SEATTLE — As Washington residents weigh the pros and cons of labeling genetically engineered foods, one question on many minds is whether current GE crops have helped or harmed the environment.
A panel of independent scientists and agriculture experts convened by the National Academy of Sciences in 2010 came down on the positive side. "Generally, GE crops have had fewer adverse effects on the environment than non-GE crops produced conventionally," their report concluded.
But as in everything to do with genetic engineering, blanket statements conceal a world of complexity. Environmental impacts vary depending on the types of genes added to each crop and the traits those genes confer.
Corn and cotton engineered to produce a natural bug killer called Bt have been clear environmental winners, reducing the amount of insecticide farmers spray on their fields. The picture is murkier when it comes to the other major category of GE crops: those engineered to survive being sprayed with the weed killer glyphosate, sold by Monsanto as Roundup. Initially, the technology led to a drop in the amount and toxicity of herbicides used on American fields, but an explosion of resistant weeds is erasing those gains.
"It's a mixed bag," said Bruce Tabashnik, head of the entomology department at the University of Arizona. "I don't think GE crops are necessarily bad. They can be good, and they can even be great for the environment -- but it depends on the inherent traits and whether they're being used in an environmentally conscious way."
Tabashnik works with cotton farmers in Arizona, where the advent of GE varieties with the built-in bug killer slashed insecticide use by 80 percent. Results haven't been so dramatic elsewhere, but Charles Benbrook, of the Center for Sustaining Agriculture and Natural Resources at Washington State University, estimates that insecticide use in the U.S. has dropped by about 123 million pounds since the new crops were introduced.
The bug killer Bt is also much less toxic than other varieties, and specifically targets pests like the pink bollworm, the bane of cotton farmers, Tabashnik said. "It's about as close as you can come to being completely benign."
Bt, widely used in organic farming, works by binding to specific proteins in the guts of certain insect larvae. It's harmless to other insects and all vertebrates -- including humans, birds, mammals and reptiles -- because they lack those proteins. While some insecticides contain nerve poisons that kill insects on contact, Bt harms only bugs that munch on a crop.
As a result, populations of "good" insects, like ladybugs and other beetles, have increased in many GE fields. The presence of insects that prey on crop pests helps farmers avoid the need to spray additional, more toxic bug killers, Tabashnik pointed out.
"If things work out as well as they have in cotton here, you can get off that pesticide treadmill and you can even run it in reverse."
Margaret Reeves, senior scientist at the Pesticide Action Network, agrees that Bt crops initially reduced the use of more toxic bug sprays, but also noted the emergence of Bt-resistant insects. That's particularly alarming to organic farmers, as they have few tools to fight insects.
Tabashnik is concerned as well -- but not surprised. Insects have evolved resistance to virtually every bug killer humanity has devised, he pointed out. While Bt remains remarkably effective against about a dozen major pests, the number of resistant species has increased from one to five since 2005. The biggest problems are among corn pests that weren't very susceptible to Bt in the first place, Tabashnik explained.
Faced with potential losses in yield, Bt corn farmers are increasingly turning back to bug-killing sprays, and insecticide use is edging up again.
To slow the development of Bt resistance, the Environmental Protection Agency requires corn farmers to plant conventionally bred seed on a small percentage of their lands. That provides a refuge for pests vulnerable to Bt, which then mate with the smaller population of resistant pests. The resulting offspring don't inherit resistance, so the cycle can be broken -- or at least delayed.
To impede evolution even more, Tabashnik and other insect experts are pressing the EPA to require Bt corn farmers to plant as much as half their acreage in non-Bt crops, a measure the agency's own scientific advisory panel recommended years ago.
"We don't imagine we can make that rate of evolution zero," he said. "But we're trying to figure out how to maximize the benefits of Bt."
If EPA's "refuge" requirement helped slow the march of Bt resistance, the ubiquitous use of Roundup and crops engineered to resist the weed killer has had the opposite effect.
"It's a train wreck, and it's entirely man-made," said WSU's Benbrook.
Some experts call Roundup "agricultural heroin," and it's easy to see how farmers got hooked.
The idea behind the technology is simple, appealing, and -- until recently -- highly effective. Adding a bacterial gene to soybeans, corn, cotton and sugar beets makes the plants impervious to the herbicide.
Farmers who grow so-called Roundup Ready crops can spray their fields with Roundup and kill only the weeds.
Glyphosate has many environmental advantages over nearly all other herbicides. It's less toxic to humans, other mammals, birds and fish. It breaks down quickly and is less likely to run off fields and pollute rivers and streams. Farmers who switched to Roundup Ready crops didn't need to plow up weeds before planting, which reduces erosion and improves soil quality.
In the first several years after the introduction of Roundup Ready crops in 1996, herbicide use dropped. But as the GE crops quickly came to dominate U.S. farms, use of glyphosate soared.
Benbrook estimates the amount of herbicides applied in the U.S. increased by 527 million pounds between 1996 and 2011, compared with what would have been used on conventional crops.
Several industry-funded analyses dispute that conclusion. The most recent, published this year, reports a 4.2 percent decrease in herbicide use on GE versus conventional soybeans in the U.S. between 1996 and 2011. The study also points to a 26.3 percent increase in "environmental impact quotient," a measure of herbicide safety.
Benbrook agrees glyphosate is ecologically superior to the alternatives. "It's one of the safest herbicides ever marketed," he said. "That's one of the tragedies about the fact that it's being driven into obsolescence."
But farmers were seduced by the simplicity of spraying a single chemical -- and assured by Monsanto that the risk of Roundup-resistant weeds emerging was low, said Carol Mallory-Smith, a professor of weed science at Oregon State University. Instead of rotating crops, they grew only Roundup Ready varieties. Instead of spraying once, they applied the herbicide three or four times a season.
Overuse of any pesticide is a recipe for resistance -- and it wasn't long before the spray stopped working against some weeds. Twenty-four species of weeds found in 21 countries and 32 U.S. states -- including Oregon -- are now glyphosate-resistant, said Ian Heap, director of the Corvallis, Ore.-based International Survey of Herbicide Resistant Weeds.