By Mindy Tape

The adage “What doesn’t kill you makes you stronger” can be applied to weeds, insects and diseases developing resistance to pesticides.

For centuries, farmers have combated pests in several ways, embracing technological advances such as cultivation, chemicals, data-driven scouting, integrated pest management and genetic modification of crops as they became available.

Resistance is not a new phenomenon, nor is it a direct result of the introduction of genetically modified organisms (GMO), according to Michigan State University researchers. Rather, it’s a natural process that happens over time as pests that are already resistant to a certain type of removal mechanism find ways to survive and reproduce.

“In weeds, what resistance comes down to is just the overuse of a particular herbicide that functions at the same place in the plants,” says Christy Sprague, MSU weed scientist.

Sprague explains that herbicides themselves don’t make a plant resistant; instead, a small percentage of plants are just naturally resistant to any given herbicide. What’s happening is called selection pressure, caused by applying the same herbicide over and over, and it has occurred in crops for nearly half a century.

“Once those resistant plants grow and they don’t die, they produce more seed and suddenly the [resistant] population expands,” Sprague says. “One of the first weeds identified in Michigan that was resistant was atrazine-resistant common lambsquarters back in the early ’70s. Since then, we’ve seen multiple weed species become resistant to herbicides.”

Even though fungicides used to combat plant diseases are generally applied less often than herbicides used to fight weeds, selection pressure may still occur.

“In terms of field crop production in Michigan, we typically don’t have to spray frequently,” says Martin Chilvers, MSU field crops pathologist. “Sugarbeets might be an exception to that, where we have to make multiple applications to combat Cercospora leaf spot, and that’s where we have seen resistance develop. In terms of fungicides, with overuse, you’re just applying selection pressure to the population, and there’s always a small portion of the population that’s going to be resistant.”

Insect resistance to insecticides works in much the same way as disease resistance. Insects can become resistant to a particular insecticide very quickly if it’s used frequently.

“House flies became resistant to DDT within a year or so of the first use because there’s just so many flies and DDT was used so widely,” says Chris DiFonzo, MSU field crops entomologist. “In certain systems, like fruit, vegetables or even home use, insecticide application is often more frequent and there are more cases of insecticide resistance.”

GMO crops didn't invent resistance

Before GMO crops were developed, farmers generally had to apply a greater variety of herbicides and insecticides more often than they do today to control insect pests and weeds. For more than 20 years now, GMO crops have allowed farmers to reduce insecticide sprays, use fewer types of herbicides and make fewer passes across their fields, allowing them to farm more sustainably both in terms of economics and the environment. While there are some similarities between weed control and insect control in GMO crops, they do differ in how they combat pests.

The GMO technologies used for weed control use seeds into which genes that are naturally resistant to glyphosate, or another broad-spectrum herbicide, have been inserted. When that herbicide is applied to a field, the weeds in it will die but the crop will survive. In recent years though, selection pressure has caused some weeds to become resistant to glyphosate, as happened with older herbicides.

“[Resistance] really has nothing to do with a GMO crop, it’s just how a management system has happened, and it’s just putting that same selection pressure on the weed species,” Sprague says. “When glyphosate-resistant or Roundup Ready crops were first introduced, there was widespread use of glyphosate without a lot of other inputs to help manage weeds. With a focus just on glyphosate, you’re going to have a development of glyphosate resistance.”

According to Sprague, researchers and agriculture companies are now looking into weed control strategies such as using one herbicide that targets specific weeds along with a broad-spectrum herbicide such as glyphosate.

“One of the biggest issues that we have now is we’ve seen more and more weeds develop resistance to different types of herbicides. We also have weed species that have developed resistance to not just one, but multiple types of herbicides,” Sprague says. “We’re running out of options for management without new herbicides being brought onto the market. We’re also trying to look back at some older ones that worked a long time ago.”

Many insect pests, such as the European corn borer and corn rootworm, can be controlled by a protein toxic to pests produced naturally by the bacterium Bacillus thuringiensis.

This toxin (which isn’t harmful to humans) is inserted into the corn genome. Having the ability to combat insects built into Bt corn nearly eliminates the need to apply any other insecticide. Unfortunately, populations of Bt-resistant field crop insects are now surfacing.

“The Bt crop is essentially the insecticide,” DiFonzo says. “It makes the insecticide inside of its tissues and any time an insect is on that plant and feeding, it is exposed [to the insecticide]. So, it’s 100% exposure, all of the time, on every acre that crop is being used. This continuous exposure can lead to the development of resistance.”

Unlike weed seeds and spores that can lie dormant in the soil after a crop is removed, insects tend to be more mobile and mate with others across the landscape, potentially diluting resistance.

“Insects are moving, and in a way, sometimes that can help, because the population moves and mixes more and that can dilute out resistance as it’s starting. But in some ways, it can also make resistance move as well,” DiFonzo says. “So, there are some differences just based on the biology of the critters.”

Crop rotation reduces resistance

Because Michigan is one of the most agriculturally diverse states in the nation, most Michigan farmers can rotate crops on a regular schedule. This practice helps reduce pesticide resistance among diseases, weeds and insects.

“From a disease standpoint, extended rotations really help manage diseases,” Chilvers says. “With foliar and soilborne diseases, if it is possible to get away from just the corn-soybean rotation, extending that with wheat or something, can actually help reduce disease pressure in the soil.”

The corn rootworm is one insect farmers can combat through crop rotation. Because it survives only on corn, rotating away from corn production on a field eliminates the pest. In some Western states, corn rootworm has become resistant to Bt corn due to selection pressure and lack of crop rotation. Michigan’s crop diversity gives farmers a better chance to rotate corn out of fields before potential issues arise.

In the eastern Corn Belt, there have been a few cases of isolated Bt rootworm field failures, often on dairy or livestock operations with limited crop rotation potential. Once the issue was identified, most of the affected farms followed crop rotation recommendations from university and crop experts, greatly reducing the prevalence of the resistant insects.

“I think our longer crop rotations delayed the resistance to rootworm traits, as opposed to what’s happened out West. Even if it’s every three to four years, it’s better than the 20-year rotations some of these farms have. Anything is better than nothing,” DiFonzo says. “If you are in Iowa, southern Minnesota or the Dakotas, you may be using a full rate of soil insecticide on top of the Bt trait that you already bought. That resistance issue almost stops once you get to Indiana and Michigan due to crop rotation.”

According to the MSU researchers, Michigan’s diverse agriculture industry that allows for crop rotation, coupled with informed farmers who follow the recommendations of the university’s agriculture scientists, gives the state a slight advantage in the fight against resistance.

MSU researchers take action

Researchers from MSU AgBioResearch and MSU Extension have collaborated with their counterparts at other land-grant universities and agriculture industry representatives to combat pesticide resistance. DiFonzo’s Handy Bt Trait Table, helps farmers across the nation select seed with traits that are effective in their location.

Chilvers, Sprague and DiFonzo have worked with other university experts and the National Soybean Board to contribute their expertise to Take Action, a nationwide educational program designed to help farmers manage herbicide, fungicide and insecticide resistance. The goal of the effort is to encourage farmers to adopt management practices that will reduce the effect of resistant pests on crops while preserving current and future crop protection technology.

“It’s great that MSU AgBioResearch and MSU Extension are able to have people who work in these areas when a lot of states don’t,” Sprague says. “I think that’s a big positive because it’s helping us be on the forefront of solutions.”

Tape is communications manager for MSU Extension’s Agriculture and Agribusiness and Community Foods Systems.