**3. On the substrate to be composted**

If initially the use of uncontrolled composting of organic agricultural wastes may have been motivated by obtaining a fertilizer for crops, as in the case of the use for this purpose of animal bedding and manure slurry, the fact is that subsequently, with the exponential growing production of industrial and urban organic waste, the emphasis has shifted to the disposal of polluting waste.

Within this last context, controlled composting methods have been developed [9], including in its motivation for the production of arthropods and worms for food and feed, and not least for the production of organic fertilizers; in fact, food security is no less relevant, and to this end the resilience of agricultural soil fertility, the restoration of depleted soils and even the acariation for agricultural production of infertile soils hitherto ignored for this purpose, without compromising environmental security, is urgently needed.

But fertility is an ambiguous concept when applied in relation to the productive capacity of agricultural soils. Actually, boosting the full productive capacity of a given plant species (or even of a given genotype of the same species), depends on rigorously reconciling its physical and chemical requirements with the soil and climate conditions in which it is located, and these can be very diverse – in poor soil conditions, a primitive variety has more yield potential than a variety improved for yield capacity [10, 11].

However, that above-mentioned objective, of full productive capacity, will not always be the most appropriate if it is not based on economic, environmental and ethical considerations, because maximizing production does not always lead to greater financial return, it frequently translates into an environmental burden and it often forgets the responsibility of the agricultural sector in the context of global food security.

With regard to soil fertilization, which underlies the subject of this Communication, the above considerations are also valid: Soil fertilizers should be required to provide an advantageous cost–benefit balance, to cooperate in

protecting the environment, and to increase the resilience of soil fertility as a basis for long-term food security. According to these requirements there seems to be difficult to find a perfect type of fertilizer.

As with the majority of organic fertilizers, the main virtue of entomocompost lies in their action in correcting the physical, chemical and microbiological properties of soils (a fundamental factor in their deferred fertility) and in supplying the mineral elements necessary for each crop, in each specific situation (an important factor for their immediate fertility) [12]. Furthermore, entomocomposting can be a relevant factor in recycling exhaustible plant nutrient resources; an example of this is mentioned by Zhang *et al*. [13], when they observe the accumulation of phosphorus in the frass of grasshoppers as a function of the stoichiometric homeostasis of the N:P ratio in their bodies.

Any entomocompost is, however, more than the frass obtained from a given substrate; apart from the frass, it contains the remainder of the substrate and residues from the metamorphosis of pupae into adults (if not the pupae themselves). This is not entirely true in the case of biological digestion by the BSF larvae, which evidences the unique behavior of abandoning the compost at the pre-pupa stage, a phenomenon with obvious advantages in pupal harvesting and which is referred to in the bibliography as self-harvesting. For this reason, for the entomocompost obtained from BSF was proposed [14] the acronym CASH (Compost After Self Harvesting).

But the constraints on the more generalized use of entomocomposts are not limited to those mentioned above. They range from lack of definition of the exact formulation of their composition, to lack of knowledge of the mineralization rate (for formulations of its fertilizing elements that can be directly assimilated by the plants) in the soil and climate situations in question, to logistical and scale limitations to supply. In reference to this latter setback, Timsina [15] states that "considering the current organic sources of nutrients in developing countries, organic nutrients alone are not enough to increase crop yields to meet global food demand".

Through bio-digestion by insects, the formulation of the substrate is largely altered in its physical, chemical and microbiological composition, with decisive consequences on the fertilizing potential of the entomocompost; it, therefore, plays a relevant role in the quality of the compost. As referred by Poveda *et al*. [16], by modifying the insect diet, not only do you get different nutritional content in the frass, but also significant changes in the actual microbiota, both aspects relevant to its ability to be used as organic fertilizer.

As an extreme situation, regarding the nature of the substrate, Koh *et al*. [17] reports that polystyrene, when digested by the coleopteran *Zophobas morio*, produced a starch-rich frass, which promoted the growth of *Hylocereus undatus* plants from both the aerial and root parts.

An entomocompost of a substrate of agroindustry origin, or of remnants and residues from agricultural production, is not seen as a threat of chemical or microbiological contamination of agricultural soil. On the other hand, a compost obtained by insect bio-digestion in industrial urban waste plants, requires analysis and possible remediation if chemical and microbiological substances harmful to soil fertility are found to be present, as for instance in the case of houseflies, with high levels of lead and arsenic in the frass [18].

