Environmental Effects of Insect-Resistant GM Crops!
Plant-eating insects can cause great damage to crops; the European corn borer, for example, can destroy up to 20 per cent of maize yields. Over the past half-century, insecticides have become the main weapon of choice against insect pests. In the 1960s, concerns about their adverse health and environmental effects led to the prohibition or restriction of the most powerful broad spectrum contact poisons such as DDT.
Alternative insecticides have been developed which attack selectively instead of indiscriminately and which break down into non-toxic substances in the environment. Though these are subject to stringent testing, concerns about their effects persist, particularly about residues in food and water and their effects on non-target insects. The development of transgenic crops to enhance resistance to insect pests has been a focus of the agricultural biotechnology sector since the 1980s.
Genes expressing toxins from the soil bacterium Bacillus thuringiensis (Bt) were first engineered into tobacco in the middle of the decade. Bt toxins are highly specific to, and highly effective against, particular groups of insects but are not harmful to other organisms. They are also biodegradable and have been used as commercial insecticides in spray formulations since the 1950s. Though environmentally benign, their utility in spray form has been limited by high production costs and variable operational persistence.
Several varieties of Bt maize, cotton, and potatoes have received regulatory approval in the United States. Bt maize and cotton are estimated to have accounted for 25-30 per cent of the total US acreage of these crops in 1999.A variety of Bt maize which also incorporates a gene for herbicide tolerance has received marketing approval in the EU, but the acreage under cultivation in member states is negligible. Other transgenic crops incorporating Bt toxin genes are currently in development, as are genetically modified plants incorporating other proteins with insecticidal properties such as lections and protease inhibitors.
Crops genetically modified to incorporate Bt genes, and thus to produce the Bt insecticide inside the plant tissues, offer growers a number of advantages over other insect pest control methods. Unlike insecticide sprays, their utility is not dependent on weather conditions; plant parts difficult to reach by spraying are also protected. Particularly in the case of crops which require heavy applications of insecticide, there are significant potential savings to growers in pesticide, labour and equipment costs.
It has also been claimed that crops modified for insect resistance offer environmental advantages compared with conventional insecticide spraying. Spraying, especially from the air or in windy conditions, tends to be indiscriminate in its impact and often affects non-target plants in the vicinity. Bt toxin produced within the plant by genetic modification only kills insects which eat the plant tissue, and only certain kinds of insects at that because of its specificity.
Contamination of ground water around farmland should also be reduced as the Bt toxin is a natural protein which is readily biodegradable into non-toxic substances. Critically, the use of Bt crops should mean that less insecticide is needed as the insecticidal material is contained in the plant tissue.
There are possible risks associated with the modification of crop plants for insect resistance. Concerns have been expressed, first, that insects will evolve resistance to GM plants. This has been a long-standing problem with conventional insecticide treatments. The main counter-measure at present against the threat of enhanced resistance consists of the provision of ‘refuges’, that is areas of non-GM crops, which can support pest populations that are not resistant to Bt toxins.
The Environmental Protection Agency in the United States insists that, when farmers grow Bt cotton, 4 per cent of the crop must consist of unmodified cotton that has not been treated with any pesticide; if growers wish to spray with other pesticides, 20 per cent of the crop must be non-Bt cotton. In the case of Bt maize, the Agency’s new regulations announced in January 2000 provide that unmodified maize must account for at least 20 per cent – and in some cases up to 50 per cent – of the total crop.
While such provisions may help to address the problem, some recent research findings have raised questions about the effectiveness of refuges. This underlines the need for continued careful monitoring of the risk of enhanced resistance to Bt crops among target insects.
Effects on Non-Target Insects:
The other main concern about insect-resistant crops concerns their possible impact on non-target insects, some of which play an important role in the control of insect pests. On the one hand, the impact of such crops might be expected to prove beneficial because, unlike sprays which may affect all susceptible insects in the field, they should directly affect only insects which attack the plant.
On the other hand, GM crops may adversely affect non- target insects by depleting the supply of prey on which they feed. This problem also arises with non-GM pest control measures and gives cause for concern only to the extent that Bt crops are more effective in killing insect pests.