Influence of multi-year Bacillus thuringiensis subsp. israelensis treatments on the abundance of B. cereus group populations in Swedish riparian wetland soils

Abstract: Bacillus thuringiensis subsp. israelensis (Bti) is a soil-borne bacterium affiliated to the B. cereus group (Bcg, a group including the pathogens B. cereus, B. thuringiensis, and B. anthracis) and is used in biocontrol products against nematoceran larvae. However, knowledge is limited on how long-term Bti application affects the structure of indigenous Bcg communities as well as the overall abundance of Bti. Based on new primers, group-specific quantitative PCR assays for Bcg and Bti in environmental samples were developed. On six occasions during the vegetation season, soil samples were collected in forest swamps and wet meadows which had been treated with Bti during the preceding 11 years as well as in untreated forest swamps, wet meadows and well-drained forests. Abundances of Bcg and Bti varied among the different sampling occasions. The highest abundance of Bcg was found in forest swamps and differed significantly from wet meadows while no such variation was found in Bti abundance. The Bti treatments had no effect on the overall Bcg abundance whereas for Bti, the abundances were significantly higher in the treated than in the untreated sites. However, abundances of Bti and Bcg didn’t correlate with the number of Bti applications, indicating that Bti use influenced abundances of Bti in the short term while in the long term the number of treatments had only a limited effect. The findings illustrate the value of such investigations for understanding the ecology of Bti applications, which can facilitate environmental risk assessment as well as approval of biological control agents.