Abstract:
This thesis determines the risk posed by the introduction of high numbers of Bacillus thuringiensis subsp. kurstaki (Btk) endospores to natural environments using a retrospective environmental risk assessment (ERA) process, in particular the effect of Btk endospores on the native microbial community in soil.
Various subspecies of the bacterium Bacillus thuringiensis (Bt) have been used extensively as 'organic' commercial insecticides for over 60 years. The effects of the insecticidal Cry crystal protein and formulation components of Bt biopesticides are currently well understood, however the effects of the Bt endospore component are not well documented.
The problem formulation phase of the ERA outlines the current knowledge regarding exposure and effect components of the risk relationship between Btk endospores and the natural environment. Through this process the native microbial community in soil is identified as an appropriate 'assessment endpoint' to measure the effect of Btk endospores on the natural environment, and specifically, population structure, metabolic function and genetic complement are identified as 'measurement endpoints'. The problem formulation results in a series of specific research objectives requiring analysis in order to characterise the exposure and effect relationships between Btk and the natural environment, and to determine risk.
The analysis phase of the ERA characterises the exposure and effect relationships between Btk and the natural environment by observation, experimentation, and literature review. Exposure characterisation initially uses the insect pest eradication spray program 'Operation Evergreen' as a case study to show that Btk endospores persisted at high levels in the environment for at least 54 months after introduction as part of a biopesticide. Secondly the environmental factors of pH, nutrient level and microbial competition are shown to modulate Btk metabolic activity in soil. Effect characterisation determines the change to soil microbial communities when Btk is introduced under conditions optimal for metabolic activity. Population structure changes are assessed using both restriction fragment length polymorphism (RFLP) and terminal restriction fragment length polymorphism (T-RFLP) methods, and metabolic function changes are assessed by the ability to utilise a range of sole-carbon substrates. The potential for change in the genetic complement of the native microbial community is assessed from the literature, specifically reviewing the potential for Cry protein encoding cry genes to transfer to novel recipient microbes. Assessment of all measurement endpoints shows either no change to the native microbial community, or a low likelihood of change.
The risk assessment phase of the ERA considers the information gathered in the problem formulation and analysis phases to estimate the nature and magnitude of the risk posed to the natural environment by introduced Btk endospores. The likelihood of Btk endospores adversely affecting the native microbial community in soil is determined to be low, and the consequences of any potential change minimal. The ERA shows that the application of Btk biopesticides to the vast majority of environments is safe, and will result in little environmental modification and no adverse ecological effects.