Entomocompost derived from manure and slurry does not normally pose the danger of soil contamination of any kind, however, in the context of insect production for feed or for food, is not at all suitable, and is subject to severe restrictions. To overcome these constraints, which are mainly dictated by the nature of the substrates, some progress has already been made.

#### *The Insects as a Workforce for Organic Fertilizers Production – Insect Frass DOI: http://dx.doi.org/10.5772/intechopen.100144*

Thus, although the use of unsafe wastes as substrate for entomocomposting can be done with efficiency, as this technic is mainly applied for food and feed purposes, it is quite uncommon to see full-scale insect rearing units using such substrates. In fact, the majority of entomocomposting units, or insect farms, are using vegetable coproducts as substrates for insect production, being its main purpose the production of insects as a protein source and the insect frass a co-product of this process. Nevertheless, and as previously demonstrated, entomocompost is a high valuable product with very good effects in soil fertility and plant health.

This way, entomocomposting should not only be considered as a process to produce food and feed, but also to produce the entomocompost as a main objective, opening the use of other, unsafe, wastes that cannot be used when the produced insects are intended for the food chain. However, such approach will require studies to evaluate food safety concerns in a one health approach, evaluating from environmental impacts and benefits, to possible impacts in the soil and plants. The so produced insects cannot be intended for the food cahin but might well find an economic value as a raw material for biorefineries such as fuel and plastic, or to be also used as fertilizers.

The use of entomocomposting technics to convert unsafe substrates such as urban organic wastes, manure or sewer sludge, is being tried in several R&D projects. Such is the case of NETA project (POCI-01-0247-FEDER-046959), a project in which a new manure and sewer water is being treated with a novel process and the sludge is being tested as a substrate for entomocomposting. This produced insect frass is being tested in vegetable and olive oil production, while the larvae are being evaluated in terms of safety, evaluating both chemical and microbiological contaminants, and being used for industrial purposes.

In order to be possible to produce insects for food and feed purposes with organic wastes as substrate, what would unlock the entomocomposting potential as a bioremediation tool, one should first show if such approach is safe. However, before proving its safety, one of the main challenges in entomocomposting organic wastes is that if we used the same insect species as for the production of food and feed, it will not be possible to differentiate insect products produced with safe or unsafe substrates. Thus, one possibility would be to develop the entomocomposting process of unsafe substrates, such as organic wastes, manure and sewer sludge, with insect species not being used for food and feed purposes. That would allow to differentiate the obtained insect product with DNA testing and would unlock a very beneficial tool for the treatment of high environmental impact organic wastes, transforming them into novel products and returning lost value to the value chain, while contributing to both economic growth and sustainability in a 100% circular economy approach.

Thus, it should be highlighted that using insects for nutrient production is not a goal in itself but can be an instrument to achieve goals in biowaste reduction and conversion, improving sustainability and optimizing the food value chain. Insects should be evaluated as a tool to increase the efficiency of resources use and to increase income, and thus, one must evaluate them beyond their nutrient value as a feed ingredient.

For example, BSF are a rich source of lipids which can be industrially extracted to obtain a pure oil with several different potential uses, from feed and food, to biodiesel and cosmetics. It has been shown that BSF fat could be a useful alternative for other commonly used fats, with specific technological properties in common with palm and coconut oils, which are increasingly associated with negative environmental impacts. In particular, the melting and crystallization behavior of BSF larval fat seems to allow replacement for traditional fats [19].

In addition, the insect exoskeleton can be processed to obtain chitin and chitosan, and its industrial scale production could offer a potential source of prebiotic oligosaccharides for pet, animal, and human nutrition [20]. Applications for chitin and chitosan go beyond nutrition, as chitosan is characterized by non-toxicity, biodegradability, film-forming capacity, antimicrobial and antioxidant properties and good barrier properties of packaging films against oxygen [21–23]. Thus, the potential for the use of insect derived chitosan to produce biodegradable plastics is being evaluated for a variety of applications, ranging from agriculture to food packaging.

Chitin-derived products have also been shown to be toxic to plant pests and pathogens, inducing plant defenses and stimulating the growth and activity of beneficial microbes. Chitin-based treatments augment and amplify the action of beneficial chitinolytic microbes [24]. Such properties prompted the development of novel crop fertilizer and crop protection products, which can be used in conjunction with one of the main insect products, the insect frass. In natural conditions, it is well known that frass deposition to soil has a great impact on soil fertility due to its high nutrient and labile carbon content and, therefore, several companies have already started to sell frass as a fertilizer [25].

