Bacillus thuringiensis (Bt): human and environmental safety
What is Bt?
Bt is short for Bacillus thuringiensis (Bt), a bacterium in the genus Bacillus. Members of the genus Bacillus are generally considered soil bacteria, and Bt is common in terrestrial habitats including soil, living and dead insects, insect feces, granaries, and on the surfaces of plants. Bt occurs in nature predominantly as spores that can disseminate widely throughout the environment. The diversity within B. thuringiensis is reflected in the fact that more than 60 serotypes and hundreds of different subspecies have been described. The two most widely used in commercial insecticides are B. thuringiensis subsp. kurstaki (Btk), which kills a wide range of lepidopteran species that are important pests in agriculture and forestry, and B. thuringiensis subsp. israelensis (Bti), used primarily for the control of mosquito and blackfly larvae.
A unique feature of Bt is that it produces crystalline structures during sporulation and these have activity against some insect species. Bt was first isolated about 100 years ago in Japan from silkworm larvae, and for over 40 years Bt has been applied to crops in the form of a Bt plant spray as an insecticide, a mixture of spores and the associated protein crystals. By 1995, 182 Bt products were registered by the United States Environmental Protection Agency (EPA), but in 1999 the total sales of Bt formulations constituted less than 2% of the total sales of all insecticides. Is Bt safe? Compared to many other methods of insect control, Bt has the distinct advantage of being very safe to humans and the environment.
What are Bt plants like Bt corn?
Bt plants have genes from Bt engineered into them (otherwise known as the ‘Bt gene’) so that the plants produce an ICP toxic to the pest species of concern. As the insect feeds on the plant, it ingests the ICP and suffers the same fate as if it ingested leaf tissue sprayed with Bt. There are presently only two Bt crops registered in the US –- Bt corn and Bt cotton.
How does Bt work and how does it kill insects?
The main insecticidal activity of Bt is due to the insecticidal crystal proteins (ICP) produced during sporulation. Different ICPs work against different insect types (e.g. some only work against certain species of caterpillars while others work only against certain species of beetles). Key agricultural pests currently targeted with Bt insecticides include bollworms, stem borers, budworms, and leaf worms in field crops and grains; the gypsy moth and spruce budworm in forests, and; the cabbage looper and diamondback moth in vegetable crops. Mosquitoes and blackflies are also controlled with Bt sprays and treatment of water breeding sites with Bt.
Bt insecticides, whether in the form of a Bt plant spray or in a Bt crop, do not function on contact as most insecticides do, but rather must be ingested by the target organism to be effective. This process takes hours to days—longer than is required for synthetic insecticides to kill insects. The active ICP binds to specific receptors on the midgut of the stomach, forming pores and leading to leakage of the midgut contents, paralysis, and death of the insect.
One level of specificity in this interaction is provided by the midgut environment of the insect, and a second by the binding of ICPs only to membranes carrying the appropriately matched receptors. Only certain insect species have the proper midgut environment and receptors to which the ICPs bind. For those organisms that do not have the proper midgut environment or binding sites (eg., all mammals), the ICPs just pass through the guts and are voided, just as hundreds of other foreign proteins are daily. Because the activity of Bts is confined to insects, they have a uniquely safe profile. For a more detailed explanation of the mode of action of ICPs and their specificity, the reader is referred to the review by Koch et al. (2015).
What does the scientific community say about Bt?
The overwhelming evidence demonstrates the environmental and human safety of Bt proteins, whether sprayed or expressed in a plant. Listed below are three scientific reviews that support these findings.
American Academy of Microbiology. 2002. 100 Years of Bacillus thuringiensis, a Paradigm for Producing Transgenic Organisms: A Critical Scientific Assessment. http://ipm.ifas.ufl.edu/pdfs/100_years_of_bt.pdf
Hammond, B. et al. 2013. Toxicological evaluation of proteins introduced into food crops. Critical Reviews in Toxicology. 43:25-42. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3835160/
Koch, M. S. et al. 2015. The food and environmental safety of Bt crops. Frontiers in Plant Science 6:283. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4413729/