Pesticides and the adoption of genetically modified crops
There is a strong association between commercialized GM crops and pesticides. Chemical pesticides were introduced to farming systems on a large scale in the late 1940s, and their use was reinforced by the 1960s introduction of green revolution plant breeding technologies.  Most farmers now use pesticides.

The commercially available GM crops are entirely focused on the role of pest management of weeds or insects. The main GM trait is herbicide tolerance (HT), with the majority of crops engineered to be resistant to the herbicides glyphosate (RoundUp Ready), and others to glufosinate ammonium (Liberty Link). The second most common trait is incorporation of the gene for Bacillus thuringiensis (Bt), a widely used bacterial insecticide approved for use in organic production. Some varieties express both traits.

The companies marketing GM crops state that they will reduce the cost of pest management, which is the main selling point, and potentially reduce the use of hazardous pesticides.

Herbicide tolerance in GM crops allows growers to use a broad-spectrum herbicide, glyphosate or glufosinate ammonium, to control weeds within the crops.  This does not necessarily mean less pesticide being used, but rather a different set of pesticides from those used with conventional crops. There could be a reduction, however, if the grower has a range of weeds which would need a range of different pesticides to control.  With GM crops, these might be controlled with a single application.  These pesticides are also significantly cheaper than the selective herbicides used with conventional crops, and this leads to considerable cost savings (both in chemicals and labour).

However, there are concerns about the efficacy of HT GM crops: for example, with the ‘Liberty Link’ seeds, glufosinate ammonium is not effective in controlling weeds to an economic level, and 90% of US farmers also spray the herbicide atrazine  (an endocrine-disrupting herbicide recently banned in Europe).

Bt toxins give resistance to certain lepidopteran pests. Bt cotton, for example, has led to initial reduction of insecticide use in some areas, however it should be noted that planting of Bt crops does not eliminate insecticide spraying, as they are ineffective against many other pests. Bt is a particularly useful insecticide which is approved for use in organic systems, and therefore the introduction of Bt as a genetic trait will only eliminate the use of an already relatively benign chemical, not the more hazardous insecticides such as organophosphates.

There have been reports of an increased incidence of sucking pests, not affected by Bt, in Bt cotton. This would be consistent with patterns of pest behaviour, where one pest is controlled it is often found that other pests can multiply at a greater rate and become more significant than previously.

It is important to know therefore whether claims for pesticide reduction bear examination, and whether the associated reductions in pesticide use are sustained. Conventional pesticide use, particularly in developing countries, is often associated with escalating levels of application.

The question of whether this increases or reduces the use of herbicides and insecticides tends to be both crop and site specific, and there are different pressures driving each. However, if a reduction in pesticide use was a consequence of GM crop production, one would expect that the rapid growth of GM crops world wide (Table 1) would have slowed or reversed the increasing sales of pesticides, which is not the case (Fig. 1). On the contrary, since 2001 there appears to have been an upturn in both insecticide and herbicide sales.

Table 1. GM Crops (million ha)



































South Africa










Source: International Service for the Acquisition of Agri-biotech Applications
(ISAAA), compiled by Genewatch UK, 2005.

Figure 1. Increasing sales of herbicides, insecticides, fungicides 1980-2004 (US$)
Fig 1.
Source: Wood MacKenzie and Allan Woodburn Associates reports for the Crop Protection Association UK

The actual quantity of pesticides used is not easy to determine from sales figures, since falling costs of chemicals, due, for example, to pesticides coming off-patent, can disguise rising quantities sold.  Similarly, figures that indicate tonnes of active ingredient sold can be even more misleading when more modern pesticides are effective at much lower doses than their predecessors.

There is no doubt, however, that world-wide pesticide use is not declining and every indication that it is still increasing.

Company sales figures show that the rise in herbicide tolerant GM crops are leading to a significant increase in glyphosate sales:

Monsanto Second-Quarter and First-Half 2005 Performance Summary:
Net sales increased 27 percent to $1.9 billion in the second quarter primarily because of higher corn and soybean trait revenues in the United States, increased corn seed sales in the United States and the Europe-Africa region, and higher revenues for Roundup and other glyphosate-based herbicides in all major world areas. For the quarter, sales in the Seeds and Genomics segment increased by 35 percent on the strength of increased seed and trait sales. There was also a 17 percent improvement in sales in the Agricultural Productivity segment, as sales of branded Roundup herbicides and other glyphosate-based herbicides increased.

