## 5 Insect Frass as an Agricultural Resource  
Mostly used as a fertilizer due to its nutrient content, insect frass also acts as a phytortifier, improving plant growth and health beyond its nutritional contribution [61; 62; 63; 64]. Furthermore, its use as a natural amendment increases soil carbon content, which is especially helpful in Mediterranean areas with little organic content [65]. Given these characteristics, frass can be incorporated into carbon farming plans to help slow climate change and promote sustainable agriculture [66]. In addition, some studies have investigated the use of _H. illucens_ frass in aquaculture diets as a partial substitute and have found encouraging results with species, such as hybrid tilapia (_Oreochromis niloticus x O. mozambique_) or white shrimp (_Litopenaeus wanname_) [67; 68].  
What makes frass such an interesting biofertilizer is its rich nutritional profile, which is easily assimilated to by plants [69; 62] and packed with essential nutrients, the presence of beneficial microorganisms that can support plant health and soil vitality [70], and its ability to release nitrogen gradually [71]. Frass has the potential to be used as a soil improver and plant fertilizer by supplying soil not only with nitrogen, phosphorus, and potassium [72], but also secondary (calcium, magnesium, and sulfur) and micro-nutrients (manganese, copper, iron, zinc, boron, and sodium) [60; 73]. However, each type of frass has unique characteristics in terms of nutritional composition and microbiota, with diet significantly influencing its nutrient content [60; 74], suggesting that its agricultural application should be tailored to the needs of the soil and crops. A comparative study conducted by [75] analyzedthe mass of _H. illucens_, _T. molitor_, and _Gryllus assimilis_ (JFC), highlighting differences in composition and microbiota. _H. illucens_ frass exhibited the highest ammonium and plant-available phosphorus content, along with high microbial activity, while _T. molitor_ frass had higher nitrate levels and lower biological activity. In contrast, _G. assimilis_ frass contained the highest total nitrogen content and a distinct microbiota. These differences suggest that the agricultural application of each type of frass should be tailored to the specific needs of the soil and crops.  
Despite frass' potential, challenges remain regarding its large-scale application as a fertilizer. Ref. [76] found that 98% of mineral nitrogen in _H. illucens_ frass is ammonium, which may pose risks of phytotoxicity at high doses and potentially reduce mycorrhizal colonization. Ref. [77] observed that a 1.5% application rate of _T. molitor_ and _A. diaparius_ frass improved plant growth and nitrogen mineralization, increasing NH\({}_{4}\)\({}^{+}\) and NO\({}_{3}\)\({}^{-}\) availability. However, a 3% application inhibited germination, likely due to ammonium toxicity or high salinity.  
When comparing insect frass to more widely recognized fertilizers, such as manure or vermicompost, the scientific literature remains limited. Studies indicate that _T. molitor_ frass has shown effects comparable to those of chicken manure, with no significant differences in edible biomass yield. However, plants treated with frass exhibited a higher flower production [78]. Furthermore, research by [79] found that frass application led to increased chlorophyll content in treated plants compared to those receiving vermicompost.  
To regulate its production and commercialization, the European Union has recently approved the use of insect frass as a fertilizer, a decision expected to boost its adoption in crop production and expand its market [80]. Valued at approximately 96 million dollars in 2023, the market is expected to exceed 135 million dollars by 2030, with an approximate compound annual growth rate (CAGR) of 6% from 2024 to 2030 [81].