ABSTRACT: Bt toxin is an insecticidal protein produced by the soil bacterium Bacillus thuringiensis. It has long been popular among organic farmers because it is a very selective pesticide, specifically targeting particular types of insects. The bacterial genes for making Bt have been inserted into a number of crops such as corn and cotton. A potential problem associated with these crops may be the development of resistance to Bt, unless preventive measures are taken. Certain types of Bt (such as that found in StarLink corn) may be more stable during digestion and could present potential allergy risks; testing of other Bt crops, however, has shown that they are less stable and break down more easily.

 
Various strains of the soil bacterium Bacillus thuringiensis produce proteins that are toxic to specific groups of insects (for example, some proteins target caterpillars, while others are specific for beetles). These Bacillus thuringiensis toxins, or Bt toxins, have been used for many years by organic farmers as a natural insecticide to control pest populations. The insects ingest the toxin, and then in the alkaline gut of the insect (typically around pH 10), the Bt proteins interact with specific insect cell receptors to create pores, effectively poking holes in the gut and killing the insect.

The genes encoding these insecticidal proteins have been identified and inserted into the chromosomes of various crops (the most well known being corn and cotton). In their current form, these crops then produce the Bt toxin in every cell of the plant (e.g., leaves, roots, pollen).

Opponents of Bt crops have raised two main concerns: the development of resistance, and potential harm to humans who ingest the resulting food product. They have also expressed concern about the effects on nontarget species, such as the monarch butterfly.

The development of resistance is worrisome because Bt toxins have been an important part of organic agriculture. It is an especially good pest control agent because it is much more specific than some of the chemical pesticides used in organic farming. For example, one of the most common pesticides used in organic farming is pyrethrin (also known as the active ingredient in Raid); it is not a very selective toxin and is active against most insects (which is why it is so useful in household applications). In contrast, Bt can be used to target a specific group of insects (e.g., caterpillars) while leaving others (e.g., ladybugs) unharmed.

So far, there has been no conclusive evidence that planting Bt crops significantly accelerates the development of resistance in insects any more than applying the toxin topically to crops (as is done in organic farming). And there are enough different types of Bt toxins available that if resistance should arise, an alternative Bt toxin could be found. I nevertheless believe that refuges are a reasonable precaution; under this system, farmers would plant sections of non-Bt versions of their crops next to their Bt versions. Non-resistant insects would then flourish in the non-Bt section and dilute out the resistance genes during mating. Farmers have been reluctant to set aside land for this purpose, however, since nothing worth harvesting comes from the refuges.

It's my opinion that the seed companies need to take an active role in preventing Bt resistance. They seem to have the resources available to make sure nobody is planting their seed without their permission (and having paid for it); it seems to me that they should also have the resources to make sure that farmers are using their product appropriately, preferably by having a third party oversee its use.

There has also been a lot of discussion about the potential health risks to those who consume Bt crops. "There's pesticide in every cell!" is a common warning offered by those opposed to Bt crops. I believe that seed companies made Bt crops available before they should have, but I also don't think there's any real risk to consuming Bt crops (in those cases where they have been approved for human consumption, i.e., not StarLink corn).

The word "pesticide" encompasses a lot of different types of materials, and to proclaim that there is pesticide in every cell implies a threat that isn't really there. Most pesticides that we are familiar with are small chemicals (e.g., DDT, malathion, diazinon) that have activity against a broad range of organisms (insects, cats, people) because they attack a system (e.g., the synapses between nerve cells) that is common to all of these organisms. But Bt is different. As mentioned above, it's a lot more selective than, for example, Raid, because it targets specific insect cell receptors and requires an alkaline environment to work. Humans, in contrast, have an acidic digestive tract and don't have the insect receptors on their cells (since we're not insects).

Moreover, because Bt is a protein, it gets digested in our stomach and is broken down like other proteins. The reason StarLink corn was not approved for human consumption is that the type of Bt used in that crop is more stable than the Bt found in other crops. So, while the others get broken down easily, StarLink does not. And it's not that StarLink Bt will then put holes in the human intestine; the concern was that the undigested StarLink protein might generate allergic responses in people.

As I mentioned above, the current Bt crops produce Bt in every cell of the plant, but it doesn't have to be that way (and that's why I think seed companies should have waited before selling Bt crops). Every gene in the plant has a switch (promoter) to turn it on and off. Certain genes get turned on in the leaves, while other genes get turned on in the roots, and still other genes get turned on in the stem. That's how plants are able to make their different tissues and parts -- they turn on genes specific for each part of the plant. What the companies should have done was to wait until they had developed crops that produce Bt only in the parts of the plant that the insects eat (e.g., the leaves). Then it might be possible to have a Bt crop that is protected against the insect pests but that doesn't put the Bt into the parts that people eat.

Tissue-specific promoters also would have avoided the questions raised by the Bt-pollen feeding experiments at Cornell. In this study, monarch butterfly larvae were fed either plain milkweed or milkweed covered with pollen from Bt corn. Predictably, the caterpillars whose diet included Bt corn pollen got sick and died. The news media often portrayed this finding as a great surprise, but every researcher familiar with the system (and even those who were not) could have predicted the outcome. If you feed a sufficient quantity of poison to an animal, the animal is going to get sick and die.

There are a number of points that indicate that these laboratory studies have little bearing on the real world:

• Monarch caterpillars generally feed at a time when corn is not even producing pollen. (It has been suggested that global warming may shift the pollen-bearing time to coincide with the feeding period, but it's not clear to me why one would shift but not the other.)

• Corn pollen generally travels only a few meters beyond the edge of the planted area.

• In nature, monarch caterpillars avoid milkweed coated with pollen. The caterpillars in the study were given no choice; they were given only either plain milkweed or milkweed with Bt toxin.