Growth Performance of Black Soldier Fly (Hermetia illucens) Fed Oriental Melon Waste and Analysis of the Frass Value

1. Introduction

The increase in the global population has led to a corresponding increase in agricultural production (UN, 2022). This escalation has resulted in the generation of substantial amounts of agricultural by-products at every stage of production, processing, and distribution. According to a report by the Food and Agriculture Organisation (FAO) in 2017, 13.8% of food waste in 2016 occurred during the production and distribution processes, amounting to approximately 400 billion US dollars (FAO, 2017). Although methods such as converting these by-products into food, feed, compost, or energy exist, they are insufficient for processing all agricultural waste effectively. Therefore, a critical need exists for environmentally friendly disposal methods enabling the upcycling of agricultural by-products.

One promising approach for efficient processing and upcycling of agricultural waste is the utilisation of black soldier fly (BSF) (Hermetia illucens). BSF larvae decompose a wide variety of organic materials (Banks et al., 2014; Oonincx et al., 2015; Mohd-Noor et al., 2017; Cai et al., 2018; Chun et al., 2019; Lim et al., 2019). After decomposition, these larvae can be processed into protein or fat sources for animal feed, and the resultant frass can be used as fertiliser in accordance with standard fertiliser regulations (Choi et al., 2013; Lee et al., 2013; Salomone et al., 2017; Spranghers et al., 2017; Meneguz et al., 2018; Chun et al., 2019; Wang et al., 2020). Additionally, BSF consume only water instead of organic matter during the adult stage. Therefore, they do not transmit diseases (Furman et al., 1959; Diener, 2010). Thus, BSFs are advantageous in industrial applications.

In South Korea, the production of oriental melons (Cucumis melo var. makuwa) in 2023 was 208,542 t, constituting approximately 11.9% of the total vegetable and fruit production,i.e., 1,753,172 t, of thenation. Approximately 95.7% of the oriental melons produced in the country are produced in Gyeongsangbuk-do (Korean Statistical Information Service, 2024), of which a substantial portion is produced in Seongju-gun (171,654 t) (Agriculture Technology Center of Seongju-gun, 2024). The extensive production in Seongju-gun led to an increase in the production of low-grade, immature, or rotten fruits. To manage this surplus, Seongju-gun purchased and processed approximately 3,399 t of low-grade or waste-oriented melons at approximately 21 billion South Korean won. Unprocessed waste oriental melons are often used as feed or compost, which has limited effectiveness, and many rely on nonproductive treatments such as landfills and storage (MAFRA, 2020).

Here, we aimed to investigate the decomposition rate of waste oriental melons and the growth of BSF larvae fed with this substratein order to process waste oriental melons in an environmentally friendly manner and utilise resources efficiently. Furthermore, we analysed the fertiliser properties of the frass produced after decomposition and conducted fertiliser efficacy tests on leafy vegetables such as green skirt lettuce (Lactuca sativa). In this study, we explored the potential of upcycling waste oriental melons using BSF, with the aim of contributing to sustainable agricultural waste management practices.