3.1 Glyphosate
The manufacturers of glyphosate-based herbicides claim that glyphosate has ‘low toxicity and environmental friendliness’. However, independent research indicates that glyphosate may not be as safe as previously thought.  Some surfactants that are commonly used as co-formulants in Roundup and other products are also hazardous (yet the use of these is not regulated)  and the effects of the formulated products can be significantly more severe than of glyphosate alone.

A PAN UK factsheet on glyphosate (see appendix 1) outlines the health and environmental hazards.

A more recent study of Roundup(1) presents new evidence that the glyphosate-based herbicide is far more toxic than the active ingredient alone. The study reports glyphosate toxicity to human placental cells within hours of exposure, at levels ten times lower than those found in agricultural use. The researchers also tested glyphosate and Roundup at lower concentrations for effects on sexual hormones, reporting effects at very low levels. This suggests that dilution with other ingredients in Roundup may, in fact, facilitate glyphosate's hormonal impacts.

3.2 Environmental effects of Pesticides
The environmental effects of pesticides are well recognised in Europe.  The regulatory process is designed to assess the risk to the environment and eliminate those that are unacceptable. There are remaining risks, however, that over the years have led to loss of biodiversity, poor soil quality, contaminated water and a build-up of persistent chemicals in the environment.  In developing countries, however, where pesticide regulation is much less developed and GM production is increasing fastest, the environmental consequences are likely to be more serious.

The environmental consequences of increasing the use of broad-spectrum herbicides such as glyphosate are potentially very serious.  While glyphosate may be less toxic to invertebrates and mammals than some selective herbicides, the fact that is kills such a wide range of plant life means that habitats and food sources at the bottom of the food chain are completely destroyed.  The resulting effects on biodiversity have been documented in the UK farm scale evaluation trials, recently completed.  The results showed that the effects on wildlife, which depended on the particular crop grown, were largely a result of changing herbicide use rather than the use of GM crops themselves.

Examples of the findings include(2)

  • The researchers aren’t sure yet whether the differences in weed and seed numbers would persist after several years of cropping with cereals, but seed stores could get severely depleted and may not recover.
  • Growing GMHT spring and winter rape, and beet might provide benefits to some insects and other small creatures. However, this boost for these animals may be short-lived as, over the years, there could be fewer weeds left for the herbicide to kill, reducing the supply of rotting weeds.
  • The small creatures that survive on seeds dropped by weeds would not fare as well in GM spring rape and beet crops. For these animals, fewer weeds would mean a shrinking food supply. If the seed stores remain depleted in the long-term, this would have serious repercussions on their survival.
  • Bumble bees and butterflies will be affected if flowering weed numbers decline. However, they can fly long distances until they find the plants they need so a decline in weeds may have little immediate impact on them. But if weeds die away over large areas over several years, the effects on nectar resources would become more important. This could happen if GMHT beet, and winter and spring rape were grown over many years and over large areas of arable land.
  • This study did not involve farmland birds, but its results do have implications for them. As growing GM spring rape and beet dramatically cut the number of weed seeds, bird populations could struggle to find enough to eat, especially later in the year. 

Some of the results suggested that there could be improvements in weed numbers within GM crops. There is, however, significant reason to suppose that some herbicide tolerant GM crops will adversely affect the environment through the changing pattern, and potential increases in, herbicide use.

3.3 Herbicide resistance in weeds
Herbicide resistance can develop in two ways – either by a genetic mutation, which can happen whether conventional or GM crops are grown, or specifically with GM crops, when gene transfer (through cross-pollination) occurs.

The causes of the former type of resistance to herbicide developing in weeds are complex, but it is a widespread and serious problem. In October 2000 there were 235 herbicide-resistant weed biotypes in 47 countries. Some were resistant to than two herbicides. By April 2005 there were 296 resistant biotypes. The problem was once only in industrialised countries where herbicides were widely used, but is now a worldwide phenomena, indicating the global spread of herbicide use.

