Unfortunately there’s no easy answer to this, since it often depends on the crops and how they’re used.
In some cases, GM crops can help farmers use fewer chemical insecticides. In others, they might lead to greater herbicide use or pesticide resistance. On balance, many scientific bodies are unconvinced that GM foods pose a special environmental threat — so long as they’re used carefully.
Here’s what the National Research Council concluded in 2010: “Generally, GE crops have had fewer adverse effects on the environment than non-GE crops produced conventionally.” But the report cautioned, “Excessive reliance on a single technology combined with a lack of diverse farming practices could undermine the economic and environmental gains from these GE crops.”
Some GM crops allow fewer pesticides: In some cases, GM crops can benefit the environment. Cotton that’s engineered to be pest-resistant can allow farmers to use fewer chemical pesticides. Likewise, the growth of Bt corn in the United States since 1996 has allowed farmers to use fewer insecticides in cornfields:
Other GM crops can lead to more herbicides — with a caveat: The story is murkier for chemical herbicides used on weeds. Many crops like soy, corn, cotton, and canola are now genetically engineered to be resistant to Roundup, a weed killer. That has led toa clear increase in herbicide use in the United States. But there’s a caveat here: the herbicide behind this increase, glyphosate, is less toxic than some of its predecessors.
Pest resistance and the risk of overuse: The National Research Council also warned against improper use of GM technology: Farmers who plant herbicide-resistant GM crops often use a limited range of herbicides on their fields, which can give rise to herbicide-resistant “superweeds.” Similarly, there’s evidence that overplanting of Bt corn has fostered a new breed of resistant insects in some fields.
That said, many conventional crops also re quire herbicides, and those ”superweeds” can appear on non-GM crop sites, too. In the end, the National Research Council wasn’t convinced that GM crops were inherently riskier, so long as they were used properly.
Other risks: It’s worth listing a few other environmental concerns, as well. The decline of the monarch butterfly in North America has been linked to the increased use of herbicide spraying on herbicide-tolerant crops. There’s also the risk that genetically engineered traits still in the testing phase could escape into nature, as apparently occurred in May 2013, when a never-approved strain of GM wheat made its way to an Oregon field.