**7. Data in the context of the trinomial "soil x plant x fertilizer"**

Most of the research on the role of insects in soil fertility has focused on specific aspects of the benefits of their frass, not necessarily obtained through the technological composting of organic waste but through their metabolism as part of the soil entomological fauna; with this last aim, numerous studies have been published, as it can be seen in the comprehensive listing by Poveda [26].

These *in situ* studies focus on the possible symbiotic effect of frass within the particular soil entomofauna, in different ecosystems, to assess their role in the ecological balance of the same, particularly with regard to nutrition and phytosanitary aspects of the crop species.

When it comes to the use of a compost derived from off-site insect digestion of organic waste, a more objective assessment of its fertilizing potential, although guided by scientifically well-founded theoretical considerations, should be further informed by evaluation in preliminary production trials in a conditioned environment, in accordance with the pre-defined end goal.

The fact that seldom these trials showed a decrease in production - as was reported by Alattar *et al*. [39], when comparing processed food waste via Microaerobic Fermentation and BSF larvae biodigestion as soil fertilizers in maize, or Gärttling *et al*. [40], with a BSF frass in a low nutrient potting soil - may not mean that, in the overwhelming majority of experimental trials, the results with entomocompost compete with mineral fertilizers; it should be noted that on the one hand, the implementation of field trials is often preceded by a prior study of the feasibility of the hypothesis and, on the other hand, that regrettably a large proportion of the trials that do not confirm the hypothesis are not reported.

Some pot tests have shown the potential of entomocompost, obtained from substrates of various kinds, to reduce mineral fertilization in several crops, as for instance: With mealworm (*Tenebrio molitor* L.) frass, in barley [25] and ryegrass [41], and with BSF frass, in basil and Sudan grass [42], chinese cabbage [43], yardlong bean [44], lettuce [45, 46], ryegrass [45, 47], maize [48] and swiss chard [49].

Also testing the potential fertilizer value of BSF frass from several origins and for different plant species, in pot experiment, comparing either with other organic composts or with commercial substrates, the results found by Newton *et al*. [42], Rosmiati *et al*. [50], Setti *et al*. [51] and Kawasaki *et al*. [52], were encouraging, resulting in yield increases when using frass in certain amounts.

Although these results are encouraging, the conclusive proof, which is specific to the conditions that characterize the experimental situations, can only be given in the light of the results of the field test. Trials of this nature are still scarce and will never allow abusive extrapolations, but they constitute the most valuable information on the fertilizing potential of entomocompounds in relation to mineral fertilization.

Notwithstanding the fact that Temple *et al*. [53] have not foud positive results in a field trial with beans, using a BSF EntomoCompost (BSFEC) from food waste, most of the field trials where entomocompost is used as a complement of the mineral fertilization, for contrasting with exclusive mineral fertilization, have shown an increased production for the mixt alternative. This was the case reported, among others: by Anyega *et al*. [54], in a 'acric ferralsol' trial with tomato, kale and fresh beans, with BSFEC from Brewer's spent grains; by Quilliam *et al*. [55] in a 'Ustic duraquert' trial with chili pepper and shallots, with an identical entomocompost, so as in the same soil, with maize; and by Temple *et al*. (*op. cit*.) in a 'Humic Gleysol' trial with bok choi, lettuce and potato, with BSFEC from Brewer's spent grains.

These results support the thesis that in the experimental situations tested to date, the percentage of CASH capable of competing with mineral fertilization alone, in what concerns the immediate fertilization for crops, is between 10% and 40% in volume. More optimistic results were seen, for example, in a demonstration field [56] with potato (*Solanum tuberosum*), comparing traditional mineral fertilization without and with CASH (from the digestion of agroindustrial waste of potato

and onion), where a 9% increase in yield was recorded and, in addition, the tuber specific weight, and the percentage of dry matter were also higher when combining both fertilization approaches.

The arguments mentioned so far, based on experimental results endorsed in the literature, justify promising perspectives regarding the role of insects in the production of organic fertilizers capable of allowing a reduction of mineral fertilizers as far as the immediate fertilization of crops is concerned; nevertheless, more important than the immediate fertilization of crops is the deferred fertility of soils, both in the resilience or increase of their fertility and in the acariation of soils rendered unproductive by anthropogenic or climatic effects.

