The larvicidal activity of Bacillus thuringiensis Cry proteins against Thaumatotibia leucotreta (Lepidoptera: Tortricidae)
Hua Li, Gustav Bouwer
School of Molecular and Cell Biology, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa.
The susceptibility of one of the most important pests in southern Africa, Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae), to Bacillus thuringiensis Cry proteins was investigated. Cry proteins were produced in Escherichia coli BL21 cells that were transformed with plasmids containing one of the six cry genes encoding the Cry proteins used in the study. The larvicidal activity of each Cry protein against T. leucotreta neonate larvae was evaluated using the droplet feeding bioassay method with the volume imbibed by neonate larvae determined by fluorescence spectroscopy. Bioassays were performed using crude extracts and semi-purified Cry inclusion bodies of BL21 transformants, with the concentration of the Cry proteins determined by a novel protein quantitation strategy. For each BL21 transformant, a better fit of the data to the probit model was obtained for the semi-purified Cry inclusion bodies than for the crude extracts. Cry9Aa and Cry1Ca had significantly lower larvicidal activity against T. leucotreta neonate larvae than the other evaluated Cry proteins (Cry1Ac, Cry2Aa, Cry1Aa, and Cry1Ab). Cry2Aa and Cry1Ac had significantly higher larvicidal activity against T. leucotreta neonate larvae than the other tested Cry proteins. The study is important because it establishes a benchmark for the susceptibility of T. leucotreta to different Cry proteins, which is a key step in the evaluation of Cry proteins for potential use in T. leucotreta control programmes.