Over a decade ago, a biotechnology company designed corn plants that poison certain pests in an effort to lower use of pesticide sprays. 

It worked for a while, but cost-cutting farmers haven't heeded scientists' warnings about overusing the genetically modified plants. 

Today, the bugs are back. And they're getting tougher. 

“The western corn rootworm has repeatedly demonstrated its ability to adapt to pest management strategies,” reads a March 17 report from a team of scientists led by Aaron Gassmann, an entomologist at Iowa State University. 

The report found a “significant correlation” between the number of years the modified corn was used, and the survival rate of the worms.  

“The evolution of resistance by the western corn rootworm could cut short the benefits of Bt maize,” the report reads.

Last year, revenues from U.S. corn crops topped $60 billion, according to the USDA. But a major chunk of that could be at risk, if this pest gets its way. 

More than 40 million tons of corn never reach market because of these worms, according to Syngenta AG (NYSE: SYT) the biochemical company that created the new strain of bug-resistant corn.  

The adult rootworms lay eggs on the surface of corn plants, which allows larvae to tunnel inside, eating as they go.

“Once corn borers enter the corn stalk to attack the plant from the inside, conventional treatments are not always effective, or even possible,” states a report from Syngenta.  

After that, it’s only a matter of time before the hollow stalk is blown over by a gust of wind or a heavy rainstorm.

The company’s scientists infused ordinary corn plants with a gene called Cry1Ab. It makes the plant produce a protein called Bt through its leaves, stalks and ears, which is deadly for the insects.

The new plant was commercialized in 1996, and worked well for a while.

But the worm would not be easily defeated.    

Scientists have been warning for more than a decade that the modified crops must be used sparingly. It’s important for farmers to keep non-modified sections of their land, to ensure the toxin-exposed bugs breed with the ordinary ones, to stop evolution of resistance.  

Agricultural scientists suggested that farmers keep 50 percent of their crops unmodified. The Environmental Protection Agency has decreed that only 10 to 20 percent of a crop should be non-modified.

But even that hasn’t happened.  

“Sometimes convincing growers to plant non-Bt maize refuges is a challenges because it requires careful planning of where to plant the refuge and could reduce yields,” reads a 2012 report from Nature Education.

Farmers loved the new, modified plants. By 2009, more than 45 percent of American maize crops were Bt corn, and more than 58 million hectares were planted worldwide by 2010.

But in 2011, damaged crops were reported in Illinois, Iowa, Minnesota, Nebraska and South Dakota -- all separate cases in which the bugs evolved on their own.

A year later, 22 scientists wrote to the EPA, urging more oversight of Bt maize usage amid more and more reports of “greater than expected damage” to the protected corn. 

“We are concerned that high commodity prices and other factors may have fueled an insurance-based approach to corn rootworm management,” the scientists wrote, adding that this “will only increase insect resistance development in the long term.”  

And today, the problem is only getting worse and farmers may have to use older methods to protect themselves. “These recent cases suggest a need to develop more integrated management solutions for pests targeted by Bt crops,” states Monday’s report.

It’s suggested that farmers sill use older methods such as crop rotation, along with the genetically modified seeds, to stem the pest outbreak.

“Sole reliance on Bt crops for management of agriculture pests will likely hasten the evolution of resistance in some cases, thereby diminishing the benefits that these crops provide,” the writers add.