1 Introduction

Bioconversion using black soldier fly larvae (BSFL) is an emerging approach for transforming nutrients from diverse organic substrates into valuable raw materials for feed, fertilizer, and various technical applications (Gold et al., 2018). BSFL’s remarkable ability to thrive on low-nutrient and unbalanced diets is attributed to their highly adaptable digestive system and associated larval digestive tract and substrate microbiome (Bruno et al., 2025). This system features distinct morphofunctional gut regions, selective acquisition of microbes from the surroundings, and considerable plasticity. This allows larvae to modulate digestive enzyme activities, nutrient absorption surfaces, and gut residence times in response to substrate quality (Bruno et al., 2025; Gold et al., 2018).

Unlike conventional livestock such as swine, poultry, and ruminants, whose metabolism and growth are primarily influenced by ambient farm temperature and feed quality, BSFL experience their environment mainly through the substrate they inhabit, feed on, and excrete into. Consequently, the microclimate within the substrate, rather than the air temperature in the farm, directly affects their physiological processes and bioconversion efficiency. This unique ecological niche means that managing substrate conditions, particularly temperature, is essential for optimising larval development and waste conversion outcomes (Harnden et al., 2016).

In commercial-scale operations, heating, cooling, and ventilation for larvae can account for 30-60% of the total energy expenditure (Bühler, 2025; DTI et al., 2023). Therefore, heat management should be a research priority to reduce operational costs and enhance the competitiveness of bioconversion products. Both excessive and insufficient heat significantly affect bioconversion efficiency (Chia et al., 2018; Li et al., 2023). Suboptimal temperatures slow metabolic processes, hinder larval development, and may result in operational challenges like poor sieving and process interruptions. Conversely, high heat can accelerate larval development but may reduce bioconversion efficiency due to increased metabolic costs, elevated microbial activity, higher mortality rates, and rapid drying of the substrate, which renders it less digestible for BSFL (Gold et al., 2022). Furthermore, excessive heat is associated with increased CO₂ emissions as CO₂ is a main product of heat-producing larval and microbial metabolisms. CO₂ emissions can reduce the environmental sustainability of the process (Coudron et al., 2024).

Heat production by BSFL and the substrate microbiome, enhanced by their interactions, urgently warrants more attention. Despite research focusing on the heat generated by BSFL, the role of microbial heat production in the substrate and the interaction between BSFL and the substrate microbiome have been largely overlooked. Here, we explore the roles of these key players in heat generation and argue that oxygen availability in the substrate is a critical driver of heat during bioconversion. We discuss how integrating oxygen management into process control designs can increase the resilience of BSFL bioconversion to improve yields, reduce energy requirements and lower emissions.

2 Heat and oxygen from BSFL

BSFL are ectothermic insects and their body temperature is largely influenced by the surrounding environment. BSFL also generate metabolic heat as a byproduct of respiration together with frictional heat from their movement. BSFL behaviour is characterized by natural aggregations within the substrate (Shishkov et al., 2019) and such dense clusters have been observed to reach higher temperatures than solitary larvae or small groups (Klammsteiner et al., 2025; McEachern, 2018). The underlying mechanism likely combines reduced heat loss (due to aggregation-based insulation/thermal mass) and increased heat production (via friction or metabolic activity). These aggregations play a crucial role in enhancing oxygen penetration within the substrate, allowing larvae to successfully breathe and feed at depths exceeding their typical 2–4-cm individual feeding zones (Barett et al., 2023).

Some BSFL aggregates can be considered a form of behavioural thermoregulation. For example, BSFL have been observed to form aggregations to prevent heat loss in response to low temperatures (20 °C) as reported by Li et al. (2023). This heat can increase the substrate temperature, presumably enhancing digestive capacity and accelerating development and growth (Cattaneo et al., 2025).

