## 9 Negative Stress-Related Effects of the Use of Insect Fass in Agriculture  
Despite the effects of insect fass in reducing the negative effect of biotic and abiotic stresses described above, some studies have reported negative effects related to these stresses in plants where fass is applied. All of these studies are compiled in Table 14.  
The most widely described negative effect related to the use of insect fass in agriculture is the spread of plant pathogens. In this sense, insect fass has been described as a vector for the transmission of phytopathogenic bacteria, such as _Erwinia tracheipilia_[103] and phytopathogenic fungi, such as _Colletotrichum graminicola_[83], _Verticillium albo-atrum_[106; 127], _F. oxysporum_ f.sp. _cubense_[90], _Fusarium proliferation_[140], _Ceratocystis lukuolia_, _C. lukuolia_[121], and _Cladosporium_ spp. or _Botrytis_ spp. Therefore, a plant health safety analysis of insect frass is required before it is placed on the market, in order to avoid the possible presence of plant pathogens.  
Furthermore, strategies for treating insect frass must be studied and established to try to prevent the spread of plant pathogens. As with other animal excrement, the composting process is an effective strategy for eliminating plant pathogens before their application to crops [144]. In the specific case of the European Union, the possible transmission of plant pathogens in the agricultural application of insect frass is less probable, due to Regulation 2021/1925 of the European Commission, which includes this insect product in Annex I of Regulation No 142/2011. Specifically, in order to market insect frass within the European Union, a prior heat treatment of at least 70 \({}^{\circ}\)C for 1 h [80] is mandatory.  
With regard to insect pests that affect crops, several mechanisms of action described for insect frass may favor its negative effects. Insect frass can be directly used as a food resource by plant-eating insects in a process of coprophogy, as is the case with frass from _F. auricularia_, which enables the pest to survive in the absence of a plant host [110]. Insect frass can also act as an attractant of insect pests through the emission of volatile compounds. In the case of the insect pest African cotton leafworm (_Spodoptera littoralis_), this attracting effect of its own frass is a consequence of the emission of the volatile compound guaiacol by the bacteria _Serratia marcescens_, _Enterobacter cloacea_, _E. ludwigii_, and _Klebsiella_ sp. [85].  
With regard to the induction of defensive plant responses, insect frass has also been described as an effective inhibitor. This is the result of an evolutionary response of insect pests to facilitate their feeding on plant biomass. For example, the insect frass produced by the Colorado potato beetle (_Leptinotars decmelineata_) contains bacteria of the genera _Acinetobacter_ and _Citrobacter_, which, upon contact with potato plant tissue, are capable of inhibiting jasmonic acid-mediated plant defense responses, such as the local accumulation of steroidal glycoalkaloids [91].  
The last negative mechanism of action of insect frass for crops is the inhibition of microorganisms used as bioinsecticides. This mechanism is a survival strategy of the insect against different entomopathogens. For example, against the entomopathogenic fungi _Metrahizium anisopliae_ and _Beauveria bassiana_, the patent leather beetle (_Odonottaenius disjunctus_) and the German cockroach (_Blattella germanica_) present in their frass bacteria that produce powerful antifungal metabolites [88; 118]. Therefore, it is important to understand the microbial diversity associated with the insect frass used in agriculture, not only to avoid spreading plant pathogens, but also to avoid negative effects on other biological strategies that may be used in crops.  
To conclude, and as a summary, an infographic (Figure 2) has been created showing all the positive and negative effects of insect frass in agricultural use, in relation to abiotic and biotic plant stresses.