Introduction In general, bacterial entomopathogens have low virulence and are opportunistic. Bacteria belonging to the genus Bacillus however have proved to be highly effective control agents as they infect and kill a range of orders. Bacilli  are Gram positive, produce spores and are found abundantly in the soil. The bacteria typically infect the larval stage via ingestion and are transmitted horizontally. However, lack of contact action means transmission is low and therefore Bacillus pathogens need frequent re-application. Fortunately, however, they are cheap to grow and produce. Detailed Example: Bacillus thuringiensis Bacillus thuringiensis (Bt) is the most widely used microbial biopesticide and is an example of inundative control. It has been deemed the safest insecticide on the market by the World Health Organisation (Ehlers, 2011). Bt targets species in the orders of Lepidoptera, Diptera and Coleoptera, pests of vegetables, fruits, cotton, rice and maize. Bt was first used in the late 1950s and by 1998, around 200 Bt products were registered in America (Harper, 2006; Lacey et al, 2001). Infection Process During sporulation Bt produces a protein pro-toxin crystal, located in parasporal inclusion bodies (Figure 1). Crystals ingested by larvae dissolve within the alkaline midgut and are cleaved by gut proteases to form delta- endotoxins. Bt is therefore selectively lethal to organisms with an alkaline gut. The toxin binds to specific gut receptors which damages the gut via pore formation and membrane transport disruption which results in cell paralysis (Lacey et al, 2001). The insect stops feeding and dies within 2-24 hours (Copping and Menn, 2000). The infection is not usually transmitted to new hosts (i.e. it is a dead end infection). Crystalline proteins are encoded by cry genes located on extra-chromosomal plasmids in the bacterial cell (Copping and Menn, 2000). More than 200 genes have been identified which code for a different toxin in Bt. Each toxin is specific to a particular insect species because it binds to a different gut receptor. Other Bacillus species successfully used as MCAs These species are used on a smaller scale than Bt but are still very important. 1. Bacillus papillae targets the order scarabaeids and in particular causes ‘milky disease’ in the Japanese beetle. This species was the first entomopathogen to be released onto the market in America and was registered in 1939 (Harper, 2006). 2. Bacillus sphaericus has been used in Europe and Africa to kill mosquito larvae, in particular Culex species (Copping and Menn, 2000). Figure 1 Crystals of Bt toxin Genetically Modified Bt Crops An alternative to applying the toxin producing bacteria directly onto a crop, is the controversial technique of taking toxin coding genes and putting them into plants. Genetically modified Bt-crops are therefore engineered to produce a toxin themselves which kills any insect feeding on it. In America, Bt-crops such as cotton, tobacco, maize and rice have been grown since 1996 (Nicholson, 2007). Transgenic plants that produce insecticidal proteins are highly effecitive at protecting the whole plant from phytophagous (plant-eating) invertebrates (i.e. protects parts inaccessible to foliar sprays). Benefits and Costs Bt has similar benefits to chemical pesticides and therefore it is no surprise that 70-80% of all spending goes on this control agent (Harper, 2006). This non-living toxin acts fast, has broad activity and is insect specific. Similar to chemical pesticides however, resistance is starting to emerge in some populations. The diamondback moth,  Plutella xylostella for example is a serious worldwide pest of cruciferae vegetables that has developed resistance to Bt biopesticides (Harper, 2006).