Metabolic heat production per unit body mass has been observed to decrease as BSFL advance through their larval stages (Gligorescu et al., 2019). Conversely, total metabolic heat production per larva and per crate increases as larvae gain mass through growth (Laganaro et al., 2021; McEachern, 2018). The metabolic heat production of individual BSFL was approximately 12 μW/mg for third instar larvae and gradually reduced to 2 μW/mg for sixth instar larvae (Gligorescu et al., 2019). These values are context-specific and thus may not be broadly generalisable. However, the overall reduction in metabolic heat generation per unit body mass is consistent and attributed to a physiological shift, where larvae prioritise building body fat and accumulating critical energy reserves necessary for successful reproduction in the next generation (Schmolz and Lamprecht, 2000).

3 Heat from substrate

Catabolic respiration is the major source of microbial heat in substrates. However, unlike BSFL, microbes can metabolise organic substrates in the absence of oxygen, either through fermentation or by using alternative electron acceptors during anaerobic respiration. Aerobic respiration, which uses oxygen, is more efficient and releases significantly more energy than anaerobic metabolism (e.g. respiration in Escherichia coli: −2870 kJ/mol glucose Gibbs free energy change $\Delta G$° vs fermentation in E. coli: −218 kJ/mol glucose) (Tran and Unden, 1998). Aerobic respiration occurs across diverse microbial taxa – including bacteria, fungi, and archaea – through both obligate aerobes and facultative anaerobes, which switch to aerobic respiratory pathways when oxygen is present.

Bekker et al. (2021) and Palma et al. (2018) suggested an important role for oxygen in microbial activity during BSFL rearing. Concurrently, Hansen et al. (2023), Erbland et al. (2021), and Klammsteiner et al. (2025) suggested a link between microbial activity and heat production. Johnson et al. (2013) explicitly elaborated on this connection, stating that oxygen is key to microbial heat production in a bioconversion system with insect larvae. Similarly, Fuhrmann et al. (2025) aligned with this view for BSFL rearing, using bulk density and pore space as proxies for oxygen availability. A lower bulk density (500 kg/m³) of the food waste was related to a 9 °C increase in substrate temperature compared to the higher bulk density (800 kg/m³).

In reality, microbial heat production in a bioconversion crate has spatial and temporal variations due to the variability of aerobic and anaerobic/anoxic areas and the dynamic nature of the bioconversion process, which changes the biotic and abiotic parameters influencing oxygen availability. The high moisture content (45–75%) in substrates (Bekker et al., 2021) may restrict oxygen availability. Particle size plays an important role in oxygen availability, while small particles might accelerate microbial decomposition via increased surface area but risk anaerobic conditions by reducing air flow (Lin et al., 2022); larger particles might improve aeration via larger pores (Azis et al., 2022), but their core may become or remain anaerobic due to insufficient oxygen diffusion (e.g. above particle sizes >0.25 mm as stated by Richard, 2004). Additionally, oxygen availability may decrease with the depth of the substrate in crates, and larval and microbial respiration may deplete available oxygen in the substrate matrix. This is similar to observations in sewage sludge composting, where pore space oxygen can drop from ambient oxygen concentration (21%) to below 5% (v/v) in as little as 15 min (Richard, 2004).

Overall, aerobic and anaerobic microbial metabolic activity is an emergent feature of BSFL substrates. The precise heat contribution from aerobic or anaerobic metabolism will depend on the substrate conditions. However, given that aerobic metabolism releases substantially more energy – and thereby more heat – it is reasonable to assume that oxygen availability within the substrate likely drives microbial heat production in many cases of heat accumulation (e.g. increases of 10 °C above ambient temperature).

4 Heat in the BSFL bioconversion system

Overall, heat generation in BSFL bioconversion is shaped by feedback loops and tipping points, with oxygen levels, altered by larval-microbial interactions in the substrate, serving as critical factors. Feedback loops are processes where a change in one part of a system causes further changes that reinforce or dampen the original changes. Tipping points are critical thresholds in a system that, when crossed, can cause the system to shift abruptly, and often irreversibly (Kéfi et al., 2022).

