Long-term survival of Bacillus thuringiensis subsp. kurstaki in a field trial

Long-term survival of Bacillus thuringiensis subsp. kurstaki DMU67R has been investigated in a field trial. An experimental cabbage plot was sprayed with DMU67R in 1993 and was allowed to lie fallow since then. The investigation reported here was carried out from 2001 to 2007 in a single square meter within the plot using a systematic randomized sampling approach. The bacterium survived at relative low densities in these 13 years after spraying. Statistical analyses revealed that the overall density decreased approximately 40% during years 8 to 13 after the application; however, the trend was not uniform and contained periods of both increases and decreases in density of DMU67R, with decreases in density notably related to conditions of low water content in the soil. Long-term survival of DMU67R in this field plot seems to include germination and growth, possibly related to growth in insect hosts, and death or inactivation during dry periods, both phases occurring during May to October where the soil temperature exceeds 10 °C.