GMOs & Pesticides

When genetically modified organisms (GMOs) were introduced to farmers’ fields in 1996, one of the major selling points was that these new crop varieties would reduce the use of pesticides. After all, if the crop plant itself was producing a toxin that killed major insect pests – as in the case of Bt crops – it seemed logical that less insecticide would be needed. And if a GMO crop that could tolerate a particular herbicide – an “HT crop,” better known as a “Roundup Ready” variety – could live through the application of a pesticide that kills every other plant in the field, it seemed logical that Roundup would be the only herbicide needed and that less of it would be applied.

For a time, those assumptions seemed to be coming true. A USDA study from 2006 cited earlier, 1997 data and said:

“Overall pesticide use on corn, soybeans, and cotton declined by about 2.5 million pounds, despite the slight increase in the amount of herbicides applied to soybeans. In addition, glyphosate [Roundup] used on HT crops is less than one-third as toxic to humans, and not as likely to persist in the environment as the herbicides it replaces.”

That decrease represented a 6.2 percent decrease in pesticide treatments in 1997 from the time before GMO crops were introduced.

In 2008, a biotech industry lobbying group, the International Service for the Acquisition of Agri-Biotech Applications (ISAAA), claimed that in 2007 alone use of GMO crops resulted in a reduction of pesticide use of over 77,000 metric tons of active ingredients. The group said that was equivalent to using 18 percent less pesticide on farmers’ fields.

However, a 2009 survey of USDA data by the advocacy group the Organic Center countered that HT crops were causing a significant increase in pesticide use, particularly in the last few years:

“GE [genetically engineered] crops have increased overall pesticide use by 318.4 million pounds over the first 13 years of commercial use, compared to the amount of pesticide likely to have been applied in the absence of HT and Bt seeds.”

The Organic Center argued that many fields of CMO crops were being overrun by new weeds that had developed a resistance to Roundup. The problem seemed to be worst in the South and the Midwest where resistant strains of Palmer amaranth, horseweed, giant ragweed, common waterhemp and six other weeds have infested fields in 16 states. So, farmers were either applying much more Roundup, were going back to applying higher rates of traditional herbicides, or were manually reducing the weeds, either by cultivating with tractors or by hiring crews with hoes and machetes to chop the weeds.

These are diametrically opposite conclusions. So, who is right?

The truth is in the details and how they are interpreted. First, we need to distinguish between HT technology and Bt technology.

HT, herbicide tolerant crops, of course, are designed to survive the application of an herbicide like Roundup or Liberty that usually kills all plant life it touches. Before GMO HT technology, a farmer could treat his field of corn, for example, with an herbicide that killed broadleaf weeds. That was fairly effective, but it didn’t do anything to kill other grass species that could compete with the corn. Or, if the farmer was growing a broadleaf plant like soybeans, he or she could choose an herbicide that affected grasses. Again, that didn’t kill other broadleaf weeds. Now, HT technology meant that every weed except the specific crop you planted would be killed – every weed, that is, until the nature developed her own herbicide tolerant plants.

In 2008, the USDA Agricultural Research Service recognized that, “As some cotton growers have relied exclusively on glyphosate (Roundup) to control weeds, it was inevitable that weed resistance to glyphosate would develop.” The report recommended that farmers use both crop and herbicide rotation systems to avoid the development of resistant weeds. They also suggested planting winter cover crops, like rye, before planting the cotton, weeding the fields by hand, or using traditional pre- and post-emergence herbicides to control any weeds that germinate and survive the other methods.

So far, there have been fewer problems with Bt varieties of GMO crops than with HT. Bt varieties have incorporated the genes from a common bacteria known as Bacillus thuringiensis into various crop hybrids. There are various strains of the Bt organism that produce naturally occurring toxins for various pest insects. What we collectively call “Bt cotton” in fact may have several different versions of Bt genes incorporated into the cotton hybrid that allow the plant to manufacture chemicals that kill different insect pests.

So, Bt cotton in India has genes to kill the cotton looper, red hairy caterpillar and the spiny bollworm. Bt cotton in the U.S. has genes to kill the cotton leaf perforator, the saltmarsh caterpillar, the cotton bollworm and six other insects common to North America.

Because the pests that attack cotton are so specific and because GMO scientists have been able to find strains of bacteria that attack those specific species, the use of broad spectrum insecticides has been reduced more than with HT technology. One 2008 study – led by USDA researcher Steven E. Naranjo – found that:

“It is estimated that between 1996 and 2005 the deployment of Bt cotton has reduced the volume of insecticide active ingredient used for pest control in cotton by 94.5 million kilograms and increased farm income through reduced costs and improved yields by US $7.5 billion, with most of the benefit accrued by farmers in developing nations… This represents a 19.4% reduction and was the largest reduction in pesticide use afforded by any GMO crop… After 11 years of Bt cotton cultivation, control failures due to resistance have not been detected under field conditions.”

But the authors cautioned that insect species resistant to Bt genes could still develop. They said it was crucial that farmers continue to plant up to 20 percent their fields in traditional, non-Bt varieties. These parts of the fields are known as “refuges.” The theory is that if any Bt-resistant insects evolve, they will mate with insects from the refuge, and then the resistant genes don’t have as good a chance of getting established.

Written by Bill Ganzel, the Ganzel Group. First published in 2009. A partial bibliography of sources is here.