Greenhouse trials of new Bacillus thuringiensis isolates for control of Chilo partellus larvae in sorghum

Eight new Bacillus thuringiensis Berliner (Bt) isolates originally recovere d from different source materials and geographic locations in Kenya were te sted against Chilo partellus (Swinhoe) on sorghum grown in a greenhouse. Al l isolates had been demonstrated previously to be toxic to this pest. Bt st rains were cultured in a liquid growth medium and preserved as powders for experimental application. Three-week-old sorghum plants (4-6 leaves) were a rtificially infested with 20 neonate C partellus and sprayed 24 h later wit h Bt suspensions. Non-infested sorghum and infested non-treated sorghum ser ved as the comparative checks within each experimental trial. Plant develop ment and dead-heart formation were monitored up to harvest; at harvest, gra in yield was recorded, In each replicate trial. all of the Bt strains teste d reduced plant damage consistently, compared with the infested, non-treate d checks. Dead-heart formation was significantly reduced and grain yields w ere significantly higher. Three novel isolates, A-3, A-C-2 and M44-2, provi ded superior levels of protection throughout. Statistically, there was no d ifference among these three Bt treatments and the non-infested check in ter ms of the number of dead-hearts (i.e., non-yielding plants) that formed dur ing the course of the experiment. In the infested, non-treated check plots, dead-hearts developed in 74.5+/-10.0% of the plants, compared with only 3. 1+/-1.2% of the noninfested plants, and 3.1+/-1.5%, 6.2+/-2.0% and 6.2+/-2. 0% of the plants treated with M44-2, A-3 and A-C-2, respectively. At harves t, the weight of grain obtained from the non-infested control plots was con sidered to be 100% of the potential yield for each experimental replicate. In the infested, non-treated controls, only 14+/-5.6% of the potential yiel d was realized. When isolates M44-2, A-3 and A-C-2 were applied, proportion ate yields of 83+/-5.4%, 93+/-2.3% and 98+/-1.2%, respectively, were obtain ed. Results showed that the Bt strains selected may have a strategic role t o play in the management of C. partellus, providing applications are timed to target young larvae feeding in the leaf whorl.