A great number and diversity of naturally occurring microorganisms are capable of causing disease in insects and other arthropods, and these pathogens have been manipulated for the purpose of pest control for more than 130 years (Lord, 2005). These efforts in applied invertebrate pathology have given rise to the field known today as microbial biological control, or simply microbial control, which is broadly defined as “that part of biological control concerned with controlling insects (or other organisms) by the use of microorganisms” (Onstad et al., 2006). The term biological control does not have a universally accepted definition. The principal disagreement relates to whether or not the term should include control by non-organismal biological factors (e.g. toxic metabolites and other natural products). We believe that the term biological control should be restricted to use of living organisms, and in this context, the simplest definition of biological control is: the use of living organisms to reduce damage by pests to tolerable levels. This definition is similar to those presented by Van Driesche & Bellows (1996), Crump et al. (1999), Eilenberg et al. (2001) and Hajek (2004), all of which limit the definition to use of living organisms. Employing this definition, it follows that biological control agents are living organisms and microbial control agents are living microbes (including viruses, which some do not consider as living, and nematodes, whose inclusion will be explained below).

Most pathogenic microbes in general produce large numbers of propagules (e.g.bacterial and fungal spores and viral occlusion bodies) that function as the infectious units.