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Oxygen availability – shaped by substrate physical properties, water content, and BSFL behaviour – centrally drives heat production by both BSFL and substrate microbes. Tipping points and feedback loops shift the system between aerobic and anaerobic/anoxic states, ultimately influencing bioconversion efficiency and larval development.

Citation: Journal of Insects as Food and Feed 2026; 10.1163/23524588-bja10369

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Heat generation dynamics arise from the interplay among larval activity and metabolism, microbial substrate metabolism, and environmental conditions. Aerobic environments in solid-state bioconversion processes promote elevated microbial activity within the substrate, which in turn elevates temperatures, creating a positive feedback loop that further stimulates microbial metabolic rates (Richard, 2004). Unlike traditional composting systems, this thermal process can be amplified by two concurrent mechanisms: heat from larval metabolism and the physical impact of larvae on the substrate. Larval movement enhances substrate aeration, particularly in larger late instar larvae, thereby intensifying microbial heat production, as seen in carcases decomposition (Johnson et al., 2013). This aeration simultaneously drives water evaporation, producing dual effects: increased oxygen availability for microbial and larval metabolism, and potential cooling through evaporative heat loss that may counteract thermal feedback.

In addition to the availability of digestible nutrients, the temperature trajectory of a bioconversion system can greatly depend on the initial physical properties of the substrate or its structural evolution during decomposition. Initial anaerobic/anoxic conditions in water-saturated substrates can transition to aerobic conditions as larval bioconversion and evaporation reduce moisture content and increase porosity. This structural shift enables higher-energy aerobic microbial pathways, creating dynamic thermal regimes influenced by microbial-larval interactions. Such feedback mechanisms are particularly evident in porous, low-density, and high-digestible-nutrient substrates (Fuhrmann et al., 2025) but may also emerge through larval-mediated modifications of denser substrates during bioconversion. In some cases, this aerobic thermal trajectory is disrupted by a tipping point, where excessive release of water or liquid compounds (e.g. oils) from substrate breakdown, coupled with insufficient water removal, can lead to pore saturation and prevent aerobic pathways. Slow water removal is reinforced by a feedback loop of low temperatures and liquid-filled pores with reduced surface areas, perpetuating anaerobic/anoxic conditions. For instance, it is sometimes observed that warm, plastic substrates rapidly transition to colder states while becoming viscous and sticky. These conditions suppressed larval growth and free movement, resulting in diminished heat production and limited larval-mediated aeration. Similarly, crates may never ‘take off’ if heat generation and surface area are insufficient to evaporate enough liquid to start high-energy aerobic trajectories. Here, ‘take off’ refers to a change from low-heat anaerobic/anoxic conditions to high-heat aerobic conditions (see Figure 1; also see Figure S1 in the Supplementary Material). The failure to initiate a thermal pathway can lead to stunted larval development, larval escape, and poor harvest separation. Notably, reduced microbial respiration under stagnant conditions may increase bioconversion because more nutrients become available for larval growth. Consequently, minor substrate variations may trigger complex and nonlinear oxygen-mediated system effects, leading to divergent and suboptimal bioconversion outcomes (Fuhrmann et al., 2025; Oonincx, 2024).

Understanding BSFL bioenergetics offers opportunities to control rearing temperatures using more sustainable and cost-effective methods than relying solely on air temperature control. For example, substrate formulation can be strategically managed to influence heat generation, especially by considering physical properties such as bulk density, porosity, and water-holding capacity, which collectively affect oxygen supply and heat production in the substrate. Additionally, factors like substrate nutrient availability, carbon-to-nitrogen (C:N) ratio, pH, and aeration rate – all typically influencing microbial respiration – can be optimised to provide greater control over heat generation. In composting research, these factors have led to the development of predictive models, an approach with significant potential for BSFL research (Petiot and Guardia, 2013).

