Organisms are expected to invest more in their immune function when the risk of disease infection is high. However, induction of a robust immune response is costly and may not be achievable in suboptimal environments. High conspecific density could simultaneously imply high infection risk and a suboptimal environment for many insect species. We focus on the economically important dipteran species (black soldier fly, BSF) that represents the insect order that has been ignored in previous research on density effects on immunity. The experimental part of the study was carried out to evaluate the effect of larval density (three density treatments: 1, 5 and 10 larvae/cm2) and temperature (three thermal treatments: 23, 27 and 30 °C) on the immune function of BSF larvae. The larvae that were reared at high compared to low larval densities and at higher than lower temperatures had significantly higher activity of phenoloxidase, an enzyme that plays an essential role in insect immune function. Sex did not have a significant effect on phenoloxidase activity and prepupal mass, pupal mass and adult mass were not affected by the levels of phenoloxidase activity of fifth instar larvae. In addition, we give an overview of larval density effects on insect immunity and show that density-dependent prophylaxis (stronger immune response in high larval density environments) is indeed common in the results of published case studies. However, cases with no correlation between density and immunity traits were as frequent. Moreover, in more than half of the studies, qualitatively different within-species patterns in different immunity traits were observed. We conclude that BSF larvae exhibit density-dependent prophylaxis, and larvae invest more into their immune system at high larval densities and temperatures than they do at low larval densities and temperatures.

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High larval densities and high temperatures lead to a stronger immune response in the black soldier fly

Black soldier fly larvae (BSFL) hold great promise for sustainable management of meat and bone meal (MBM), a kind of organic waste. Harvested BSFL frass can be used as soil amendment or organic fertilizer. This study evaluated the quality and microbial profile in the frass of BSFL, fed with fish MBM containing 0% (CK), 1% (T1), 2% (T2) and 3% (T3) of rice straw. Results suggested straw addition into fish MBM had no significant impacts on BSFL weight; however, straw addition remarkably affected waste reduction and conversion efficiency, as well as physicochemical properties including electric conductivity, organic matter (OM) and total phosphorus contents in frass. Fourier transform infrared analysis indicated that increasing levels of cellulose and lignin might not be fully degraded or transformed by BSFL when more straw was introduced into substrates. Straw addition had hardly significant influences on microbial richness or evenness in BSFL frass, only T3 treatment remarkably elevated the phylogenetic diversity value more than the control. Bacteroidetes, Proteobacteria, Actinobacteria and Firmicutes were the most dominant phyla. Genera Myroides, Acinetobacter and Paenochrobactrum maintained high abundances in all frass samples. Elements including OM, pH and Na were key factors in shaping the microbiological characteristics of BSFL frass. Our findings helped to understand the effects of fish MBM waste manipulation on BSFL frass qualities and contributed to the further application of BSFL frass.

Effects of straw addition on the physicochemical and microbial features of black soldier fly larvae frass derived from fish meat and bone meal.

Previous work showed that low-temperature thermal pre-treatment of waste activated sludge (WAS) under different temperatures (30, 60, 75, 90 °C) and durations (2, 4, 8, 16 h) prior to black soldier fly larvae (BSFL) rearing could improve the larval growth. In this work, the biochemical profiles of WAS-fed BSFL and the characteristics of biodiesel produced were reported. The BSFL fed with pre-treated WAS had high protein contents (44%–54%) and moderate lipid contents (21%–25%). The WAS pre-treated at 90 °C for 16 h produced BSFL with the highest protein and lipid yields, achieving 69% and 71% of increments, respectively, as compared with the controlled WAS. Subsequently, high conversion of lipid into biodiesel (>90%) could be achieved through the acid-catalyzed transesterification process. The BSFL biodiesel was dominated by saturated fatty acid methyl esters (FAME), with C12:0 being the abundance. This unique FAME profile enabled BSFL biodiesel to have low iodine value, high cetane number and more oxidatively stable as compared with other reported biodiesels like palm oil and rapeseed oil biodiesels. The BSFL biodiesel also satiated the biodiesel standards of EN 14214 and ASTM D6751, except for the flash point, which was still higher and safer than petroleum diesel. In short, the thermal pre-treatment of WAS was effective in promoting BSFL growth and its lipid yield, ultimately leading to higher biodiesel production. 

