Epidemiological studies are important for successful use of entomopathogens for pest management. Although microbial control of insects is applied epidemiology, there are few epidemiological-ecological studies, which partially explains why the use of entomopathogens has ended in failure. Isolated studies using preserved strains of entomopathogens in a laboratory might contribute to lack of efficacy under field conditions. An option is regional exploration of entomopathogens as the basis for epidemiological diagnoses, because this would enable identifying the most prevalent species and isolates that presumably have greater potential for use in pest management. Field epidemiological/ecological diagnoses allow identifying factors that induce the development of epidemics, including the range of hosts, plants, and cropping systems. Understanding their prevalence, dispersal, and pathogenicity is key to successfully detect pathogens in the field. Basic epidemiological characterization includes measuring n-variables associated with different categories or factors which as a whole explain the intensity of an epidemic (epidemiological system). Although epidemiology of entomopathogens has been studied in different regions of the world, variability (species and strains) and differing weather conditions make local epidemiological studies necessary. An agroecosystem for citrus production is a good model for epidemiological studies of entomopathogens. Fungi and phytosanitary diagnoses have been studied on citrus in the Yucatán Peninsula, Mexico. Future field studies, complemented with laboratory studies including characterization of pathogenic, physiological, and cultural aspect, with new methodological analytical approaches (i.e., Logistic and Weibull models), will aid rational use of entomopathogens in the field.