Fig. 2

Glyphosate, the most widely used herbicide, is now seeing an increase in resistant weeds. The first incidence of glyphosate resistant weeds probably appeared around 1996 when GM planting began. In the GM cotton growing areas of Tennessee, resistant marestail infested over 200,000 acres of cotton in 2002, with impacts on 36% of all cotton in the state(3). Glyphosate resistance is now reported everywhere where HT GM crops are commercially grown.  Even if HT CM crops disappeared, we would be left with the legacy of glyphosate resistance which was not present before GM crops were introduced.

It is universally accepted that widespread use of a limited number of chemicals is likely to lead to resistance.  Thus herbicide tolerant GM crops, specifically designed to encourage use of a single chemical, will inevitably lead to herbicide-resistant weeds which will before long eradicate the only “benefit” that these crops provide..

The transfer of the herbicide resistant gene to weeds has always been dismissed as a negligible risk by GM proponents.  However, recent research(4) has shown that it has in fact occurred during some GM trials in the UK.  While the reviewer of the report suggested the frequency of this occurring is likely to be low and the consequences negligible, there is insufficient evidence to make these assertions.  The frequency of resistance occurring does not need to be high to cause problems: one resistant weed can spread rapidly.  The frequency of this occurring is also likely to be dependent on the amount of genetic material released into the environment, therefore the more GM crops there are, the greater the risk of gene flow.

This route of creating resistance may or may not be significant in the development of “superweeds”, which is occurring anyway.  It is crucial, however, in demonstrating that gene flow occurs and that safety measures are insufficient.

The consequence of herbicide resistant weeds is that alternative herbicides need to be used. Those being recommended in the USA include more hazardous chemicals, such as 2-4,D and paraquat (both WHO class II moderately hazardous pesticides) and pendimethalin (potential carcinogen).  Farmers are also being advised to always use the full dose of glyphosate to reduce the chances of resistance developing. Both of these strategies will also inevitably contribute to increasing herbicide use.

3.4 Bt resistance
Bt is used as an insecticide in organic systems, and it is a significant concern that through its incorporation into the crop, and global increase in use, insect resistance will emerge to this important pest management product.

Resistance-management techniques have been developed to address this, which include planting “refuges” with a minimum of 20% non-GM blocks (required by US regulators). These strategies appear so far to have been effective in the USA, with no reports of Bt resistant insects.  However, these strategies do involve the use of insecticides.

In China, research suggests that resistance is becoming a problem, as well as increases in other insect pests:

  • resistance build-up towards Bt in the main target pest, cotton bollworm: susceptibility of bollworm to the Bt toxin fell to 30 percent after 17 generations under continuous feeding with Bt cotton leaves. The resistance of the bollworm increased 1000 times when the feeding was continued to the 40th generation.
  • a significant reduction of the parasitic natural enemies of cotton bollworm.
  • an increase of secondary pests: e.g. cotton aphids, cotton spider mites, thrips and others, replaced the cotton bollworm as primary pests in some of the cotton fields. These factors have forced farmers to continue the use of chemical pesticides, and increased the possibility of outbreaks of certain pests due to the destabilized insect community. (Professor Xue Dayuan Nanjing, Institute of Environmental Sciences)

and other incidences of Bt resistance have been documented. Both the development of Bt resistance and resistance-reduction strategies may account for why insecticide use does not appear to have declined with the increase in Bt resistant crops and leads us to question whether there is really any benefit to Bt GM crops at all.

There are serious concerns about GM crops per se – the ability for genes to escape into the environment and the consequent damage to wild species and the safety of introducing GM products into the food chain.

In addition to these concerns, the impact of GM crops on the use of pesticides, and their consequences for both the environment and public health are potentially very serious.  Because of the current restrictions on GM planting in European countries, these impacts are effectively being transferred to countries that are much less able to protect their own environment and their own population from the hazards of pesticides.

Appendix 1 >

  1. Sophie Richard, Safa Moslemi, Herbert Sipahutar, Nora Benachour, and Gilles-Eric Seralini, Environmental Health Perspectives, Vol. 113, No. 6 June 2005,
  2. Managing GM crops with herbicides: effects on farmland  wildlife, Maria Burke, Farm Scale Evaluations Research Consortium and Scientific Steering Committee
  3. Robinson, E. UT Scientists make new marestail list, Farm Press Online 15 August 2002 – reporting on research of Dr Robert M. Hayes, University of Tennessee
  4. The potential for dispersal of herbicide tolerance genes from genetically-modified, herbicide tolerant oilseed rape crops to wild relatives