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Several hundred strains of Bacillus thuringiensis (Bt), isolated in New Zealand from samples of soil and sheep fleece, were tested for toxicity to larvae of the blowfly Lucilia cuprina Wiedemann. Characterization of the Bt strains revealed that three of the more active strains produced Cry1Ba (an insecticidal protein present in Bt mother cell crystal inclusion) that was toxic to blowflies. These strains were evaluated for the ability to prevent experimentally induced fly strike in a bioassay by using first instars. Results with undiluted spore/crystal preparations were variable, but they generally prevented fly strike on sheep maintained on pasture for 3-6 wk. Spore viability was satisfactory throughout the trials and environmental factors (e.g., precipitation and UV radiation) seemed to have minimal effect on persistence. The loss of fly strike protection in these experiments correlated with the movement of spore/crystal toxicity away from the skin as a result of wool growth. Solubilized protein preparations were not as potent as spore/crystal preparations and fly strike protection lasted only from 1 to 3 wk. Vegetative forms of the Cry1Ba-producing strains of Bt did not establish on the fleece of sheep, did not produce significant sporulation, and no protection against fly strike was achieved. Escherichia coli expressing recombinant Cry1Ba protein was toxic to larvae in vitro but did not effectively protect sheep from fly strike because blowfly larvae were able to establish readily 8 d posttreatment. In a single field experiment involving 80 sheep per group, a spore/crystal preparation from a Bt strain expressing Cry1Ba provided less protection from naturally acquired fly strike than afforded by a commercially available dip.

A new staining method is described using naphthalene black 12B and Gurr's improved R66 Giemsa for staining all known crystal types produced by Bacillus thuringiensis.

Although the use of Bacillus thuringiensis (Bt) as a foliar spray is minimal within New Zealand at present, a considerable effort is being made genetically to incorporate Bt δ-endotoxin genes into a wide range of important crops, including pasture species, perennial horticultural crops and annual field crops. For several years it has been clear that the major threat to the sustained efficacy of Bt in New Zealand is the evolution of insect resistance. Recognition of this threat has prompted consideration of a range of strategies for preventing and/or managing its possible evolution in pests of specific crops. However, such strategies do not address the critical issue of how we might limit the consequences of Bt resistance if it did evolve, particularly the evolution of resistance in polyphagous pests. Highly mobile polyphagous pests, such as leafroller species, might develop resistance to Bt on one transgenic crop and then disperse, nullifying the effectiveness of a wide range of Bt transgenic crops expressing the same or similar Cry proteins. Pests with resistance to CryIA proteins in transgenic plants might also display significant resistance to Bt biopesticides. To prevent such an eventuality in New Zealand we propose a plan for deploying Bt genes among crops according to an assessment of the risk and consequences of Bt resistance evolving in the insect complexes on these crops. The major elements of this plan are: (i) assess the risk of Bt resistant insects evolving and dispersing out of the crop to infest others; (ii) characterize the diversity of Bt protein binding sites in the guts of key polyphagous pests; (iii) use the above information to deploy Bt genes among different transgenic crops in a pattern that minimizes the chance that any future Bt resistant cosmopolitan pests will be broadly cross-resistant to other Bt crops and/or Bt biopesticides.