5.4 MYTH:

GM Bt crops reduce insecticide use


GM Bt crops merely change the way in which insecticides are used

GM proponents claim that GM Bt crops reduce insecticide use, as farmers do not have to spray chemical insecticides. But this claim does not stand up to analysis, since the Bt gene turns the plant itself into an insecticide and because pest adaptation makes the GM pesticide less effective over time, making it necessary for farmers to revert to the use of chemical pesticides after just a few years. The genetically modified insecticide is present in active form in every part of the crop, including the parts that people and animals eat.

So Bt crops do not reduce or eliminate insecticides. They temporarily change the type of insecticide and the way in which it is used – from sprayed on, to built in. But in the long term, use of chemical pesticides must be resumed, as long as the industrial agricultural model is followed.

Even if we choose to ignore this factor and only consider the temporary reduction in chemical insecticide sprays due to Bt crops, the figure is unspectacular (see 5.2, above) – a reduction of 64.2 million pounds (29.2 million kg) over the first thirteen years of GM crop cultivation in the United States. This reduction is swamped by the massive increase in pesticide use resulting from the adoption of GM herbicide-tolerant crops, which has caused farmers to spray 383 million more pounds (174 million kg) of herbicides than they would have done in the absence of GM herbicide-tolerant seeds (herbicides are technically pesticides).9

Even the modest reduction in chemical insecticides attributed to GM Bt crops is proving unsustainable, due to the emergence of pests resistant to Bt toxin and secondary pests, as explained below. Moreover, there is a question mark over whether Bt crops can truly be said to have reduced chemical insecticide use in view of changes in the types of insecticides used and in the methods of application.

5.4.1. Resistant pests are making Bt technology redundant

GM Bt insecticidal crops express the Bt toxin in every cell for their entire lifetime, constantly exposing pests to the toxin. This is different from the traditional use of natural Bt as a spray, where the targeted pests are only exposed for a brief period before the Bt breaks down in daylight. Exposing pests to a pesticide for long periods of time inevitably speeds up the emergence of resistant pests, since selective pressure eliminates all but the most resistant pests, which then reproduce and pass on their genes.

For this reason, Bt crop technology sometimes enjoys short-term success in controlling pests but is soon undermined by the emergence of pests resistant to the toxin.43,44,45 By 2009, the western corn rootworm had evolved resistance to a Bt maize specifically engineered to target the pest that was first commercialised only six years previously.46 Bt-resistant rootworm populations have been reported in Iowa46,47 and Illinois.48

5.4.2. The “refuge” concept breaks down

Farmers are encouraged to plant “refuges” of non-Bt crops as a resistance management strategy to delay the emergence of Bt-resistant pests. The idea is that the non-Bt crop acts as a refuge where Bt-susceptible pests can survive, ensuring the existence of a population of Bt-sensitive pests to mate with any Bt-resistant pests that survive in the adjacent field where the Bt crop is under cultivation. The theory is that the Bt-susceptible pest population will dilute out the Bt-resistant population that survives in the Bt crop, assuring that the predominant population is Bt-susceptible.

But a study on rootworm resistance in Iowa found that refuges were redundant in the case of substantial Bt-resistant rootworm populations, as the pests were able to live and reproduce in Bt maize fields. The study concluded, “Even with resistance management plans in place, sole reliance on Bt crops for management of agriculture pests will likely hasten the evolution of resistance in some cases.”46

Also, the effectiveness of refuges relies on the Bt crops expressing doses of Bt toxin that are high enough to kill pests, and the non-Bt refuges remaining free from Bt toxin-expressing genes. But cross-pollination between GM Bt maize has been found to cause “low to moderate” Bt toxin levels in the refuge plants,49 making refuges less effective.

5.4.3. Secondary pests attack Bt crops

Nature abhors a vacuum. So even when Bt toxin succeeds in controlling a primary pest, secondary pests move into the ecological niche. For instance, in the United States, the Western bean cutworm has increased significantly in Bt maize fields.50 In China and India, Bt cotton was initially effective in suppressing the target pest, the boll weevil. But secondary pests that are resistant to Bt toxin, especially mirids and mealy bugs, soon took its place.51,52,53,54,55,56

Two studies from China on GM Bt insecticidal cotton show that GM Bt technology is already failing under the onslaught of secondary pests:

A study of 1,000 farm households in five provinces found that farmers noticed a substantial increase in secondary pests after the introduction of Bt cotton. The researchers found that the initial reduction in pesticide use in Bt cotton cultivars was “significantly lower than that reported in research elsewhere” and that “more pesticide sprayings are needed over time to control emerging secondary pests” such as aphids, spider mites, and lygus bugs. In addition, a quarter of the farmers thought Bt cotton yielded less than non-GM varieties. Close to 60% said that overall production costs had not decreased, due to the higher price of Bt cotton seed.57

Field trials conducted over ten years in northern China show that mirid bugs have increased in cotton and multiple other crops, in proportion to a regional increase in Bt cotton adoption. The researchers’ analyses show that “Bt cotton has become a source of mirid bugs and that their population increases are related to drops in [chemical] insecticide use in this crop.” Moreover, mirid bug infestation of other food crops (Chinese dates, grapes, apples, peaches, and pears) increased in proportion to the regional planting area of Bt cotton.58

It is clear from these developments that GM Bt technology is not a “silver bullet” solution but is economically and environmentally unsustainable, as farmers who have paid premiums for Bt insecticidal seed have had to return to spraying costly and toxic pesticides.