#### **4. Insects as agents to produce organic fertilizers**

With all the economic and environmental advantages in the search for agricultural production that is compatible, in a sustainable way, with global demand, entomocomposts have been affirming themselves as an important alternative for reducing (if not replacing, in some cases) synthetic mineral fertilizers. To this end, several insect species have been evaluated for their proficiency in composting organic substrates.

In a careful literature review, Poveda [26] presents two thorough lists of studies on the use of insect frass as fertilizers, indicating, for each case, the plants, the benefits and the mechanisms by which these benefits were expressed.

In addition to providing the necessary mineral nutrient elements for plants, the benefits provided by adding insect frass to the soil are diverse in nature, such as: increased germination, sprouting, growth and nutritional content of plants; increased tolerance to abiotic stress; activation of the plant's defense mechanisms against pathogens and pests; increased nitrogen in plant tissue; reduced oviposition of pest insects and; increased microbial activity in soils.

However, these advantages are not all concentrated in a single species so, although frass from various species may have a relevant role as a complementary fertilizer in specific situations, few species have shown the potential to produce an entomocompost with the potential to be an alternative to avoid completely mineral fertilization in all situations. One of this specie contradicts the thesis of Lardé [27], supported by Smetana *et al*. [28], that one species cannot be suitable for the huge diversity of organic substrates - BSF has proven to be quite "cosmopolitan", living comfortably in any type of organic substrate experienced so far [29–33]. Note, however, that even in cases - in experimental situations - where entomocompost shows the potential to provide a reduction in synthetic mineral fertilizer, the fertilizing effect of entomocompost can still be enhanced if associated with appropriate soil handling technology. In this context, Dulaurent *et al*. [34], in a pot trial with frass, reports a significant increase in nutrient content in the plant by the addition of earthworms (*Lumbricus terrestris*), by promoting an acceleration of the recycling of fertilizing elements from the frass.

But if the possibility of biotic associations or of physical or chemical corrections of entomocompost for preferential purposes (within the versatility of its benefits

#### *The Insects as a Workforce for Organic Fertilizers Production – Insect Frass DOI: http://dx.doi.org/10.5772/intechopen.100144*

in crop nutrition and soil fertility resilience) is a proven reality in the experimental and commercial field, in the field of genetic enhancement of insect species for entomocomposting only the first steps have been taken.

Advances in this field are predictable and particularly desirable, notably when it comes to insects with the ability to adapt to a wide diversity of substrates. This is the case of BSF, which is able to efficiently biodigest manure from various livestock species (from polygastric, monogastric or fish species) as well as residues and remnants from crops or from agroindustry.

The methodologies for this could be very diverse, but the simple continuous selection of pupae fed on the digestion of specific substrates can lead (as it has happened with the generality of animal and plant species already submitted to human-induced selection) to the differentiation of specialist genotypes more competent than generalists, probably because it should be anchored in an evolution towards more targeted physiological mechanisms that are necessarily less energydemanding; this option, which would certainly not meet with the disagreement of the detractors of transgenics, would only require, as an additional effort, the separation of breeding facilities for flies, even though it may be slower in results, but "constant dripping wears the rock away".

## **5. Entomocomposting technologies**

Insect production has grown a lot in the last decade. This new sector emerged with the support of FAO-UN who first referred to this field in the beginning of the decade and started an insect rush in several countries, with the development of new business. However, by then, both the business and the process where not mature yet and it took several years of development to see the first full-scale insect rearing unit being built by a handful of companies. However, the legal framework had not grown at the same rhythm, what promoted a lower growth. At this point, different companies have developed their entomocomposting technics in parallel, and even using the same insect species and substrates the processes can follow completely different approaches.

Besides that, although several approached have been made to create technologies to produce insects, and entomocompost, at a small scale, and although it can be applied at the farm level, it is only economically relevant at a large and controlled scale, ensuring both food safety and traceability.

