There have been endless arguments about the potential harm that GM crops could cause. In fact, all agricultural practices have an impact - farming is not a "natural" process. When you see a picturesque field of rapeseed in flower, you are actually looking at a field containing virtually no genetic diversity (all the plants are from the same seed stock) with its own specific impact on the environment. Our countryside, a patchwork of neat fields separated by hedges, was all made by people. The "natural" English countryside finally disappeared when Henry VIII had the forests cut down to build his navy.
Organic farming, which claims to be more "environmentally friendly" than conventional methods, allows practices that can be harmful, such as the use of copper-based fungicide sprays and rotenone for pest control. Some GM crops, on the other hand, are developed to lessen the need for pesticides and other sprays with spectacular reductions in the use of noxious chemicals. Bearing in mind both impact on wildlife and the significant numbers of conventional farmers around the world who are poisoned every year using these chemicals, as well as their effects in the water supply, this is a major advantage for such GM plants.
Stories about harm to Monarch butterflies and other insects from GM maize turned out not to be true: an authoritative set of papers showed that the butterflies were alive and well in the US corn belt (although a quarter of them died in a freak snow storm in Mexico).
Claimed threats of “superweeds” have not materialised; the incidence of outcrossing of herbicide-tolerant GM constructs is no greater than for “non-GM” resistance varieties. There appear to be no compelling arguments for believing that GM crops are innately different from their non-GM counterparts. Their environmental impacts on invasiveness, weediness, toxicity and biodiversity fall into categories familiar from the cultivation of non-GM crops - and are much less than the environmental consequences of releasing in the UK, foreign plants bought from garden centres, as has recently been pointed out.
Mark K. Sears, Richard L. Hellmich, Diane E. Stanley-Horn, Karen S. Oberhauser, John M. Pleasants, Heather R. Mattila, Blair D. Siegfried, and Galen P. Dively (2001). Impact of Bt corn pollen on monarch butterfly populations: A risk assessment. Proceedings of the National Academy of Sciences, 98, 11937-11942 (http://www.pnas.org/cgi/content/full/98/21/11937?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=Monarch+butterflies&searchid=1110540089670_1917&stored_search=&FIRSTINDEX=0&journalcode=pnas)
Richard L. Hellmich, Blair D. Siegfried, Mark K. Sears, Diane E. Stanley-Horn, Michael J. Daniels, Heather R. Mattila, Terrence Spencer, Keith G. Bidne, and Leslie C. Lewis (2001) Monarch larvae sensitivity to Bacillus thuringiensis-purified proteins and pollen. Proceedings of the National Academy of Sciences, 98, 11925-11930 (http://www.pnas.org/cgi/content/full/98/21/11925?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=Monarch+butterflies&searchid=1110540089670_1917&stored_search=&FIRSTINDEX=0&journalcode=pnas)
J. Mark Scriber (2001). Bt or not Bt: Is that the question? Proceedings of the National Academy of Sciences, 98, 12328-12330 (http://www.pnas.org/cgi/content/full/98/22/12328?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=Monarch+butterflies&searchid=1110540089670_1917&stored_search=&FIRSTINDEX=0&journalcode=pnas)
G. J. Persley (May 2003; updated 14.4.04). New Genetics, Food and Agriculture: Scientific Discoveries – Societal Dilemmas. International Council for Science (http://www.icsu.org/1_icsuinscience/INIT_GMOrep_1.html)
Other papers on this set of experiments can be downloaded from http://www.pnas.org/cgi/search?fulltext=Monarch+butterflies&submit.x=8&submit.y=9