## 8 Insect Frass Against Agricultural Pests and Pathogens
### Against Insect Pests
In addition to being used as a vehicle for entomopathogens, insect frass has been widely studied as a source of volatiles with great potential for controlling insect pests in plants. These volatiles include pheromones from pest insects present in their frass, which act as powerful attractants for entomopathogens, such as nematodes. This mechanism has been described in the frass from the citrus root weevil (_Diapperes abbreviatus_), which emits a specific pest insect pheromone that actively attracts the entomopathogenic nematodes _Steinerema diapperesi_ and _Heterorhabditis indica_, even when applied in citrus orchards [114]. Therefore, the frass of the insect pest itself can be used for its control by actively attracting entomopathogenic nematodes. Similarly, other works have described how frass from various insect pests actively attracts their predators. For instance, for the frass from the lepidopteran pest tomato pinworm (_Tuta absoluta_), different volatile compounds have been described that actively attract the pest's predator. These compounds are the monoterpenes \(\alpha\)-pinene, \(\alpha\)-phellandrene, 3-carene, \(\beta\)-phellandrene, and \(\beta\)-ocimene, which attract the polyphagous hetropteran _Nesidiocoris tenuis_[89]. Similar effects were reported with frass extracts produced by the insect pest larger grain borer (_Prostephanus truncatus_), which contained volatile compounds that actively attracted its predator, the specialist beetle _Teretrius nigrescens_[138]. These insect frass volatiles can also act as attractants of parasitoids of insect pests, a field of study that has been widely addressed in the 21 st century. In vivo studies have reported the attractiveness of coleopteran pest frass and its extracts to parasitoids that control the pest [102; 112; 132]. Furthermore, coleopteran frass has been used for the active control of insect pests. This is the case of the frass produced by the lesser grain borer (_Rhyzpertha dominica_), which has been described as an effective attractant for the generalist parasitoid _Theocolax elegans_ when applied to stored wheat grains [125]. In the case of lepidopteran insect pests, this mechanism of action has been described for various species in vitro, such as _S. frugiperda_[117] or _T. absoluta_[86]. Specifically, in the case of _T. absoluta_, the volatile compounds responsible for attracting the parasitoid _Dolichogerida gechelidioviris_ were identified as \(\alpha\)-pinene, \(\beta\)-myrcene, \(\alpha\)-phellandrene, \(\alpha\)-terpinene, \(\beta\)-ocimene, methyl salicate, and (E)-\(\beta\)-caryophyllene [86]. These attracting effects on parasittoids have also been confirmed by applying fass directly to plant material, bringing the use of this fass closer to actual commercial use to reduce the losses caused by these pests. In apple fruits, the application of fass from the codling moth (_Cydia pomonella_) effectively attracted its parasitoid, the wasp _Hyssopus pallidus_[96]. Similar results were reported for the wasp _Glyptapanteles favicaxis_ on poplar leaves (_Populus nigra_) through the application of fass from _L. dispar_[116]. Some of these volatile compounds with attractive properties for entomopathogens, predators, or parasittoids may be produced by microorganisms present in the fass. However, the volatiles obtained via these microorganisms have not been described with these attracting mechanisms so far. These microorganisms have been studied in the fass from the large pine weevil (_Hylobius abietis_), which produces volatiles that reduce the infestation of Scot's pines (_Pinus sylvestris_) by the insect pest. The bacteria isolated from this fass are capable of producing the volatile compound 2-phenylethylanol, which acts as an antifeedant compound against _H. abietis_[133]. In addition, the filamentous fungi _Opliustoma canum_ and _O.