Western corn rootworm is toppling GM corn in America

November 2011

Western corn rootworm
Picture from Wikimedia Commons
The fact that western corn rootworm is toppling GM corn in America is bad news because the corn has had a 'Bt'-toxin gene inserted which is supposed to kill this major pest.

Modern corn crops, which cover tens of millions of acres of the American mid-west, look very different from those of a generation back. Now, American corn grows “head high and bristling. The stalks stand should to shoulder like an army without rank, their sharp-edged, sword-like leaves forming a nearly impenetrable wall. A modern corn field would have rebuffed Cary Grant in 'North by Northwest' because there are no rows to speak of, only a dense lattice of intersecting spears.”

There's not much scope in these fields for weeds, wildlife, or old-fashioned agricultural chemical treatment, never mind film-stars.

Below-ground, corn of that height and weight needs strong healthy roots to hold it upright. Western corn rootworm, a beetle whose larval form eats away the corn roots, is the No.1 enemy of many farmers: the weakened plants topple over when the wind blows, and the yield is decimated.

Chemical pesticides against rootworm have always been limited because once the grubs are underground they're well-concealed and protected. Add to this that the pest has “amazed scientists” by its ability to evolve resistance to chemicals. The answer conjured up by Monsanto was, of course, GM corn which generates its own insecticidal protein to kill any rootworms munching its roots. The first such GM corn hit the seed market in 2003 and was genetically transformed to produced a protein, 'Cry3Bb1', based on toxins from the bacterium Bacillus thuriengensis ('Bt'). This corn now represents 40% of Monsanto's sales.

Early on, US regulators recognised that planting huge areas of a plant generating a prolonged and continuous presence of the same insecticide would encourage resistant pests to emerge. It was also known before the Cry3Bb1-generating corn was even released that the level of toxin produced was not high enough to kill any more than half the larvae on the plant; such non-lethal exposure to a toxin is certain to stimulate resistance most efficiently. Since then, scientists have proved that it is quite possible to breed Bt-resistant strains of insects in the laboratory in as little as three generations. US law therefore demands that farmers plant 'refuges' of non-GM corn close by their GM crops. The idea behind this is that a resistant rootworm will have evolved paired resistance genes, both of which must be present for adequate protection of the pest. Since the insects living in the refuge will have no reason to evolve resistance genes, their lack will dilute any unwanted genes when individuals from the GM and non-GM areas mate.

Early on, there was considerable disagreement about the size of the refuge necessary to limit the emergence of resistance. Scientists said it would have to be 50% of the entire crop. A refuge this size, entailing more work for farmers and lower productivity, would clearly have reduced the GM corn's attraction for farmers and profitability for the seed suppliers. Accordingly, regulators caved in to industry demands and set the required refuge at 20% for the first Cry3Bb1-toxin containing corn. Since then, new GM varieties which have more than one toxin stacked in them are allowed to be grown with only 5% refuge. The sad truth is that only half of corn farmers are actually adhering to these refuge specifications. Add to this that farmers have been encouraged by regulators to spray the refuge with insecticides, negating any potential for the delay of resistance because dead beetles don't reproduce.

At the end of the day, the refuge strategy to delay insect resistance to Bt was never subject to long-term studies and its effectiveness has never been more than hypothetical even when correctly implemented.

Little wonder that Cry3Bb1-resistant western rootworm are now toppling corn in America.

The first field-study of rootworm damage in Bt corn backed up by laboratory studies of beetles collected from the site has shown that, in Iowa, Bt-resistance has become a reality in only 6-8 years. Several States across the US are experiencing similar problems. Such reports have been growing in number for several years, and as the re-emergence of the pest is publicised, more farmers are coming forward with evidence of crop damage.

One agricultural entomologist described it as “very, very significant damage”.

Besides the failure of the refuge system due to non-compliance, the problem has been helped along by poor farming practice: farmers have not been rotating their corn crops with other plants to break the life-cycle of the pests in their fields, but instead relying on the Bt-toxin to deal with them. The bad habit of growing nothing but Bt corn year-on year has been encouraged by other factors: increasing corn prices, the boom in demand for corn-based ethanol and other biofuels, plus, indirectly, by the absence of GM-food labelling which has encouraged Bt-corn into the lucrative food processing industry. Also, the assumption that paired resistance genes would have to evolve to render the beetles immune to the Bt crop seems to be wrong: if only a single, 'dominant', gene is sufficient to confer resistance, it can't be diluted by mating, and the refuge strategy is worthless.

