AgBioResearch

In 1994, modern agriculture in the United States changed dramatically when the U.S. Food and Drug Administration approved the first genetically modified organism, or GMO, for commercial cultivation on American farms.

More GMO crops followed. In 1995, the U.S. Environmental Protection Agency (EPA) approved the first crop genetically modified to produce Bt toxin, a naturally occurring insecticide made by the bacterium Bacillus thuringiensis. A year later, soybeans genetically modified to resist the highly effective herbicide glyphosate (often sold under the tradename Roundup) appeared on the market.

These GMO crops, and those that followed, gave farmers new tools to deploy against two of their oldest foes: insects and weeds. The benefits were many. According to a 2016 study by PG Economics, an agriculture advisory and consultancy firm based in the United Kingdom, they reduced the volume of pesticide sprays by over 8 percent and reduced greenhouse gas emissions from agricultural equipment by over 500 kilograms in the United States alone. The use of GMO crops also improved soil health by making no-till farming practical.

Today, about 94 percent of soybeans and 89 percent of corn grown in the United States are herbicide-resistant, according to the U.S. Department of Agriculture (USDA) Economic Research Service. These statistics also show that Bt corn and Bt cotton comprise 81 and 85 percent of their crops, respectively. And many modern cultivars now contain both Bt and herbicide-resistant traits.

GMO technology has not come without controversy. Since the introduction of GMO crops, consumers, policymakers and scientists alike have raised concerns over their potential negative effects on the environment. Critics claim that GMO crops have caused the emergence of herbicide-resistant superweeds, the rise of secondary pest insects to fill the void left by those decimated by Bt toxin, and a reduction in biodiversity in areas surrounding agricultural fields.

The rise of superweeds

Since the introduction of glyphosate-resistant crops, about 38 weed species worldwide have been identified that have developed resistance to glyphosate. As a result, these so-called superweeds can continue to infest fields and siphon nutrients from the valuable crops planted there, leading farmers to use other costlier – and potentially harsher – herbicides to control them.

Questions quickly arose regarding the role the expanded use of GMO crops played in the development of superweeds.

Bernard Zandstra, professor in the Michigan State University (MSU) Department of Horticulture, has spent his career studying weed control in fruit and vegetable crops.

“Herbicide resistance in weeds comes from the regular, repeated application of the same herbicide, rather than the presence of genetically modified crops,” Zandstra said. “Glyphosate, for example, gives us a very convenient, clean and safe system. For the first 10 or 15 years [it was available], you could spray it once [and be done]. But its overuse caused resistant weeds to develop.”

Zandstra points to a finding consistent across much of the research, that herbicide resistance in weeds, far from a new phenomenon linked with the advent of GMO crops, has been a long-understood consequence of pesticide overuse. Glyphosate-ready crops merely made it easier to rely on a single herbicide for all weed management.

James Hancock, professor emeritus in the MSU Department of Horticulture, said that cases of GMO traits being transferred to non-GMO plants in the wild are rare. He added that while the overreliance on one herbicide has spurred the development of resistance to it, genetic modification did not have a direct role.

“The idea of herbicide resistance escaping from a GMO crop into the wild is an understandable concern,” Hancock, an MSU AgBioResearch expert in plant breeding and the biosafety of GMO crops, said. “In terms of being able to survive under the stresses of the wild, our domesticated crops are wimps. They’re bred to thrive under specific, human-maintained conditions. So if they hybridize with wild plants, those offspring will almost always be weaker and less capable of surviving [than the parent plants].”

Before glyphosate-based herbicides became available, farmers relied on a suite of chemicals for weed control. Individual herbicides were effective against a narrow range of plants, and farmers used them in rotation to effectively manage weeds. Rotation helped control the emergence of resistance by exposing weeds to a wide range of stresses.

According to Zandstra, when used in conjunction with appropriate spraying practices, GMO crops remain invaluable to many farmers.

“GMO crops, like anything else on the farm, are a tool,” he said. “When used in the context of good agronomic practices, such as rotating herbicide sprays, they become a great tool for the farmer and the consumer by making farms more efficient and economical. I recommend using another nonglyphosate herbicide alongside glyphosate, for example, so that if you do have some resistant weeds in the field, you ensure you aren’t leaving them behind to flourish.”

Even if herbicide-resistant weeds were to render some current weed control technologies ineffective, Hancock said farmers and researchers would find ways to adapt to the changes.

“Weed resistance just returns us to where we were before we had access to herbicides like glyphosate,” Hancock said. “That doesn’t create a new problem, it just brings an old one back that was being handled in different ways.”

Out with the old pests, in with the new

As major insect pests succumb to Bt crops, other secondary pests that aren’t affected by Bt toxin often take advantage of the lack of competition.