5.4.4. Bt cotton farmers don’t always give up insecticides

GM proponents often assume that farmers who adopt Bt crops give up chemical insecticides – but this is not necessarily the case. Tabashnik (2008) reported that while bollworms have evolved resistance to Bt toxin in one type of GM cotton, this has not caused widespread crop failure because “insecticides have been used from the outset” to control the pest.45 So claims of reductions in insecticide use from Bt crop adoption are unreliable unless there is evidence that the farmer does not use chemical insecticides.

Moreover, most Bt crops currently commercialised or in the pipeline have added herbicide tolerance traits and so are likely to be grown with herbicides.59 It is with good reason that one independent scientist has called GM crops “pesticide plants”.60

5.4.5. Hidden chemical insecticides in Bt maize

Studies claiming reductions in insecticide use due to Bt crops have previously focused on insecticides that are applied to the soil or sprayed onto the plant after it has begun to grow. They may neglect to mention a different, potentially environmentally destructive type of pesticide: those that are applied to the seed before it sprouts.

According to a study by US entomologists, all commercially available rootworm-directed Bt maize seed is now treated before it is planted with the controversial chemical insecticides known as neonicotinoids. The authors suggested that the adoption of Bt maize “may shift insecticide use patterns” from sprayed insecticides to such seed treatments.61

So GM Bt crops may have done little more than help cause a shift in the type and means of application of chemical insecticide, rather than reducing or eliminating such chemicals. Where insecticides used to be applied to the soil or the plant while it is growing, now they are applied to the seed before planting.

Dr Doug Gurian-Sherman, senior scientist at the Union of Concerned Scientists, commented that neonicotinoid treatments on Bt maize seed aim to kill the insect pests that are not well controlled by Bt toxins. He added that these seed treatments are not confined to Bt maize: most maize seed, apart from organic, and an increasing proportion of the seed of other row crops, is now routinely treated with neonicotinoids.62,63

Neonicotinoids are systemic insecticides, meaning that they spread throughout all tissues of the crop plant as it grows and are even present in the pollen and nectar. Like the Bt toxin engineered into GM plants, neonicotinoids differ from sprayed insecticides in that they are persistently present in the growing plant and always active. Because of this long exposure period, pests are more likely to develop resistance to them, and non-target and beneficial insects are more likely to be exposed, too.

Neonicotinoids are toxic to a wide variety of beneficial creatures, including some that help protect crops.64,65 They have been found to have highly toxic effects even at very low doses because they persist over long periods in soil and water.66 The rise in the use of neonicotinoid seed treatments has been implicated in bee die-off and colony collapse.67,68 Bees living near agricultural fields have been found to be exposed by multiple routes, including contaminated wild flowers growing near fields, and neonicotinoids have been found in dead bees.68

The chief – seemingly the only – concern of defenders of Bt crop technology is the volume of insecticide applied as sprays after planting. If that volume decreases, they consider that Bt crops reduce insecticide use. But they are not reporting the whole story. The case of neonicotinoid seed treatments shows that it is necessary to consider other types of insecticide applications, how toxic the insecticides are (based on peer-reviewed research, not industry data), how they behave and persist in the environment, and the acreage over which they are applied.62

Given the extreme toxicity of neonicotinoids to bees and other beneficial organisms, their high degree of persistence and spread, and the vast acreage over which they are applied, it is questionable whether seed-treated Bt crops have had a beneficial effect on insecticide use.

5.4.6. Conclusion

Studies claiming that Bt crops reduce insecticide use have failed to take into account important aspects such as:

  • The toxicity to non-target and beneficial organisms of the engineered Bt toxins
  • The amount, type, and toxicity of insecticides actually used by farmers in the field even when Bt seeds are used – reflecting pest resistance and ineffectiveness of refuges
  • Changes in the way insecticides are used, such as the transition from sprayed pesticides to use of insecticidal seed treatments.

Also, when evaluating the impact of GM Bt crops on insecticide use, a more useful comparator than chemically-grown non-GM crops would be non-GM crops under organic or integrated pest management, where insecticide use is reduced or eliminated. This would quickly make clear which farming methods can best reduce insecticide use while maximizing yield and farmer incomes.

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