed by leaves with 27% to 36% and stems with the lowest from 27 to 36%; this may be related to the availability of compounds, such as carbohydrates, that the larvae can consume and bioconvert into biomass. In the case of the stem, although it contains a high amount of carbohydrates, these are not accessible to the larvae, preventing them from feeding on them. The carbohydrates may be bound in lignocellulosic compounds, as only 9.5% of the total carbohydrates in the stem were reduced compared to the significant reductions of 85% in tomatoes and 50% in leaves. On the other hand, BSFL larvae can degrade lignocellulosic compounds. In this study, the leaf and stem substrates showed less weight reduction and gain than the tomato fruit. However, a 33% reduction in the stem substrate was observed, suggesting that the larvae can adapt to different substrates by releasing substances that degrade lignocellulosic compounds. This observation supports previous findings that BSFL enhances carbohydrate metabolism when feeding on lignocellulosic waste. This is facilitated by enzymes, such as β-glucosidase and α-glucosidase, produced by microorganisms in the larvae’s gut, which are key to the degradation of lignocellulose [55]. These results align with other studies, where fruit waste had a conversion efficiency of 70.8%, fruit–vegetable waste of 65%, and winery and brewery by-products of 53% and 42%, respectively, all higher than stem waste [51].

Fruit waste obtained a higher FCR value, 3.29 to 4.13, similar to other substrates such as avocados, fecal sludge, and kale [32]. Those values are considered suitable for the type of waste the larvae feed on. However, the results of the FCR obtained in all the leaf and stem substrate doses were higher. Those values are considered inefficient for food conversion by the BSFL and comparable with other substrates of the same nature. This may be due to the substrate composition since leaves have a high concentration of cellulose, hemicellulose, and lignin and even more in the stems, preventing its ingestion by the BSFL [32]. The ECD values presented in this study are comparable with those of other authors. For example, in research using ground coffee to feed BSFL, a 5% ECD was obtained, whereas, in this study, this value was higher in all doses and substrates [65]. However, pumpkin, red cabbage, and red onions have presented values similar to tomato waste, with values above 20% [28]. On the other hand, the values of BR for stem substrates were lower than that of leave and fruit; this result is comparable with other results with similar substrates, such as corn stover (1.4%), fermented maize stover (5.0%), mushroom root waste (5.4–5.6%), and soybean curd residue (5.0–11.0), and these values may be due to the available food and components for the larva’s ingestion and the amount of cellulose, hemicellulose, and aligning, which cause low larval performance. Moreover, tomato leaf substrate results are similar to fruit and vegetable waste, vegetable canteen waste, abattoir waste, and poultry manure, among others [66]. On the other hand, it has been found that when vegetables are included in the diet BSFL, the levels of hydroperoxides and antioxidant capacity in the hemolymph of the larvae are reduced, thus reducing oxidative damage in the larvae [37], so the inclusion of leaves and stems in the fruit substrate could be a viable option for the joint treatment of the three substrates by the BSFL.

4.4. Proximal Substrates and Larval Composition

The percentage of carbohydrate reduction across all substrates, including the control, was observed to decrease, with the most significant reduction occurring in the fruit substrate (85%), followed by the control (83%), leaves (50%), and finally the stems (9.5%). Studies have shown that larvae utilize carbohydrates to generate fatty acids, with the concentration directly related to the amount of carbohydrates available in the substrate [67]. In this case, the fruit substrate had the highest concentration of carbohydrates, while the leaf substrate contained the lowest. In the case of the stem, carbohydrates may be bound to lignocellulosic compounds, which are not accessible to the larvae for consumption (Table 1). This is consistent with the results showing the highest total fat content in the fruit substrate, followed by the control, leaf, and finally stem substrates (Figure 4).

On the other hand, the protein content in the substrate influences the protein levels in the larvae. However, using vegetable proteins to replace animal proteins reaches a certain threshold, as adding a protein source to the substrate does not always significantly impact larval development or pupation [68]. The results of these studies indicate that the highest protein content in the larvae was observed in those grown on leaf substrates, followed by fruit and control diets, with stem substrates showing the lowest protein levels. Nevertheless, no clear pattern emerges that allows for the determination of the optimal protein content in tomato cultivation substrates necessary for successful larval growth.

The ash quantities obtained in this study are consistent with those reported in other research, which suggests that the amount of ash is directly related to the mineral content of the substrate. Additionally, it has been noted that a higher ash content in the substrate is often associated with a greater protein yield, making it a suitable substrate for larval cultivation [69]. In this context, the protein content observed in the leaf substrate may be linked to its ash content, as it exhibited the highest protein concentration and the second-highest ash content, surpassed only by the larvae cultivated on tomato stems.

5. Conclusions

The reduced degradation of tomato plant residues, such as leaves and stems, is attributed to the composition of these substrates. Leaves and stems contain higher levels of lignin, which black soldier fly larvae (BSFL) can break down and utilize for feeding, even though this process may cause a delay in their growth. Pre-treatment may be necessary to improve the digestibility of these residues for the larvae. Alternative pre-treatment methods, such as solid fermentation, should be explored.

The results of this study provide valuable insights into the performance of LMSN (larval meal produced from BSFL) when fed various tomato plant residues. These findings help identify and rule out any potential adverse effects of tomato plant waste on LMSN. For future research, we plan to explore the combination of different tomato plant residues to minimize the need for their separation. Additionally, we aim to extend this approach to other tomato varieties or grafts to propose it as a universal biological treatment for tomato crop residues.

First, understanding the impact of each type of tomato plant residue on LMSN as a food source is essential. Tomato plant waste is one of the most challenging materials to treat, often leading to its disposal in sanitary landfills, which makes it commercially unviable and offers no market value. However, the results of this study suggest that it is possible to produce LMSN from tomato plant waste, lowering production costs while generating larval biochemical components comparable to those made with a control diet.

Furthermore, this study demonstrates that larval biomass can be produced using tomato plant residues not commonly used in black soldier fly larvae cultivation. Notably, the process did not require water for cultivation, and the resulting larval biomass has applications in other sectors. The high protein and fat content of the larvae make them attractive for animal feed production. Additionally, the residues from the larval cultivation process can be utilized as biofertilizers in agriculture, offering a sustainable solution for waste management.

6. Perspectives

Further research should focus on optimizing the treatment methods for tomato plant waste to make it more cost-effective and scalable. Additionally, exploring the broader applications of the larval biomass produced, particularly in animal feed and biofertilizer production, holds great promise for enhancing tomato crop residue management’s economic and environmental sustainability.

Ultimately, this study contributes to the growing body of knowledge on sustainable waste management and highlights the value of black soldier fly larvae as a resource for circular economy practices.