Bt toxins don't act in a vacuum

February 2016 

Having predictably triggered a range of new pest problems with its 'Bt' insecticidal GM crops [1], the biotech industry has been busy developing seeds which generate multiple Bt-toxins to solve the problems it created in the first place.

Concerns have already been raised about the lack of safety testing of single-Bt crops, in particular the assumptions and generalisations underlying the excuses not to test [2]. Even more tenuous 'reasons' are being trotted out to obviate testing of next-generation Bt crops which, so far, have up to six different insecticides in them.

Besides the thoroughly unsound corner-stone that species-specific receptors must be present to activate a Bt toxin [2], regulators are basing their approvals on previous safety assessments of single purified Bt proteins produced by microbes. The possibility of multiple, plant-generated Bt toxins failing to behave as predicted from the bacterial versions has not been entertained. Similarly, the likelihood of one Bt-protein becoming more (or less) toxic, in the presence of other Bt toxins or agrichemicals has never been on the safety agenda.

Contradictions in the published data on Bt cross-reactivity have, it seems, been used as a reason to exclude inconvenient evidence rather than to stimulate further study.

Swiss environmental scientists have pointed out that seven out of eight published studies on the effects of combined Bt toxins demonstrated either increased or decreased harm to test animals. Why these should happen can only be speculation, because the mode or modes of action of Bt toxins are in doubt [2].

Besides interactions with each other, Bt efficacy may be altered by many other factors.

Conditions within the gut, such as acidity level, can alter Bt solubility and therefore toxicity. 

Digestion can change Bt activity, for example, to produce smaller more aggressive strands of the Bt toxin or to inactivate it altogether.

Although new strategies for next-generation Bt crops include enhancing their toxicity using enzymic 'helper proteins', the possibility of similar effects from natural helper-proteins in the form of dietary plant-enzymes or digestive-enzymes has been blanked.

It may be very significant that death of the pest after Bt-induced gut damage has been linked to microbes entering the body. Interactions between Bt and anti-microbial agents (which can suppress or enhance pathogenic strains) have not been assessed. Most Bt crops on the market are also genetically transformed to tolerant and accumulate glyphosate herbicide (Roundup): glyphosate is an anti-microbial and could have profound effects on the toxicity of Bt.

The Swiss reviewers note that many of the Bt interactions were "entirely unpredictable and occurred when their individual components did not illicit a response at all or only a sublethal response when tested in isolation". In fact, regulators have no grounds to plead ignorance. Interactions between Bt toxins and other proteins or chemicals are "actually widely recognised and reported in the literature". They have been discussed in the scientific community since the 1980s.


Ignoring interactions in a living system is bad science. Dismissing science which disproves your desired outcome is dishonest. When the science you're ignoring or dismissing suggests people could be harmed or, even, die, it's criminal. Your MEP should be demanding clear answers to all questions on the combined effects of Bt toxins.


[1] BT CROPS - A DEAD-END STREET? - November 2015



  • Christoph Then, Potential synergies that can enhance Bt toxicity in SmartStax, TestBiotech Background 28.06.11
  • Angelika Hilbeck and Mathias Otto, November 2015, Specificity and Combinatorial Effects of Bacillus thuringiensis CryToxins in the Context of GMO Environmental Risk Assessment, Frontiers in Environmental Science 3

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