Watching its share price tumble as the news broke, Monsanto was quick to trivialise the finding. According to spokesmen: the rootworm issue is a fairly limited problem confined to hotspots of a few thousand acres, only two GM varieties of corn are affected, its monitoring hasn't found resistance, the product is performing well on more than 99% of acres planted (Comment.). In explanation, the company suggests that 'performance enquiries' have been limited to areas where there's a high level of insect pressure which overwhelm the plants' defenses. It was soon pointed out that a similar, PR-based, explanation was immediately offered when weed-resistance to Roundup herbicide was first noted in a GM herbicide-tolerant crop. Moreover, the science as reported by one university entomologist is that there's no correlation between the level of pest numbers and crop damage. At one point the company said “It appears (the study) has demonstrated a difference in survival in the lab, but it is too early to tell whether there are implications for growers in the field” (Comment. Perhaps its PR department was getting confused because the new study is a field study).

And guess what? Monsanto is using the (non-existent!) emergence of resistance to plug its latest GM corn stacked with two rootworm toxin genes.

The efficacy of stacked GM corn may be a forlorn hope because the mechanism of insect resistance to Bt is not well understood. One researcher, who had tried to develop a Bt variant designed to bypass the gut binding mechanism which leads to insect death after ingestion of the older Bt-toxins, unexpectedly found the technique ineffective. He concluded “insects can probably adapt to modified Bt toxins used alone, or in combinations with other toxins”. Indeed, broad-spectrum resistance to Bt toxins has been documented in laboratory studies, albeit on rare occasions.


A lot of niggling questions are raised here.

The observed level of damage isn't correlating well with the observed level of pests. Rootworms must actually eat a bit of the root before they will be poisoned and die. Is chronic, low-level, root damage encouraging secondary infections and weakening plants even there's no obvious sign of pest pressure?

Unlike agrochemicals whose application level can be controlled and targeted, crop-generated levels of Bt toxins are dependent on both the health, age and part of the plant, and on many environmental factors. Low levels of the toxin are known to encourage pesticide resistance.

Is the extra, unnatural, protein produced by the GM plants acting as a food to encourage increased feeding by resistant pests? This could result in a much faster increase in the Bt-resistant population than would happen by chance.

None of these problems will be addressed by the planting of refuges (the regulators' tactic), nor by stacking up toxin genes (the biotech industry's tactic).

It's noteworthy that much the same story has already been told for GM cotton and the troublesome bollworm pest (see NOVEL AND EVERYWHERE – GMFS News Archive, September 2007)

The regulators should perhaps be insisting on good old-fashioned crop rotation as a much bigger priority than demanding refuges, whether the industry likes it or not.


  • Aaron J. Gassmann et al., 2011, Field-Evolved Resistance to Bt Maize by Western corn Rootworm, PloS ONE July 6:7
  • Mark Steil, Corn pest worries region's farmers; rootworm adapts to resistant plant, Minnesota Public Radio News, 4.09.11
  • Mark Steil, Pest resurgence casts doubts on benefits of modified corn, Minnesota Public Radio News, 20.09.11
  • Scott Kilman, Monsanto Corn Plant Losing Bug Resistance, Wall Street Journal, 29.08.11
  • Mike Gray. Severe root damage to Bt corn confirmed in northwestern Illinois,, 24.08.11
  • Jack Kaskey, Monsanto Corn Falls to Illinois Bugs as Investigation Widens, Bloomberg, 2.09.11
  • Georgina Gustin, Monsanto biotech corn not killing pests, research finds, St Louis Post Dispatch, 2.09.11
  • Georgina Gustin, More Monsanto corn is hit by pest, St Louis Post Dispatch, 24.09.11
  • Rady Ananda, Monsanto GM Corn in Peril: Beetle develops Bt-resistance, People's Voice, 24.08.11
  • Verlyn Klinkenborg, Children of the Corn, The New York Times Style Magazine, 21.09.11
  • Dr. Eva Sirinathsinghji, Bt Resistant Rootworm Spreads, Institute of Science in Society Report 31.10.11,

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