In any case, the medium- and long-term promotion and resilience of the fertility of the soils, which should be fostered by insect frass, would be translated, as for the generality of organic fertilizers, by the improvement of the structure of the soils and its capacity to retain water and crop nutrients and as well as by symbiotic interaction with the soil microbial flora and with the plant. Many knowledge within this perspective is still needed, but also a lot have been accumulated, allowing for hopeful evidences, such as: better use efficiency of P and K [57]; improved soil fertility and defense against pathogens [58]; suppression against *Pythium ultimum* [59]; influence on soil N availability [60]; stimulation of soil microbial activity and diversity [25]; not impairing hygienic properties of soils [47]; improvement of microbial activity [41]; increased dehydrogenase activity [61]; or increased enzyme activity (dehydrogenase and β-glucosidase) [46].

These data augur well, but medium and long-term field trials are indispensable for continued soil fertility management, since organic matter resilience is not its greatest virtue, particularly in tropical and sub-tropical climates.

Despite being still at the beginning of its career as a biodigester, for the production of organic fertilizers, and beyond the benefits of the utmost relevance in the perspective of safeguarding the environmental balance and food safety, research and experimental development has already given concrete proof of its potential as an indispensable partner in the resilience and recuperation of soils for agricultural production.

## **8. Conclusions**

The role of insects in the biological digestion of organic substrates, with a view to the fertilizing potential of entomocomposts, has raised a growing commitment from the scientific and technical community in the field of agriculture and environmental protection; however, despite the accumulation of positive results from the application of this type of fertilizer, significant progress is still expected in this sector, with the improvement of the genetic capacity of insects, of the pre-treatment of substrates and of the entomocomposting technology, so as the adequacy of fertilization techniques.

In the context of the organic fertilizers, entomocomposting takes precedence over other composting methods, mainly because of the speed of the organic waste digestion process, drastically reducing composting time and thus the risk of environmental pollution, besides advantages such as soil health, pest control, sprouting and germination potential.

Various insects have been tested for their potential in digesting substrates of a very different nature, giving rise to entomocomposts with positive results, in reduce mineral fertilizers, in crop production, or as correctors of certain chemical and/or microbiological deficiencies, not to mention physical soil deficiencies, for which any organic fertilizer is capable of dealing with.

Nevertheless, more results are expected with further research into entomocomposting technology, with the discovery of new insect species and their genetic improvement for the biodigestion of different organic substrates, and with new techniques for the enhancement of the fertilizing effect of composts, in order to make available suitable formulations for different "soil x plant x fertilizer" interaction situations.

Until now, as shown in the tests presented in this analysis, the greatest success in the contentious debate "organic vs. mineral" has been achieved in situations of compromise, where the organic fertilizer has the complementary role, by its relatively low and unbalanced nutrient content, notwithstanding its biological interaction with plant and soil microrganisms, its action in improving the soil's physical properties and its capacity to retain water and nutrients – so, as advocated by Ronald and Adamchak [62] or Amman K. [63], and as Saint Tomas d'Aquino said so well, '*in medio stat virtus*'.

Furthermore, although growing exponentially, increased production of organic waste for entomocomposting is unlikely to be sufficient to ensure global food security on its own, as it is a direct function of population growth; suggestions based on success rates reported in the literature for insect frass - ranging from 10 to 40% by volume - may be realistic to be expected, at least in the medium term. In fact, if the potential of the triple valence of entomocomposting (protection of the environment, food security and resilient soil fertility) can already be categorically stated, the use of entomocompost as a fertilizer still faces the major constraint of the lack of scale of its production.

Considering all these possibilities, insects must be recognized not only as a nutrient source but also as a tool. The value of insects can surpass the production of nutrients and the use of its by/co-products to increase its profitability. In the near future insects could be used in manure and household waste treatment approaches, decreasing the environmental impact of livestock production and landfill volumes [5, 64, 65]. This approach would open a completely new opportunity for insect rearing, that is distinct from insect production for animal nutrition which must comply with safety and hygiene regulations.

Increased sustainability of animal and food production can be delivered by insect use, not only through the development of new feed resources but also by contributing to the reduction and conversion of wastes into novel raw materials for bioindustry and biorefinery approaches.

There is still much to do in this regard but, in rural areas, the proposal of a circular economy system in the management of agricultural, livestock and forestry production, with circularities within private farms to be extrapolated (cooperatively) to the regional level with agroindustry and an industrial entomocomposting unit, deserved to be weighed up.
