Following the evolution of composting technology, the process of digestion of a biological substrate by insects (entomocomposting) represents the last stage; however, from its initial context of producing an organic fertilizer, the role of entomocomposting has been imposing itself (due to increasing demographic pressure) mainly in the safe disposal of organic waste (in rampant growth) and in the breeding of insects for food and feed, for the sake of food security. Both these last goals converge in the first, as the safest disposal of the compost is its use as organic fertilizer; but the organic substrates are of a diversified nature, as are the species of insects which have already proved themselves in entomocomposting; hence, for each of the purposes in view, the choice is vast and, in the same way, the entomocompost composition is wide-ranging. Furthermore, various types of organic substrates, in addition to a microbial flora with symbiotic effects, may sometimes be able to transmit to the frass a harmful load of heavy metals and/or, depending on the composting insect agents, the presence of microorganisms harmful to crops and to humans and animals; in these situations, the former should be encouraged, and the latter counteracted through appropriate composting technology. Directives and legislation in this area, if properly considered, constitute a fundamental basis for ensuring the appropriate use of this particular kind of organic fertilizer. Apart from the production of insects for food and feed, where the choice of which insect is determined at the outset, the preference for the insect to be used in entomocomposting should be considered according to its proficiency in biological digestion of the organic substrates available for this purpose and the fertilizing quality of the frass produced. Although a multitude of species have been evaluated, to date, for the digestion of organic substrates, most have been used in assessing their specific potential for certain functionalities of frass related to crop nutrition and health, but there are few which, either by prolificacy, proficiency or rapidity in digesting substrates, exhibit capacity to compete in rural environment; nevertheless, new species could be evaluated in the framework of the research of competitors for entomocomposting of all or each substrate type and for each of the main anticipated objectives, meanwhile, genetic improvement to obtain new strains specialized for different organic substrates has already started to take its first steps. In addition to the binomial “insect x substrate” the composting technology constitutes the third fundamental factor for the efficiency of the process. Insects use as a composting agent has been suggested several decades ago, but it was only in the last decade that this process grown from the garden to the factory. Within rural areas, entomocomposting could play a key role within a circular economy, where recycling and reusing potentially polluting wastes safely returns to the land the enduring fertility that enables the sustained production that generated them, requiring no particularly upscale installations, equipment or technical training; it can, therefore, be adapted to any size of agricultural holding, from smallholdings to large industrial holdings, on the other hand, and in order to obtain a controlled production and high quality entomocompost, it is needed to implement industrial technologies and the composting unit can achieve a very high production per square meter, comparing with traditional composting methods. However, whether from the perspective of agriculture, livestock or forestry, the production of waste for entomocomposting always falls far short of the necessary scale, and therefore always requiring the use of biodigested organic waste from agricultural industries, provided that the necessary precautions are taken; in any case, it always constitutes added value, due to the products it generates, in addition to the inestimable value of the productive disposal of potentially polluting products. Despite all the advantages mentioned above, the controversy over the organic vs. mineral fertilizer option persists, often fuelled by myths on both sides, but the successes already achieved with insect entomocomposts, such as the black soldier fly (Hermetia illucens L.) or the mealworm (Tenebrio molitor L.), in field trials, which are gradually adding up, anticipate an important role for insects in safeguarding global food and environmental security.
The role of organic fertilizer from natural dye waste and mycorrhizal inoculation on the growth of Indigofera tinctoria L.