Large scale insect production is an industrial sector in which several tons of vegetable by-products are converted by insects every day. Contrary to most composting technologies, insect production generally does not use piles of by-products. It processes them into controlled mixes of raw-materials ready to be digested. This raw material processing allows a steady rhythm of conversion and production. In most cases insects are thus reared inside plastic boxes of different sizes in large controlled environment warehouses. The time needed for composting and the number of insects to be used to convert each ton of by-products change from insect specie to insect specie and between companies. The main insect species to be used for food and feed are *Tenebrio molitor* and *Hermetia illucens* (BSF), however the last one is more prone to be used as an entomocomposting tool, as it have a large range of vegetable by-product conversion capabilities. BSF can convert decaying by-products in as few as 7 days, depending on the technology used, and some of the already existing BSF production units can convert as much as 100 tons of vegetable by-product every day into 20 tons of insect frass, while also producing 17 tons of insect larvae. However, this numbers and process greatly change between companies, which all apply different technologies, even when producing the same insect species.

Therefore, insect production has not only to achieve economically viable production at scale, investing in new full-scale insect production units, but it must also be standardized, to obtain a steady production and uniform product. Standardization is key not only in relation to a single production unit, but also between different producers. Insects as a food and feed resource, and also as a plant nutrition source, would greatly benefit from standard quality and nutritional values when considering the same insect species and product. This would increase farmers trust in this novel fertilizer. However, different insect producers may use different insect species and rearing substrates, as well as different production and processing techniques. This results in different products, with different nutritional values and properties, entering the market.

Nevertheless, as the insect rearing industry is only in its infancy, we believe that in the future the production and processing of insects and frass will tend to be more similar between operators, as different production processes and technologies attain relevance in the sector. One opportunity to increase standardization and quality of insect products might be technology transfer between companies, enabling rapid growth of this novel sector and allowing investors and new operators to enter the market without the need to invest in the development of processing technologies. Technology transfer from other companies and research institutes that have spent recent years in R&D will have processes providing the most suitable solutions, avoiding the need for new producers to start from scratch, costing time and money as well as decreasing the chances of success for new businesses.

### **6. The role of entomocomposting in the context of a circular economy in rural areas**

For Zink & Geyer [35] "the proponents of the circular economy have tended to look at the world purely as an engineering system and have neglected the economic part of the circular economy"; to this assertion, the facts have been demonstrating, convincingly anchored in science, that the linear economy alternative, in turn, blatantly belittles the environmental part.

The circular economy is "a new economic model operating in closed circuits, catalyzed by innovation throughout the value chain" [36], and, within the agrarian economy, whether in plant or animal production, entomocomposting is an innovative alternative, more efficient than traditional composting, to reduce the import of feed and fertilizers and energy losses, with added advantages in terms of safeguarding the environment.

This is how the entomocomposting of crop remains and residues, so as livestock production wastes, is a multifaceted pivotal factor of the greatest relevance to different circularities within farms, as shown in **Figure 1**.

The circularities represented in the diagram are multiple and interlinked and are not necessarily closed. In fact, there will always be a need for outsourcing, both for supplementation of feed and fertilizer, in quantities compatible with optimizing the efficiency of the entomocompost and the feed value derived from pupae.

The protagonist in this diagram is BSF, for the peculiarities that distinguish it, in a positive way, from the other composting agents, namely:


*The Insects as a Workforce for Organic Fertilizers Production – Insect Frass DOI: http://dx.doi.org/10.5772/intechopen.100144*

#### **Figure 1.**

*Multiple circularities driven by BSF on farms.*


But beyond these, two singularities, particularly relevant in agricultural and livestock holdings, distinguish them from other insects:


Finally, and following the results reported by Yildirim-Aksoy *et al*. [37, 38] in trials with channel catfish (*Ictalurus punctatus*) and hybrid tilapia (*Oreocromis niloticus* x *O. Mozambique*), frass obtained by biodigestion by BSF of suitable substrates, more than an organic fertilizer, can be a feed in aquaculture.

Notwithstanding the fact that entomocomposting by BSF has already proven to be highly efficient in recycling and reuse capacity in the plant and animal production circuit, the deficit generated in the export of plant and animal products to the market means that feed and fertilizers have to be imported. To this end, in rural areas, initiatives (possibly of a cooperative nature) for the production, on an industrial scale, of entomocomposts and larvae (or pupae) for soil fertilization and feed, allow the circularity of the production system to be extrapolated from the individual sphere to the community sphere; With this type of initiative, the agroindustry will play a relevant role, with additional advantages in terms of capacity and fluidity of the system, broadening the scope of circularity at regional level.
