## 4 Biological Control and the Three Ps: Predation, Parasitism, and Pathogens
### Pathogens  
Bacteria, fungi, algae, protozoa, or viruses, naturally occurring or genetically engineered, are pathogens of pest crops, and hence, are considered microbial pesticides [120]. The main advantages of this type of pesticide are their safety and specificity. Due to their toxic mode of action that is specific to a single group or species of insects, they have non-toxicity and non-pathogenic natures towards humans, wildlife, and non-target organisms, particularly those that are unrelated to the target pest or other beneficial organisms [121, 122, 123, 124]. Of course, there are limitations as well. New formulations and storage methods are needed for microbial pesticides. And because of their specificity, some microbial insecticides are not widely available or are relatively expensive [120, 125, 126]. _Bacillus thuringiensis_ is a naturally occurring soil bacterium and the most widely used microbial biopesticide in IPM programs [127, 128]. The bacteria and its toxins have been used for decades to control various insect pests such as caterpillars, mosquitoes, black flies, or beetles [121, 127, 129, 130], although further research is needed to control _B. oleae_ or _P. oleae_, as they are approved for use in olive groves, for example, in Spain [6, 131, 132, 133]. During sporulation, _B. thuringiensis_ produces an endotoxin called Cry protein, which, once ingested, causes an osmotic imbalance within the cell, resulting in the loss of the integrity of the midgut epithelium and thus killing the insect [134]. However, these toxins do not prevent insects from attacking the roots or internal parts of the plant when used as sprays [135]. These limitations have led to an interest in the development of novel biotechnological solutions to overcome these problems, as will be shown below. Symbiosis disruption in olive pests refers to the interference or alteration of the mutualistic relationship between the insect and its endosymbionts [136; 137]. Endosymbionts are microorganisms that live inside the body of the olive fruit fly and play crucial roles in its development, nutrition, and other physiological processes [138]. _B. oleae_ has been studied regarding its endosymbionts for over a century [139] and it has been identified as bacteria of the genera _Lactobacillus_, _Micrococcus_, _Pseudomonas_, _Streptococcus_, _Citrobacter_, _Proteus_, _Proteus_, _Proteus_, _Haftia_, _Klebsiella_, _Serratia_, _Pantoea_, and _Xanthomonas_ through traditional microbiological approaches [140; 141; 142; 143]. Molecular analyses have also shown the presence of _Aectobacter tropicalis_[144], _Pseudomonas putida_, _Asaia_ sp. [145], _Enterobacter_ sp. [146], or _Tatumella_ sp. [147], among others. However, the most notable endosymbiont associated with this pest is Candidatus _Erwinia daciola_, a non-culturable bacterial species that represents nearly the entire symbiotic population associated with the olive fruit fly [148; 149]. It is an endosymbiotic bacterium found in the _olf_ and is essential for its development and survival. This bacterium produces nutrients and other compounds that are important for the growth of fly larvae, thus influencing their abilities to complete their life cycles [150; 151]. Additionally, disrupting the symbiosis between _B. oleae_ and its endosymbionts has the potential to impact the fly's fitness, reproduction, and overall population dynamics. Symbiosis disruption can be achieved through various methods, but chemical treatments are, perhaps, the most studied ones, and Ca. _Erwinia daciola_ is the most evident target to implement this approach. In this respect, several compounds have been tested such as viridiol, propolisis, or copper oxychloride [152; 153; 154; 155]. The former is a secondary metabolite biosynthesized by beneficial fungi belonging to the genus _Trichoderma_ (Hendrik) [156; 157], while copper oxychloride is widely used in olive groves for disease control [158; 159; 157].