## 7 Insect Frass Against Abiotic Plant Stresses  
Climate change is increasing the abiotic stresses affecting crops, which causes significant agricultural losses (as discussed in Section 2). So far, several studies have been carried out on the use of insect frass to reduce the negative effects of abiotic stresses on plants, which are summarized in Table 1.  
In response to plant stress due to drought, several studies have been carried out on the use of insect frass from the mealworm (_T. molitor_) and the black soldier fly (_H. illucens_) as a strategy to reduce its negative effects. The frass from _T. molitor_ is characterized by a chitinous layer derived from the pleiotropic intestinal membrane of the insect. chitin in this frass has been reported to enhance drought tolerance in bean plants due to its ability to retain water [69]. Furthermore, by using unsterilized frass from the same insect species, researchers determined that certain microorganisms present in the frass induced drought tolerance responses in bean plants, complementing the effects previously reported for sterile frass [61]. In the case of insect frass from _H. illucens_, its potential to increase plant tolerance to drought in barley and lettuce has been described, although the underlying mechanism remains unclear. In both crops, it was described how the application of insect frass to the soil enhanced photosynthetic capacity of plants under drought stress, as well as decreased accumulation of polyphenic compounds, a sign of a lower presence of stress in these plants [109; 131]. Regarding flood stress, only one study has been carried out that uses insect frass to try to reduce it. Using both sterile and non-sterile _T. molitor_ frass, it was determined that certain microorganisms present in non-sterile frass promoted flood tolerance in bean plants. This suggests that the observed tolerance was microbially mediated, as no benefits were reported with sterile frass application [61]. Under salinity stress, plant tolerance has been successfully increased through the application of insect frass, although only one study has addressed this topic to date. The application of frass from _T. molitor_ to the growing medium increased the tolerance of bean plants to saline stress. This effect was attributed to both the presence of microorganisms capable of inducing plant tolerance and the ability of chitin in the frass to bind salt ions, as demonstrated using sterile and non-sterile frass treatments [61]. \begin{table}
\begin{tabular}{c c c c} \hline
**Reference** & **Journal** & **WoS Citations** & **Scopus Citations** \\ \hline
[137] & Occologia & - & 148 \\
[61] & Applied Soil Ecology & 98 & 109 \\
[116] & Ecological Entomology & 98 & - \\
[120] & Environmental Pollution & 69 & - \\
[97] & Plant Physiology & 56 & - \\
[142] & Journal of Chemical Ecology & 49 & - \\
[96] & Journal of Chemical Ecology & 44 & - \\
[103] & Environmental Entomology & 43 & - \\
[77] & Applied Soil Ecology & 37 & - \\
[98] & Journal of Chemical Ecology & 32 & - \\ \hline \end{tabular}
\end{table} 
Table 2: Number of citations of the 10 most cited articles. Finally, several studies have been carried out on the use of insect frass to combat the presence of heavy metals in the soil, using different mechanisms of action. The insect frass of _T. molitor_ has been shown to absorb heavy metals such as cadmium (Cd) in its own structure when applied to the soil [108]. Additionally, when this frass is processed into biochar via pyrolysis at 600 \({}^{\circ}\)C, its structure enables the sorption of lead (Pb), copper (Cu), zinc (Zn), and chromium (Cr) present in the soil [120]. Another mechanism of action described for mealworm frass in soils contaminated by heavy metals is complexation, which works for Zn, but not for Cd or Pb [93]. Therefore, both mechanisms of action (sorption and complexation) can act simultaneously when frass from _T. molitor_ is applied to the soil. This dual effect has been described for the removal of Zn, Cu, Cd, and nickel (Ni), using both _T.