Managing Invasive Forest Insect Pests with Bacillus thuringiensis
Many strains of the Bacillus thuringiensis (Bt) group of bacteria produce insecticidal proteins, referred to as Cry toxins, in crystal inclusions during sporulation. The Cry toxins present in the crystals play a major role in determining the insecticidal potency and specificity of strains of Bt. Several derivatives of the Bt strain HD1 subsp. kurstaki (Btk) play a major role in the microbial control of forest pests such as the gypsy moth (Lymantria dispar) and the spruce budworm (Choristoneura fumiferana) in North America. Bt products can be as effective as conventional broad-spectrum insecticides with the advantage of having little or no effect on non-target organisms. However, microbial control of some other insect pests such as the Douglas-fir tussock moth (Orgyia pseudotsugata) and the browntail moth (Euproctis crysorrhoea) has been less effective. Elucidation of the processes that govern the insecticidal activity and specificity of different Bt Cry toxins will make possible the development of new microbial insecticides with enhanced performance against target insect pests.
Research on the mode of action of Bt has revealed that activated Cry toxins bind to specific receptors on the cellular lining of the gut in larvae and then insert into the membrane, creating pores. Until recently, it was generally accepted that the effect of Cry toxins was due to pore formation and subsequent lysis of gut cells. However, Bt Cry toxins may also promote cell death in another manner. Many bacterial pore-forming toxins have been reported to damage cells by inducing the shedding of receptors,which is believed to increase the virulence of bacteria that produce pore-forming toxins. Recent studies of the processes involved in promoting the insecticidal activity of Bt in gypsy moth has revealed that Bt toxins trigger massive shedding of membrane-bound receptors such as aminopeptidase N (APN ) and alkaline phosphatase (ALP). Although a direct relationship has not been established, shedding may cause extensive damage to the gut epithelial cells as well as promote the cytocidal action of Bt Cry toxins. Since Bt receptors are essential for insecticidal activity, our research has been focused on the characterization of these receptors and the elucidation of their precise role in the mode of action of Bt. These research efforts have led to the identification two major Bt toxin receptors in the gypsy moth and to the development of a simple and reliable screening assay for evaluating the insecticidal activity of new Cry proteins obtained through discovery or by genetic manipulation. Further studies are needed to determine if resistance to Bt in insect species can be attributed to the absence of specific toxin-binding receptors for certain Cry toxins or is due to other factors. The results of these studies will facilitate the commercialization of new microbial insecticides derived from natural Bt strains as well as a new generation of Bt-based products generated by protein engineering, with altered specificity and enhanced toxicity towards forest insect pests.
Deciphering the mode of action of Bt, in particular how receptor/toxin interactions affect the insecticidal activity and specificity of Bt towards specific insect pests. Developing screening assays for evaluating the insecticidal activity of new proteins obtained through discovery or protein engineering. Bridging scientific discoveries with applied research to develop products with improved entomocidal activity and specificity towards target pests.
Krofcheck, Daniel J.; Valaitis, Algimantas P. 2010. Microimaging of Bacillus thuringiensis toxin-binding proteins in gypsy moth larval gut using confocal fluorescence microscopy. In: McManus, Katherine A; Gottschalk, Kurt W., eds. Proceedings. 20th U.S. Department of Agriculture Interagency Research Forum on Invasive Species, 2009. Gen. Tech. Rep. NRS-P-51. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station: 83.
Valaitis, Algimantas P. 2009. Bacillus thuringiensis insecticidal toxins induce shedding of GPI-anchored APN by activation of phosphatidylinositol-specific phospholipase C. In: McManus, Katherine A; Gottschalk, Kurt W., eds. Proceedings. 19th U.S. Department of Agriculture Interagency Research Forum on Invasive Species, 2008. Gen. Tech. Rep. NRS-P-36. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station: 83.
Valaitis, Algimantas P. 2008. Bacillus thuringiensispore-forming toxins trigger massive shedding of GPI-anchored aminopeptidase N from gypsy moth midgut epithelial cells. Insect Biochem. Mol. Biol. 38: 611-618.
- Algimantas P. Valaitis, Research Biologist, U.S. Forest Service, Northern Research Station
- John Podgwaite, Microbiologist, U.S. Forest Service, Northern Research Station
- Donald Dean, The Ohio State University
Last Modified: 11/17/2010