Dispersal ofBacillus thuringiensisvar.kurstakiin an experimental cabbage field

Abstract

The field population dynamics of Bacillus thuringiensis var. kurstaki DMU67R isolated from cabbage leaves was investigated. Two experimental cabbage plots were spray inoculated with a suspension of sporulated bacteria containing 6.5 × 107colony-forming units (cfu)/mL DMU67R or with a sterile medium control. To study transport between phyllosphere and soil, plastic covering was used during spraying to ensure that DMU67R was applied only on leaves (8.5 × 106cfu/g) or on soil (1.2 × 104cfu/g). Numbers of DMU67R declined five log units during the first 4 weeks after spraying of leaves; the initial half-life (1st week) was 16 h. In topsoil, however, a long-term persistency was demonstrated with a half-life exceeding 100 days. Dispersal by rain splash from topsoil to lower leaves of cabbage was demonstrated. After 1 year, analysis of the top 15 cm of soil showed that 77% of DMU67R remained in the 0- to 2-cm topsoil layer. When DMU67R was applied on leaves, larvae of Pieris brassicae were killed within 7 days after spraying. Germination of DMU67R was demonstrated in dead P. brassicae larvae but not in any leaf or soil sample. Pitfall sampling demonstrated dispersal of DMU67R by means of carabid beetles (up to 135 m) and other surface-active insects carrying 102–103 cfu/g wet mass.Key words: Bacillus thuringiensis, Pieris brassicae, cabbage, soil, dispersal, population dynamics.