Low-temperature thermal pre-treated sewage sludge for feeding of black soldier fly (Hermetia illucens) larvae: Protein, lipid and biodiesel profile and characterization

Effects of straw addition on the physicochemical and microbial features of black soldier fly larvae frass derived from fish meat and bone meal

Simple Summary Within the fly family (Diptera), various species transmit diseases to humans and farm animals or are pests of many crop plants. Most species of flies, however, play key ecological roles in sustaining life on Earth, and several species are reared at different scales and for various beneficial purposes worldwide. Here, we review the special place of fly rearing in relation to the tremendous role that Diptera rearing technology has contributed to the development of our state of knowledge of genetics, sterile insect technique (SIT), biological control, and quality control. We summarize information on the rearing of flies in the fields of the animal feed and human food industries, pollination services, pest control, medical wound therapy treatments, criminal investigations, and as model organisms for the development of several biological and medical fields. We document the importance of fly rearing technology as a basis for many other rearing systems approaches, making the history of Diptera rearing as a launching point for most other insect rearing systems. Abstract Flies (Diptera) have played a prominent role in human history, and several fly species are reared at different scales and for different beneficial purposes worldwide. Here, we review the historical importance of fly rearing as a foundation for insect rearing science and technology and synthesize information on the uses and rearing diets of more than 50 fly species in the families Asilidae, Calliphoridae, Coelopidae, Drosophilidae, Ephydridae, Muscidae, Sarcophagidae, Stratiomyidae, Syrphidae, Tachinidae, Tephritidae, and Tipulidae. We report more than 10 uses and applications of reared flies to the well-being and progress of humanity. We focus on the fields of animal feed and human food products, pest control and pollination services, medical wound therapy treatments, criminal investigations, and on the development of several branches of biology using flies as model organisms. We highlight the relevance of laboratory-reared Drosophila melanogaster Meigen as a vehicle of great scientific discoveries that have shaped our understanding of many biological systems, including the genetic basis of heredity and of terrible diseases such as cancer. We point out key areas of fly-rearing research such as nutrition, physiology, anatomy/morphology, genetics, genetic pest management, cryopreservation, and ecology. We conclude that fly rearing is an activity with great benefits for human well-being and should be promoted for future advancement in diverse and innovative methods of improving existing and emerging problems to humanity.

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How Rearing Systems for Various Species of Flies Benefit Humanity

The treatment of municipal wastewater produces clean water and sewage sludge (MSS), the management of which has become a serious problem in Europe. The typical destination of MSS is to spread it on land, but the presence of heavy metals and pollutants raises environmental and health concerns. Bioconversion mediated by larvae of black soldier fly (BSFL) Hermetia illucens (Diptera, Stratiomyidae: Hermetiinae) may be a strategy for managing MSS. The process adds value by generating larvae which contain proteins and lipids that are suitable for feed and/or for industrial or energy applications, and a residue as soil conditioner. MSS from the treatment plant of Ladispoli (Rome province) was mixed with an artificial fly diet at 50% and 75% (fresh weight basis) to feed BSFL. Larval performance, substrate reduction, and the concentrations of 12 metals in the initial and residual substrates and in larval bodies at the end of the experiments were assessed. Larval survival (> 96%) was not affected. Larval weight, larval development, larval protein and lipid content, and waste reduction increased in proportion the increase of the co-substrate (fly diet). The concentration of most of the 12 elements in the residue was reduced and, in the cases of Cu and Zn, the quantities dropped under the Italian national maximum permissible content for fertilizers. The content of metals in mature larvae did not exceed the maximum allowed concentration in raw material for feed for the European Directive. This study contributes to highlight the potential of BSF for MSS recovery and its valorization. The proportion of fly diet in the mixture influenced the process, and the one with the highest co-substrate percentage performed best. Future research using other wastes or by-products as co-substrate of MSS should be explored to determine their suitability. 

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Massimiliano De Mei

Papers

Black soldier fly (Hermetia illucens L.) as a high-potential agent for bioconversion of municipal primary sewage sludge