Introduction

The spore-forming, Gram-positive bacterium. Bacillus thuringiensis (Bt), is a ubiquitous soil micro-organism with a world-wide distribution (Martin and Travers, 1989). Many thousands of isolates have been discovered since Ishiwata first isolated Bt from diseased silkworm larvae in 1901 (Nakamura and Dulmage, 1988; Beegle and Yamamoto, 1992; Feitelson et al, 1992), and it is possible to group these isolates into at least 34 serovars (also called subspecies) (de Barjac and Fraction, 1990). However, biomolecular techniques, such as multiplex polymerase chain reaction (PCR), permit a more rapid identification of strains (Bourque et at., 1993), and it is now possible to target specific DNA sequences with the use of this technology.

The delta-endotoxin produced by Bt is the most widely used biological pesticide – 2.3 million kg was used annually according to Rowe and Margaritis (1987) – and it is the ‘front-runner’ in attempts to commercialize biological control agents (BCAs); albeit that Bt is only a BCA in a limited sense, since it is the product of a living organism (the delta-endotoxin) which is utilized, rather than the organism itself.

The precise ecological role of this cosmopolitan invertebrate pathogen in the natural environment remains speculative (Smith and Couche, 1991). Bt reportedly does not produce spores or crystals in infected host cadavers, it is relatively poor at spreading natural infections in the field and rarely causes epizootics (Fuxa, 1989). Its ubiquitous occurrence in the soil (Martin and Travers, 1989), phylloplane microflora (Smith and Couche, 1991), and in man-made ecosystems (Meadows et al., 1992) suggests that it may have a primary function other than as a pathogen.