The most potent corn pesticides are losing effectiveness, thanks to overuse and heavy reliance on GM corn.

So-called "superweeds" are taking over American farmland. Impervious to chemical herbicides, like Roundup (the most widely used herbicide in the U.S.), these weeds don't die when exposed to chemicals. Instead, they just grow taller and thicker and become so sturdy, they've been known to destroy farm equipment.

As farmers struggle to cope with these monster weeds, they turn to stronger, more potent pesticides, but even those are starting to lose effectiveness.

A new study in the journal Weed Science finds that weeds are becoming resistant to 2,4-D, just as the U.S. Department of Agriculture is poised to approve a new crop of genetically modified (GM) seeds that have been designed to resist 2,4-D. The toxic herbicide made up roughly 50 percent of the Vietnam-era defoliant Agent Orange, and in addition to potentially being contaminated with cancer-causing dioxin, it's been linked to non-Hodgkin's lymphoma. Farmers who are regularly exposed to the chemical are also more likely to have children with birth defects.

The scientists looked at a weed called waterhemp, which grows in corn-belt states like Nebraska and Illinois. After being exposed to 2,4-D regularly for 10 years, the weed no longer died after applications of the chemical, the researchers found. Even the heaviest dousings of 2,4-D were insufficient to kill 50 percent of the weeds in the researchers' test field.

The scientists even collected a handful of waterhemp seeds and grew the weed in a greenhouse. They exposed the seeds to varying levels of 2,4-D and, after 28 days, saw that herbicide resistance had increased tenfold over that time period.

Dow Agro Sciences is currently seeking approval for its 2,4-D–resistant, GM corn called "Enlist." Currently, 27 million pounds of 2,4-D are applied to non-GM corn, but the nonprofit Center for Food Safety estimates that 2,4-D-resistant corn would nearly quadruple that number, leading to roughly 100 million pounds of the chemical being used every year, leading to more resistant weeds.

Ironically, biotech companies sell these seeds under the premise that they will allow farmers to use fewer pesticides. But this is the second pesticide that seems doomed to fail, if past experiences are to be believed. When Monsanto introduced its "Roundup Ready" corn and soy, genetically modified to resist the active ingredient in Roundup (also manufactured by Monsanto), 4.4 million pounds of the pesticide were applied to U.S. farm fields every year. By 2010, that number had risen to 57 million pounds. In that same time period, the varieties of weeds resistant to Roundup grew from just one to more than 24, all of which now cover over 60 million acres of U.S. farmland.

Protect yourself from these toxic herbicides that don't even work by…

• Demanding organic. Organic farmers don't use GM seeds, or the toxic herbicides they're designed to resist. Instead, they use commonsense weed-control methods that are healthier for farmers and for you.

• Voting. Voters in California have the chance to deal a major blow to biotech firms this November, when Proposition 37 appears on their election ballots. The measure would require any food that contains ingredients made from GM corn, soy, cotton, or canola to disclose that on the label. Even though a "USDA Organic" label says essentially the same thing, such a label would allow people who can't afford or find organic foods to avoid GM ingredients—and the pesticides they're soaked in. And because California is such a huge state, laws that pass there typically spread across the country.