IntroductionEntomopathogenic microbes, occurring naturally, are virulent insect pathogens that include viruses, fungi, and bacteria. A wide range of lethal parasites also infect insects. Some of these organisms serve as important natural regulators of insect populations (Lacey et al., 2001). Appreciation of insect diseases and the possibilities of using insect disease agents in biocontrol programs have a long history (Steinhaus, 1957;Tanada, 1959). Commercially useful agents include viruses, fungi, bacteria, protozoans, parasitoids, nematodes, and predators, all deployed in the biocontrol of insect pests, weeds, and plant diseases. The following example illustrates this point.The control of the rhinoceros beetle, Oryctes rhinoceros, is among the successes in microbial control (Caltagirone, 1981). This insect was responsible for severe damage to oil palms in Asia, including Malaysia, Fiji, and Western Samoa. After considerable efforts with parasites and predators, a search for O. rhinoceros diseases led to the discovery of a new virus called Rhabdionvirus oryctes (Hüger, 1966). The virus was introduced into Western Samoa and several other islands, where it became established. Oil palm losses were very effectively reduced (Hüger, 1966). Other biocontrol programs were not as successful. The use of Bacillus thuringiensis var. kurstaki (Btk) against the diamondback moth in cole crops (cabbage, broccoli, etc.) enjoyed a large, albeit short-lived success. Btk was developed into commercial products that competed with traditional chemical control throughout the 1980s. However, the overuse of these products created field resistance, the first field resistance to a Bt product to be recorded (Tabashnik et al., 1990).Many factors affect the relative success and failures of biocontrol of insect pests, including costs, the context of comprehensive integrated pest management programs, education of users, government activities, and political and environmental concerns (Lomer, 1999). Viewed from a technical perspective, however, successful biocontrol depends on biological issues. These issues span a range of biological organization from the ecological level of microbe-host population dynamics to the molecular and cell biology of host defense mechanisms.One of the most important barriers to successful deployments of microbial control agents may lie in insects' robust and complex innate immune effectors. Insect innate immunity comprises a number of host defense effector systems. The insect integument and alimentary canal are formidable physical barriers to microbial invasion. Once past these barriers, invading microbes are confronted with fast-acting cellular defense actions,