COMMENT from Dr. Brian John: A nice concise summary of the environmental effects of Bt plants.
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Dark Side of GM Plants 
Anna Kizilova 
Source: Science News, May 19 2009 
http://www.russia-ic.com/education_science/science/science_overview/911/
(translated from the Russian original)

Over 125 million hectares in the world are currently under transgenic or genetically modified plants, and about one-fifth (21-23%) of these plants are resistant to pests. In most plants such resistance is a result of genetic engineering via introduction of a gene of soil bacterium Bacillus thuringiensis into plant genome plants produce bacterial protein (Bt-toxin), which is toxic for insects. 

Biologists consider mentioned toxins to be among most environmentally friendly means for plant protection, since plants produce them in concentrations, which are harmless for homoiothermal animals. Moreover, these toxins are selective they kill not all insects, but only certain species: biologists have Bt-toxins, killing flying insects, moth slugs or beetles. Bacterial toxins are used in agriculture for over 50 years farmers spray plants with them. Bt-plants (corn, cotton plant, potatoes, rapeseed, rice, broccoli, peanuts, eggplant, etc) are considered to be quite safe and, due to cheaper farming, cover more lands than their non-modified relatives. 

Well, the [coin] has two sides. Recent research showed that plants with Bt-toxins might have negative effect on our environment. Some Bt-plants appear to lose the battle against those pests they were designed to resist. Toxin concentrations in plants drops with time, and leaves always have higher toxin content than fruits. That is why pests, which feed on fruits, do not eat enough toxins to die. In this case, plants should be sprayed with insecticides for additional protection. 

Insecticidal crystal proteins from Bacillus thuringiensisUnexpectedly, Bt-plants are very attractive for aphids or plant lice, much more than non-modified plants. Bt-toxins do not affect plant lice, since these insects take nutrients from plant vessels, containing no Bt-protein. Biologists cannot say why plant lice prefer GM-plants to other plants; possible explanation is composition of volatile substances, different from ordinary pants; color intensity and some other biochemical properties. 
After harvesting, fields are covered with silage, roots and seeds, which means some of the toxins get inside soil. In soil, Bt-toxins keep their toxicity up to 350 days and can be fatal for soil invertebrates. Such prolonged persistence, which caused surprise of many scientists, is due to silt and humus particles, which bind to toxins and protest them from microbial degradation. 

Moreover, before microbes start degrading toxic plant debris, said debris should first be eaten by soil invertebrates. Well, these debris do not attract even toxin-resistant eaters, and mechanism of this phenomenon is still unclear. One explanation is that transgenic plants have higher content of lignin, a polymer, not easy to hydrolyze invertebrates consider GM-plant residues to be less edible. 

That is why, ecologists warn about possible negative effects of Bt-plants on the environment, and that not all such plants effectively resist pests. Meanwhile, genetic engineers work on creating genetically modified plants and animals for production of biologically active substances, e.g. vaccines, hormones and pharmaceuticals. These substances target not insects, but mammals and human beings; however, no one knows how long these substances stay intact in nature, as well as no one have ever estimated possible damage from these substances.