A well-known instance of this occurred in China, where widespread use of Bt cotton allowed farmers to effectively control the destructive cotton bollworm while reducing pesticide use. It dramatically improved yields and cut pest management costs. The bollworm’s decline, however, allowed the population of mirid bug, historically a minor pest of cotton plants that is not effected by Bt toxin, to increase. This again led to increased pest control costs as farmers contended with a new threat that their previous practices couldn’t contain.

“Bt toxin is only effective against particular species, leaving a wide array of insect pests that aren’t impacted by it,” Hancock said. “It makes logical sense that if you kill a major pest, but the chemical you’re using doesn’t kill other pests, those secondary pests will rise to take the first one’s place. But it remains in farmers’ best interests to control that first major pest, and then develop other solutions to confront the new problem.”

While the use of Bt crops has helped farmers control a number of serious pests, Hancock said it was never intended as a total or permanent solution to all insect pest issues in agriculture. Effective insect control will likely always require a suite of integrated pest management practices, with Bt crops playing a significant, but not all-encompassing role.

Biodiversity and landscape simplification

As the popularity of GMO crops has risen, so too have concerns that the crops could reduce the biodiversity of both the agricultural landscape and the surrounding wild ecosystems.

“Any time you have a successful crop variety – GMO or not – that everyone wants to plant, you inevitably reduce the biodiversity in farm fields,” Hancock said. “GMOs are no different in this regard than any other effective cultivar, but GMO crops tend to have traits that make them particularly successful for farmers. At the same time, however, there are already hundreds of GMO crop varieties available, so farmers aren’t being limited to just a handful.”

The USDA and EPA employ an extensive four-tier testing process to ensure Bt and other pest-resistant crops don’t directly harm nontarget insect species.

“Our regulatory system guards against the release of harmful crop varieties,” Hancock said. “It’s also unlikely a breeder would be willing to release something that would have an impact on the natural ecosystem.”

While GMO crops undergo years-long, thorough vetting processes, some questions still remain. MSU AgBioResearch entomologist Doug Landis has studied the phenomenon of landscape simplification and its effect on monarch butterflies for several years.

Due to the effectiveness of herbicide-resistant crops, plants like common milkweed have been all but eliminated from most crop fields. While beneficial to crops, the loss of milkweed has been linked to new challenges facing insects like the monarch butterfly, which has experienced a population decline of about 80 percent in the last two decades. Many factors have been connected with the decline, with no single, definitive cause emerging. But Landis believes the simplification of agricultural landscapes may play a role.

“Monarchs overwinter in Mexico, but they breed during the summer in the north central U.S. and parts of Canada,” said Landis, University Distinguished Professor in the Department of Entomology. “They depend on this breeding period to build up their numbers for the migration south, and the best information we have suggests a principle reason for monarch decline is the reduced abundance of milkweed in that north central region.”

The loss of milkweed from crop fields has forced monarchs to seek out milkweeds in more dangerous grassland settings, where predators abound. In grasslands, 60 percent of monarch eggs can be lost in a single day, compared to just 10 to 20 percent in an agricultural setting, said Landis.

He is quick to point out that monarchs survived for thousands of years before agriculture came to North America. He adds that rebuilding natural systems may allow them to survive and thrive again.

“Ecologists talk about the importance of having enemy-free space for the survival of young of any species,” Landis said. “We believe such spaces existed in grasslands thousands of years ago, when fire and larger animals disturbed the landscape and created patches for new milkweed to grow.”

Landis and his research team are exploring ways to recreate this in the modern landscape. One approach under investigation is selectively mowing small patches of milkweed in roadside grasslands to encourage the development of younger milkweed shoots preferred by monarchs.

Reintroducing a diversity of weed and pest management practices, rather than relying on just a few, will benefit the entire ecosystem.

“The problem is relying on one or two practices, like spraying glyphosate or dicamba (another widely used herbicide), across vast areas of land,” Landis said. “It’s a recipe for resistance and landscape simplification, which has knock-on effects for the ecosystem. Reintroducing diversity, both in practices and in the way we structure the landscape, brings resilience to the ecosystem that’s lost when we rely on just one or two things.”

The new difficulties in weeds, pests and biodiversity encountered in modern agriculture don’t stem directly from the use of GMO crops, but rather from treating the crops’ traits as a final solution to weed and pest management issues. Treating GMO crops as one among many tools in a management plan will help limit the spread of superweeds and secondary pests, as well as preserve landscape biodiversity.

“GMO crops are a very powerful, safe technology when used alongside good agronomic practices,” Zandstra said. “They’re tools that have helped us feed our society and helped growers earn a living, and that contribute to the plentiful, inexpensive food we enjoy in this country.”