# Endophytic Entomopathogenice Fungi: A Valuable Biological Control Tool against Plant Pests
## 2 Ecology and Mechanisms of EPF as Endophytes  
The term endophyte includes fungi and bacteria that develop within plant tissues without causing any noticeable symptoms of disease in the plant [46]. It is worth noting that endophytic fungi are abundant in plant species, while the characteristics and the level of the endophytic colonization differ depending on the plant tissue. The ecological functions of these fungi, which are related to their endophytic behavior and their competence in the rhizosphere, may hold enormous significance for crop protection and agronomy [47]. Numerous investigations revealed that EPF are not only effective against insect and mite pests but they also improve the plant's response to other biotic stresses. The latter happens through the induction of systemic resistance or through the production of compounds such as insecticides, antifungals, herbicides and antivirals, during multiple biocide activities. The benefits of these processes involve the promotion of plant growth [32; 48; 49; 50], plant nutrition [51; 52], root development [14; 53; 54], and relief from abiotic stresses such as salinity [55] or Fe deficiency [15; 32]. Reduced plant damage is achieved by endophytic EPF through many mechanisms. The most common cases that have been recorded are the retardation of the developmental rate of the pest [16; 56; 57], inhibition of insect food consumption rate [58; 59; 60; 61], reduction of larval survival [62; 63; 64] and decreased reproduction rate [65; 66]. However, it should be mentioned that many hidden and unexplored mechanisms may exist in these complicated tritrophic (EPF-plant-insect) interactions. Endophytic EPF colonization is not an incidental opportunity given that, upon colonization, specific chemicals are secreted by both the host plant and the endophytes. Various types of secondary metabolites, including alkaloids, flavonoids, phenolics and others, are produced by plants as a defense mechanism against pathogen infections, as well as in response to probable endophytic colonization, which is perceived, at least in its early stages, as a potential pathogenic threat [67]. Secondary metabolites thus represent obstacles for the colonization of endophytic fungi. To overcome this, endophytic fungi produce detoxification and degradation enzymes, including \(\beta\)-1,3-glucanases, chitinases, amylases, lacases, and cellulases. In addition, fungal metabolites play an important role in pathogenicity-related and other interactions between the fungus and its insect host. They mediate inter- or intraspecific communicative functions or aid in mitigating abiotic and biotic stresses [10]. Moreover, endophytes produce various secondary metabolites, which have a unique species-specific bioactive structure (i.e., benzopyranones, phenolic acids, quinones, and steroids) [53], and which are widely applied as agrochemicals, antibiotics, antiparasitics, and antioxidants [54]. The production of metabolites in host plants creates opportunities for the production of plant-based drugs. Although this has given a new impetus to natural-product-based drug discoveries, no endophyte has thus far been used as an alternative source of plant metabolites, as their production by the fungi rapidly diminishes in vitro. Studying the interaction between endophytes, and plant hosts and endophytes, would aid the sustained production of plant metabolites by endophytes, thereby alleviating the dependence on plants for such bioactive compounds [19; 40]. Initially, the conidia of EPF form germ tubes, which gradually become hyphae. EPF enter the plants either through natural openings or directly through the epidermal cell walls with the help of enzymes and/or mechanical pressure. Upon entering the plant, the hyphae grow in the space between the parenchyma cells or even in the xylem vessels. In most cases, evidence of endophyte-host mutualism has not been conclusive. Nevertheless, plant communities would likely not overcome many environmental stresses without this symbiosis.