2. Materials and Methods

3. Results and Discussion

3.1. Analysis of growth and feeding performance of BSF larvae fed with oriental melon waste

BSF larvae fed oriental melon waste required 44 days to reach the end of the larval stage. Compared with the developmental time of BSF larvae fed calf feed or food waste, that of BSF larvae fed oriental melon waste was delayed by 24 days (Table 1). The developmental time of BSF is correlated with various growth conditions, such as density, pH, and substrate texture (Parra Paz et al., 2015; Liu et al., 2018; Meneguz et al., 2018; Scala et al., 2020). Substrate quality is an important factor affecting the developmental time of BSF (Gobbi et al., 2013; Oonincx et al., 2015; Jucker et al., 2017). If the larvae do not receive sufficient food, they may not receive the necessary nutrients for proper growth, resulting in smaller larvae and a longer pupation period (Nijhout, 2003). According to a nutritional analysis of oriental melon waste, it consists of approximately 84.1% water (Koo et al., 2023b). This high water content indicates insufficient nutrients for BSF larval growth, and the developmental time of the BSF larval stage will be delayed. However, the survival rate of larvae fed oriental melon waste was 82.1±13.73%. The survival rate of BSF larvae was lower than that of the larvae fed calf feed and food waste (99.5±0.25%, 98.5±1.15%). However, no significant differences were observed in the survival rates of the three substrates (Table 1). Jucker et al.(2017) reported that low levels of carbohydrates in the substrate may lead to a lower survival rate of BSFs. The amount of carbohydrates inoriental melon is 11.7%, and itscarbohydrate content is highest afterits water content (MFDS, 2024). This result suggests that oriental melon waste does not affect the larval survival rate of BSF.

Table 1.

Larval developmental time, larval survival rate, and fecundity rate of adult black soldier flies fed different substrates

3.2. Growth performance of BSF larvae fed different substrates

After harvesting the BSF larvae, the average weight of those fed oriental melon waste was 0.10±0.021 g (Fig. 1A), and those of BSF larvae fed calf feed and food waste were 0.16±0.018 g and 0.27±0.028 g, respectively. The weight of BSF larvae fed oriental melon waste was 1.6 times lower than that of BSF larvae fed calf feed and 2.8 times lower than that of the BSF larvae fed food waste. The average length of BSF larvae fed oriental melon waste was 17.3±1.38 mm. The test larvae were significantly shorter than the BSF larvae fed calf feed and food waste (18.8±0.93 mm and 22.6±0.89 mm, respectively) (Fig. 1B). These results indicate that oriental melon waste is not suitable for BSF larval growth, compared with calf feed and food waste, because of its poor nutritional content. Koo et al.(2023b) also showed that BSF larvae fed two fruit by-products, mandarin waste and apple pomace, were smaller than BSF larvae fed calf feed as a control. Further, the growth performance of BSF larvae is significantly related to good-quality nutrient content, especially protein (Gobbi et al., 2013; Nguyen et al., 2013; Oonincx et al., 2015; Jucker et al., 2017; Tinder et al., 2017). The reduced weight and length of BSF larvae fed oriental melon waste could be attributed to the poor nutritional composition of the oriental melon waste, particularly its protein content of only 1.2% (Koo et al., 2023b).

Fig. 1.

Growth performance of BSF larvae fed oriental melon waste. BSF larvae (6 day-old from eggs) were oriental melon waste, calf feed and food waste respectively. (A) Change in weight of BSF larvae fed oriental melon waste. (B) Change in length of BSF larvae fed oriental melon waste. Data are shown as the mean values ± standard error of the mean (n = 20). Different letters indicate significant differences in the weight or the length of control and test animals (p<0.05, analysis of variance)

3.3. Analysis of feeding performance of BSF larvae fed oriental melon waste

The reduction rates of oriental melon waste, food waste, and calf feed by BSF larvae were 80.5±0.22%, 79.0±1.21%, and 62.1±0.46%, respectively (Table 2). These results show that BSF larvae are more efficient at reducing oriental melon waste thanfood waste or calf feed. Gold et al.(2020) reported that BSF larvae could reduce 46.7–60.0% of fruit and vegetable waste. Another study reported that BSF larvae fed mandarin waste and apple pomace could reduce these wastes by 61.5% and 48.0%, respectively (Koo et al., 2023a). Therefore, BSF larvae appear to be particularly effective in reducing oriental melon waste, which is likely due to their semi-liquid physical properties. This waste has a high water content and soft, semi-liquid texture. Furthermore, the crude fibre content of oriental melon waste is substantially lower than that of mandarin waste and apple pomace (Koo et al., 2023b). Bruno et al.(2020) noted that BSF larvae prefer to digest semi-liquid substrates because fibres are negatively correlated with the waste reduction ratio (Liu et al., 2018). The bioconversion efficiency of BSF larvae fed with oriental melon waste was 11.8±0.37%, which was significantly lower than that of food waste and calf feed (Table 2). However, Surendra et al.(2020) noted that BSF larvae fed different fruit and vegetable wastes showed bioconversion efficiencies ranging from 4.1% to 10.8%. The bioconversion efficiency observed for oriental melon waste was within a range similar to that of other fruit and vegetable wastes.

