Science in Society Archive

Bt Risk Negligible?

The United States Environment Protection Agency has renewed the registration of Bt corn based on new studies that claim the impact of Bt-corn pollen on monarch butterfly is "negligible". But Dr. Mae-Wan Ho and Prof. Joe Cummins say that is playing fast and loose with scientific evidence.

When John Losey and colleagues in Cornell University published their findings that Bt-corn pollen harmed monarch butterflies, the concern raised was not just over monarch butterflies, but on all other (non-target) species in the environment. (Bt-corn is transgenic corn expressing a protein, Bt, isolated from the soil bacterium Bacillus thuringiensis, which kills specific insect pests.) Unfortunately, amid the enormous publicity generated, the broader issue became reduced to the impact of Bt-corn on monarch butterflies.

So, when the US Environment Protection Agency (EPA) issued a data call-in on December 1999 to consider re-registration of Bt corn, the Department of Agriculture's Agricultural Research Service (USDA-ARS) responded by sponsoring a Monarch Research Workshop in Feb. 2000 to identify research priorities regarding Bt-corn and monarch butterflies. A request for proposals was announced in April, after which a steering committee selected projects to be funded from a grant pool provided by the USDA-ARS and the Agricultural Biotechnology Stewardship Technical Committee.

The results of those projects were published in the 9 October 2001 issue of the Proceedings of the National Academy of Science USA, giving clearance to the EPA's re-registration of Bt-corn for an additional seven years, announced 16 October [1].

The first paper in the series is on risk assessment [2] based on data collected in all the studies. Here, we are immediately reassured by the statement, "This 2-year study suggests that the impact of Bt corn pollen from current commercial hybrids on monarch butterfly populations is negligible."

On reading the fine print, one discovers that,

But, these observations are not considered sufficient to cause concern, why not?

What matters in the first instance, we are told, is the probability of exposure, Pe, which is equal to three numbers I, o, and a, all less than one, multiplied together,

Pe = I o a

I is the proportion of monarchs that visit cornfields, o is the overlap of pollen-shed with susceptible larval stages and a, the adoption rate of Bt corn.

Furthermore, the actual risk R depends, not just on the probability of exposure, Pe, but also on Pt, the proportion of monarchs that would be exposed to pollen levels that exceed the lethal threshold, yet another number less than one.

R = Pe Pt

By such means, the risk becomes reduced to less than 1% or, better still, less than 0.1%.

Should we be impressed with this kind of special pleading in the light of clear evidence that the Bt protein is harmful, and admitted to be so?

The authors devoted several pages of unconvincing arguments in order to come up with estimates of the numbers, the only reliable one is perhaps the adoption rate, a, of Bt corn. All estimates are for the year under study, and could drastically change in future in any case.

The studies on toxicity are on acute effects, ignoring both long-term cumulative and non-linear effects that are well known in the ecological literature, especially with regard to extinction and population density. These short-term studies on monarch butterflies do not address impacts on other non-target species, nor on the multiplier ecological consequences of all the impacts interacting with each other.

Bt has already been shown to have the following additional negative impacts, as reviewed elsewhere by the two research teams who found clear evidence that Bt-corn pollen harmed the larvae of Monarch butterflies [3]:

In addition, Bt-toxins are actual and potential allergens for human beings. Field workers exposed to Bt spray experienced allergic skin sensitization and induction of IgE and IgG antibodies to the spray [5]. A Bt strain that caused severe human necrosis (tissue death) killed mice infected through the nose within 8 hours, from clinical toxic-shock syndrome [6]. Both Bt protein and Bt-potato harmed mice in feeding experiments [7].

Of course, proponents of Bt-crops can easily reduce all of those impacts, one at a time, to "negligible" levels by the same kind of exercise carried out in the 'risk assessment' described above; ignoring the complex interactions between the individual impacts which can have catastrophic long-term consequences.

The proponents will still claim that the benefit of Bt-crops in reducing the use of broad-spectrum insecticides outweighs the risks. Unfortunately, that claim is not borne out by the evidence [3]. During the past five years, the percentage of field corn treated with insecticides in the US has remained at approximately 30%, despite a significant increase in the hectares of Bt corn planted. Corn borer is not a serious pest, and only 1% to 2% of the crops in Iowa was treated with insecticide for corn borer between 1995 and 1998. Most farmers in Iowa and Minnesota had never used insecticides for the corn borer. Only during years when corn borer densities are high do transgenic crops out-perform the non-transgenic.

Article first published 12/11/01


  1. EPA press release Biotechnology Corn approved for Continued Use Oct.2001
  2. Sears MK, Hellmich RL, Stanley-Horn DE, Oberhauser KS, Pleasants JM, Mattila HR, Siegfried BD and Dively GP. Impact of Bt corn pollen on monarch butterfly populations: A risk assessment. PNAS 2001, 98, 11937-42.
  3. Obrycki JJ, Losey JE, Taylor OR and Jesse LCH. Transgenic insecticidal corn: beyond insecticidal toxicity to ecological complexity. BioScience 2001, 51, 353-61.
  4. Saxena D and Stotzky G. Bt toxin uptake from soil by plants. Nature Biotech.2001, 19, 199.
  5. Bernstein I, Bernstein J, Miller M, Tiewzieva S, Bernstein D, Lummus Z, Selgrade M, Doerfler D and Seligy V. Immune responses in farm workers after exposure to Bacillus thuringiensis pesticides. Environ Health Perspect 1999, 107,575-82
  6. Hernandez E, Ramisse F, Cruel T, le Vagueresse R and Cavallo JD. Bacillus thuringiensis serotype H34 isolated from human and insecticidal strains serotypes 3a3b and H14 can lead to death of immunocompetent mice after pulmonary infection. FEMS Immunol Med Microbiol 1999, 24,43-7
  7. Fares NH and El-Sayed AK. Fine structural changes in the ileum of mice fed on dendotoxin-treated potatotes and transgenic potatoes. Natural Toxins:1998: 6: 219-33.

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