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Eight oil-based and four aqueous experimental formulations of Bacillus thuringiensis var. kurstaki, were sprayed in a laboratory chamber over potted seedlings of white spruce and balsam fir. Foliar deposits were assessed before and after 3 mm rain with a rainfall intensity of 5 mm/h, using a force-feeding bioassay (FFB) method of foliar extracts on the spruce budworm larvae. In addition, a total protein assay (TPA) method was used to compare the two sets of results.
The FFB method showed that formulations that provided high deposits (IU/cm2) on foliage, showed some residual activity after the rainfall, whereas formulations that provided low deposits showed no activity after the rain. This trend was observed regardless of the formulation type, oil-based or aqueous. The TPA method was more sensitive than the FFB method because with all formulations, some protein deposits (ng/cm2) remained on foliage after the rain. In general, the oil-based formulations showed greater rainfastness than the aqueous formulations, a finding that was not as evident in the FFB method. The differences in the two sets of data were due to the fact that the FFB method depended on the biological response of an insect, thus resulting in high variability in deposits, whereas the TPA method provided a direct estimation of protein deposits that were less variable.
Bacillus thuringiensis, formulations, spectrophometric analysis of tracer dyes, rain-fastness of deposit, foliar extract, force-feeding bioassay, total protein assay
Research Scientist and Project Leader, Forestry Canada, Forest Pest Management Institute, Sault Ste. Marie, Ont.
Research Assistant, Forest Pest Management Institute, Sault Ste. Marie, Ontario
Associate Research Fellow, Formulations Research and Development, Abbott Laboratories, Agricultural Research Center, Long Grove, IL