Table 2.

Analysis of feeding performance of BSF larvae after being fed oriental melon waste

3.4. Fertiliser nutrient composition analysis and fertilising effect experiment of frass produced by larvae of BSF fed oriental melon waste

Fertiliser analysis was conducted by harvesting the frass produced by BSF larvae fed oriental melon waste, the organic matter content of which was 40.29% and the salt content was 0.23%. Additionally, testing for eight heavy metals revealed either trace amounts or no detection, confirming that all the contents met the official fertiliser standards for BSF frass (Table 3). Compared with frass from BSF larvae fed with food waste, that from oriental melon waste-fed BSF larvae showed an approximately 42.38% lower organic matter content and approximately 2.05% lower salt content. An efficacy analysis was conducted on the leaf length, root weight, fresh weight, and dry weight of green skirt lettuce (Lactuca sativa) treated with the frass of BSF.

Table 3.

Analysis of contents of frass produced by BSF larvae fed food waste and oriental melon waste

larvae fed oriental melon waste. The results showed a significant increase in leaf length, root weight, fresh weight, and dry weight compared with those of the untreated group (Table 4). A comparison of lettuce treated with frass from BSF larvae fed general food waste and that from BSF larvae fed oriental melon waste feed revealed no significant difference in root weight. However, the leaf length, fresh weight, and dry weight were significantly higher in the group treated with frass from oriental melon waste-fed BSF larvae. These findings suggest that the organic matter content of the BSF frass is sufficient to support the growth of green skirt lettuce. Contrastingly, the higher salt content in frass from food-waste-fed BSF larvae likely adversely affected lettuce growth compared with that from oriental melon waste-fed BSF larvae. Supporting evidence from other studies indicates that frass from BSF larvae fed food waste increased grass growth compared with that in untreated grass. However, as the application rate increased, the higher salt content led to a decrease in grass weight (Choi et al., 2013).

Table 4.

Growth performance of lettuce after applying frass of BSF larvae fed oriental melon waste

4. Conclusions

BSF larvae offer an eco-friendly method for decomposing agricultural by-products and efficiently upcycling organic matter. We evaluated the feasibility of using BSF larvae to process oriental melon by-products, specifically surplus oriental melons, produced in Seongju- gun, South Korea. When fed oriental melon waste, the growth period of BSF larvae was prolonged compared with that of BSF larvae fed food waste or calf feed. However, the survival rates remained unaffected. The weight and length of BSF larvae fed oriental melon waste were less than the control. The larvae were capable of decomposing up to 80.5% of the melon waste, achieving a bioconversion rate of 11.8%. Analysis of the frass resulting from the decomposition of oriental melon waste revealed that the organic matter, salinity, and heavy metal levels complied with fertiliser regulatory standards. Further testing of leafy vegetables indicated that compared with untreated soil, frass application significantly increased leaf length, root weight, fresh weight, and dry weight. Compared with frass from larvae fed with food waste, those of melon-fed BSF showed significantly higher leaf length, fresh weight, and dry weight, likely because of differences in salinity levels. These findings suggest that when using BSF larvae for industrial purposes, it may be more effective to combine melon waste by-products with other organic materials to develop an optimised feed source for larvae.

Acknowledgments

This study was supported by the Research Program for Agriculture Science and Technology Development (Grant No. PJ01596002), National Institute of Agricultural Sciences, Rural Development Administration, Republic of Korea.

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