It is important to note that heat production does not necessarily equate to heat accumulation. For BSFL substrate temperatures that exceed ambient conditions, heat generation must surpass the rate of heat dissipation to the environment. Similar to composting, this is most likely to occur when the substrate mass is high and the surface-area-to-volume ratio is low. A key issue is that most BSFL laboratory research is conducted at much smaller scales (a few kilograms or less, using crates smaller than 40 × 60 cm, and a low number of replicates) compared to industrial rearing operations (tens to hundreds of kilograms, crates of 40 × 60 cm or larger, and hundreds to several thousands of replicates). Consequently, research setups typically do not experience the same degree of heat accumulation as industrial settings. This discrepancy creates a significant challenge when translating laboratory-based research findings to industrial applications, as substrate temperature profoundly impacts rearing outcomes (Li et al., 2023). Where large-scale testing is not feasible, composting research has partially addressed this issue using incubators with temperature regimes that mimic the metabolic heat found in large-scale systems. Adopting a similar approach can benefit the BSFL research by providing relevant data for industrial applications.

5 Further research

As argued in this article, oxygen conditions are a key driver for heat generation and thermal trajectories in BSFL bioconversion. This has implications for process performance, reliability, product composition, and sustainability due to associated emissions and the energy needs and costs of temperature control. The following research priorities focus on understanding and controlling heat generation, providing an opportunity to improve BSFL bioconversion across several metrics:

1. (1) Systematically quantify the heat contribution from aerobic and anaerobic microbial metabolic pathways across a range of substrates with varying nutrient compositions to better understand their generalisability and implications for bioconversion efficiency.
2. (2) Assess realistic oxygen-related variables, such as bulk density, height and porosity, of BSFL substrates, and investigate how these parameters evolve throughout the bioconversion process to optimise heat management and maximise both yield and process stability.
3. (3) Examine whether ventilation serves as a significant source of oxygen, thereby fuelling microbial respiration and heat production during BSFL-driven bioconversion.
4. (4) Investigate whether the C:N ratio influences microbial heat production by altering microbial metabolism, as the C:N ratio is a key determinant of microbial activity, growth, and metabolic pathways.
5. (5) Investigate how heat generation alters commercially relevant product characteristics such as larval mass, larval antimicrobial properties, frass antifungal properties, and larval microbial loads.
6. (6) Investigate the effects of laboratory, pilot, and commercial-scale production settings on heat generation during BSFL production.
7. (7) Expand the work on altering and inhibiting substrate microbes and manage heat production by fermentation before BSFL bioconversion or additives (e.g. acids).
8. (8) Evaluate the impact of anaerobic/anoxic conditions in BSFL substrates on odour formation (e.g. via sulphate reduction), nitrogen loss through denitrification, and the accumulation of potentially harmful fermentation byproducts, such as butyric acid, as these factors may affect process efficiency and larval health.
9. (9) Develop practical heat management strategies for BSFL bioconversion across substrates.

Author contributions

M.J. Zorrilla: Conceptualisation, writing (original draft), visualisation; A. Fuhrmann: Conceptualisation, writing (original draft), visualisation; A. Gligorescu: Conceptualisation, writing (original draft); N. Puniamoorthy: Conceptualisation, resources, writing (review and editing), supervision; A. Mathys: Resources, writing (review and editing), supervision; M. Gold: Conceptualisation, writing (original draft), visualisation, supervision.

Conflict of interest

The authors declare no conflicts of interest regarding this manuscript.

Data statement

All data referenced in this opinion paper are derived from previously published sources.

Funding

Adrian Fuhrmann’s salary was supported by the National Research Foundation, Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) programme. Anton Gligorescu‘s contribution was funded by the Danish Council for Independent Research (grant number DFF-1127-00081B). The research also received funding by the Intra-CREATE Thematic Grant (grant number NRF2020-THE003-0003) as well as as well by A*STAR under its New Zealand-Singapore Biotech in Future Foods Research Programme (R25IBIR004).