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Finding New Quality Control Benchmark for BSF Mass Rearing Facilities.

December 2020

Throughout my PhD studies, it was clear that the amount of eggs obtained per generation varies tremendously if parameters such as pupae weight, temperature, and diet are not maintained stable. However I haven’t seen - until now - a study showing a direct impact of pupae weight on pupation rate, weight lost during pupation, egg individual weight, sex ratio, and overall production. 

 

The study recently published by Georgescu et al. (2020) indicates that pupae weight may have a strong influence on the population fitness, where pupae weight was positively correlated with pupation rate and fecundity (i.e. the number of eggs collected per generation). Moreover, the sex ratio was also affected, with more females emerging from the heaviest pupae. 

 

If you are passionate about evolutionary biology as I am, you know how this kind of data is important to start to understand how those flies are physiologically adapting under artificial settings. According to the Resource Elasticity model developed by Wang et al. (2013),  if available resources are not limited to an organism, as in mass production facilities where optimal conditions are enforced to maximize production, an organism must allocate resources to reproduction instead of survival. The data found by Georgescu et al. (2020) confirms this statement by showing that females from heavier pupae laid more eggs and had a shortened lifespan compared with the females from lighter pupae. A decade ago, Tomberlin et al. (2009) also demonstrated that heavier males and females lived an average of 2 days less compared with the lightest group. 

The paper examines the relationship between distinct diets (chicken feed, grass cuttings, and fruit and vegetables) and the BSFL microbiome. The authors ask whether the BSFL microbiome consists of a stable in-born microbiome or is driven by bacteria present in the feed (as suggested by Boccazzi et al. 2017).  The question is worth asking because the BSFL microbiome is likely pivotal to their ability to consume a wide range of organic material, decrease pathogens in their environment, and degrade pharmaceutical products.

 Klammsteiner and co-authors raised the BSFL neonates on a standard Chicken Feed diet for 6 days before transferring them to their respective feeds. The larvae were regularly sampled and dissected to remove their guts for DNA extraction and sequencing. The microbial community of the feed was also analyzed, although unfortunately these authors only sampled the fresh feed and did not track changes in the microbial community over time.

The study found that the microbial community differed between diets, with the grass cutting diet showing a distinct Bacilli-dominated composition compared to the other two. However, the authors could not find evidence for substantial time- or diet-associated changes in the BSFL microbiome. In this case, the evidence points to a core microbiome that remains relatively stable across feeds and over the larvae development time. This core microbial community represented 44% of all sequences and consisted of Actinomyces sp., Dysgonomonas sp., and Enterococcus sp. and an unclassified Actinomycetales.

Moreover, the heaviest pupae presented the highest pupation and eclosion (emergence) compared with the other two weight classes. This could mean that flies would emerger within a short period of time and at high densities, which would increase the chance of a male-female encounter, and consequently, mating rate. 

Although the paper presented a good amount of data that can easily enhance many processes in BSF facilities, information such as the population fertility (by the % of eggs hatched), and how cost-effective the diet was among the weight pupae groups were not provided or evaluated.

 

For me, the main take-home from this paper confirms what I found during my PhD research: pupal mass may be a practical quality control benchmark in BSF mass-rearing facilities. Therefore, as stated in a study published by Hoc et al. (2019), adult flies eclosing from heavy pupae may have a higher mating success than those of lower weight, due to being a female-dominant population and presenting a higher population density. 

 

Studies as one mentioned here are essential for any BSF industry to be able to mass-produce larvae at a stable and optimal level. Projects that aim to fully understand the nutritional requirements to enhance fecundity and fertility in the adults must be a priority in any R&D department, as much as those addressing abiotic conditions (i.e. lighting, temperature, RH(%)) or structural changes (i.e. cage designers, population density).

Dr Aline Malawey

Entomologist

Paper reviewed: 

GEORGESCU, B., STRUȚI, D., PĂPUC, T., LADOȘI, D., & BOARU, A. (2020). Bodyweight loss of black soldier fly Hermetia illucens (Diptera: Stratiomyidae) during development in non-feeding stages: Implications for egg clutch parameters. Eur. J. Entomol, 117, 216-225.

References:

Tomberlin, J. K., Adler, P. H., & Myers, H. M. (2009). Development of the black soldier fly (Diptera: Stratiomyidae) in relation to temperature. Environmental Entomology, 38(3), 930-934.

Wang, R. W., Wang, Y. Q., He, J. Z., & Li, Y. T. (2013). Resource elasticity of offspring survival and the optimal evolution of sex ratios. PloS one, 8(1).

Hoc, B., Noël, G., Carpentier, J., Francis, F., & Megido, R. C. (2019). Optimization of black soldier fly (Hermetia illucens) artificial reproduction. PloS one, 14(4).

Figure 1. Mean female and clutch weights, and the number of eggs per clutch recorded for the different weight classes. Source:Georgescu et al. (2020)

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Figure 2(B). Trends in the weights of the prepupae leaving the substrate recorded for the three different weight classes. Source:Georgescu et al. (2020)