Growth and Metabolism of Black Soldier Fly Larvae on Substrates of Different Quality

Research Objective

This study investigates how the nutritional quality of rearing substrates influences the growth and metabolic efficiency of BSF larvae. In particular, it focuses on the larvae’s ability to assimilate carbon and energy from different feedstocks and how efficiently this assimilated energy is converted into biomass. The central question is whether BSF larvae can thrive on low-quality organic waste substrates and, if so, how their metabolic performance compares to when they are fed more nutrient-rich diets.

To explore this, the researchers compared the larvae’s performance on three types of substrates: a high-quality, nutrient-dense control diet (commercial chicken feed, CF), a low-quality organic waste product (anaerobically digested sewage sludge, DS), and various combinations of the two. The aim was to understand how these differing substrates impact larval growth, metabolic rate, energy use, and ultimately their potential for use in sustainable bioconversion systems.

Experimental Design

The experimental setup involved five feeding regimens, each representing a different ratio of high- and low-quality substrates. These included 100% chicken feed, three intermediate mixes (75/25, 50/50, and 25/75 CF/DS respectively), and 100% digested sludge. This gradient allowed the researchers to systematically evaluate the effects of decreasing feed quality on the larvae’s development and metabolism.

A range of biological and physiological metrics were assessed. Larval weight and development time were measured to evaluate growth performance. Respiration rates, assessed via carbon dioxide (CO₂) emission, served as a proxy for metabolic activity. Additionally, carbon and energy assimilation were tracked, allowing the calculation of net growth efficiency (NGE), defined as the proportion of assimilated energy that is retained as biomass. The researchers also partitioned total energy use into growth and maintenance costs to better understand the larvae’s energy budget under different feeding conditions.

Key Findings

Growth Rate and Final Biomass

Larvae fed exclusively on chicken feed exhibited the fastest growth rates and reached their maximum weight within approximately 12 days. As the proportion of digested sludge in the feed increased, growth rates progressively declined. Those reared entirely on DS showed significantly delayed development and required over 21 days to reach harvest size. Even then, their final mass was substantially lower than that of larvae reared on nutrient-rich feed.

Growth trajectories followed a classic sigmoidal pattern across all diets, but the curve was increasingly compressed and delayed in sludge-heavy diets. This indicates not only slower development but also a lower growth potential when the nutritional profile of the substrate deteriorates.

Assimilation Efficiency

One of the more surprising results was that the larvae’s ability to assimilate carbon and energy remained relatively consistent across all diets. This suggests that BSF larvae are physiologically capable of extracting nutrients from even low-quality substrates. 

However, while assimilation rates did not variate much, the downstream fate of that assimilated energy did. On nutrient-rich diets, a significant portion of assimilated energy was channelled into growth. On the contrary, with low-quality diets, the larvae diverted a much greater share of energy toward basic maintenance processes, such as cellular respiration, rather than building biomass.

Respiration and Energy Expenditure

Respiration rates, measured by CO₂ output, increased in direct proportion to the amount of sludge in the substrate. This trend indicates that more energy was required just to maintain basic metabolic functions when larvae were fed poor-quality diets. The metabolic cost of survival became increasingly burdensome as substrate quality decreased, resulting in reduced energy availability for growth.

The partitioning of metabolic costs showed a significant rise in the energy allocated to maintenance activities (e.g., enzyme production, detoxification, and waste processing) in DS-based diets. This shift in energy allocation explains the sharp decline in growth performance, despite stable assimilation rates.

Net Growth Efficiency (NGE)

Net growth efficiency, a key measure of how effectively assimilated energy is turned into new biomass, was found to be highly sensitive to substrate quality. On 100% chicken feed, the NGE was approximately 50–58%, meaning that more than half of the energy absorbed from the substrate was used for growth. However, on 100% digested sludge, this efficiency dropped below 20%, representing a more than three-fold reduction. Even mixed diets with moderate proportions of sludge showed a marked decline in growth efficiency, with NGE values dropping to around 26–35%.

This sharp decline in NGE demonstrates that although BSF larvae can technically survive on low-quality waste, they do so with significant energetic inefficiency. Much of the energy they assimilate is consumed in non-productive metabolic processes, leaving only a small portion for actual growth.

Carbon and Energy Budget Dynamics

The researchers also developed a full carbon and energy budget model for each dietary treatment. On high-quality feed, most of the assimilated carbon was retained in larval biomass, with only a small fraction lost through respiration. In contrast, under sludge-based feeding regimes, the majority of assimilated carbon was respired rather than converted into body tissue. A similar pattern emerged for energy: less energy went into biomass production, and more was diverted to maintenance processes as sludge content increased.

This shift in energy and carbon allocation highlights the metabolic cost of processing complex, nutrient-poor materials like sewage sludge. The larvae’s bodies were forced to expend more energy simply to extract usable compounds and detoxify waste, leaving less for growth.

Interpretation and Implications

The findings make clear that black soldier fly larvae possess a remarkable capacity to survive and function on a wide range of substrates, including low-quality organic wastes. However, their metabolic efficiency is strongly influenced by substrate quality. As the nutrient content of the feed declines, the larvae increasingly allocate their assimilated energy to maintenance functions at the expense of growth. This results in slower development, lower biomass yields, and a significant drop in conversion efficiency.

From a practical standpoint, these results have important implications for both insect farming and sustainable waste management. While BSF larvae can indeed be used to valorise waste materials such as sewage sludge, doing so with high efficiency likely requires some degree of nutritional supplementation. Blending nutrient-poor waste with more digestible organic matter, could enhance larval growth performance and improve the overall efficiency of the bioconversion process.

In summary, the study provides strong evidence that substrate quality is a critical determinant of metabolic performance in BSF larvae. While they can adapt to low-quality diets, this adaptation comes at a cost. For insect farming operations aiming to maximize biomass output and economic viability, attention must be paid not only to the volume of feed provided, but also to its nutritional composition.

https://doi.org/10.1016/j.wasman.2020.12.009