How to Resist Resistance

When I started working at the Southern Region IPM Center in 2006, I wrote several stories of projects in the region that were in the process of completion. One of them involved a mosquito-management program in Houston, Texas. Populations of the southern house mosquito (Culex quinquefaciatus) were surviving treatments of malathion, the most widely-used pesticide to control it.

The Southern Region IPM Center funded a one-year IPM Enhancement project in 2005 to find an alternative to the insecticide. The researchers found that malathion would still work—as long as it wasn’t used all the time. The key to switching from one chemistry to the other was not to wait until the majority of mosquitoes were resistant, but to switch while there was still a large enough mosquito population that would be susceptible to one of the chemicals.

The key to resistance—whether it’s to insecticides, fungicides, herbicides or antibiotics—lies in the genes. The basic biological purpose of any organism is to survive and perpetuate the species. Sometimes species have to evolve in order to survive.

That fact is the basis behind glyphosate-resistant weeds, or “superweeds,” as some have named them. The fact that an undesirable pest has become resistant to a popular control is nothing new, nor is the long-standing battle against using pesticides on crops. I’ve read articles praising Roundup Ready crops as well as articles condemning them. I’m going to do neither in this blog post. I’m simply going to tell a story of how glyphosate resistance has come to be, and what may help stop the resistance cycle.

In a field, weeds can be as lethal to developing crops as a hungry insect. They often grow at a faster rate than the crop, preventing sunlight from reaching the crop. They also suck up water, and during dry spells, can spell doom for a thirsty seedling. Some weeds, once mature, are so dense that they damage harvesting equipment. If weeds mingle in with the harvested crop, in most cases, the crop is worth less at the market, ultimately making the price of the crop higher at the supermarket.

Farmers have many tools at their disposal to keep weeds at bay. Some experts have found that not tilling the soil keeps weeds down more than tilling. Others put down plastic covers that would smother emerging weeds but would have holes for emerging crops. Herbicides are another tool that growers can use, and many use herbicides in concert with the other tools.

Farmers generally apply several types of herbicides. First, the farmer uses a pre-emergent herbicide (applied before the crop emerges from the soil) to kill weed seeds. However, because the farmer has to be careful not to kill the planted seed, they must apply pre-emergents that won’t kill the type of crop that has been planted. A broadleaf pre-emergent would not be used on a field of spinach, allowing broadleaf weeds such as pigweed to grow.

Once the crop has emerged, a grower may choose to apply a post-emergent herbicide. Glyphosate (Round Up) is only one example of such a herbicide. Unfortunately, the weeds that really need to be killed are the ones that are growing right next to the crop. In that case, it’s nearly impossible to spray a weed without spraying the crop. So Monsanto gave growers another tool to use: glyphosate-resistant, or Roundup Ready, crops. Roundup Ready crops have a gene taken from a bacterium that allows the plant to tolerate the lethal effects of glyphosate.

Even before the availability of Roundup Ready crops, growers preferred using glyphosate to many of the other herbicides. Glyphosate inhibits an enzyme that only plants have, so it is less toxic to use than some of the other herbicides. It also stays within the plant and doesn’t persist in the soil. For those reasons, glyphosate has never been threatened with EPA cancellation, as methyl bromide was and atrazine is now threatened with. But the main reason why glyphosate remains so popular with growers and homeowners, aside from the fact it is effective, is that it’s really cheap.

Roundup Ready crops didn’t create superweeds. They created the ability to rely on one herbicide. Any time someone relies on one herbicide—or one pesticide of any kind—that practice is going to encourage pesticide-resistant pests.

And so they developed—glyphosate-resistant pigweed, horseweed, duckweed and others.

Despite the media-hype about superweeds, weed scientists aren’t ready to retire glyphosate. Even with the potential risks, Roundup Ready crops have increased the acres that are not tilled or tilled only lightly (a pest management technique called “conservation tillage”) according to a multi-state study in 2009. Conservation tillage reduces the number of weeds and other environmental hazards, such as erosion, pollution and water usage. In fact, just recently, a USDA Agricultural Research Service study in Kansas and Colorado found that no-till makes soil more stable than plowed soil.

However, herbicide-resistant crops present challenges as well. Any tool in the pest management toolbox comes with its own risks, especially if used in isolation. IPM encourages the use of several tools together to manage pests rather than eliminate them. For example, a new project in Arkansas to fight glyphosate-resistant pigweed will combine hoeing with herbicide applications to prevent pigweed from producing seed. Pest management practices used together are more effective than one practice used by itself.

One Response

  1. […] Times article from Sunday, May 16, 2010, describes the herbicide resistance scenario I discussed in last Friday’s blog and discusses how herbicide overuse has contributed to glyphosate-resistant weeds. As he states in […]

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