Corn plants, genetically modified by Monsanto to be resistant to corn rootworms, which are now grown widely in the USA have been discovered to be falling victim to the pests, which have developed a tolerance to the ‘natural pesticide’ inside the plant. image expired

The BT resistant corn accounts for around a third of the US crop. Iowa State University entomologist Aaron Gassmann, who discovered the resistant western corn rootworms in four northeast Iowa fields, warned the problem could result in farmers ramping up efforts to control the pests and using older, more harmful chemicals on their crops.

Dr Gassmann said, "These are isolated cases, and it isn't clear how widespread the problem will become, but it is an early warning that management practices need to change."

This latest discovery has lent more weight to sceptics who point out that growers of other hitherto-resistant crops are now encountering similar problems. ‘Roundup Ready’ crops, which were introduced in the US in the 1990s and are resistant to Monsanto’s Roundup herbicide (glyphosate) are now blighted by ‘superweeds,’ which have also developed to become Roundup Ready, and have spread over millions of acres in more than 20 US states.

Brett Begemann, executive vice president of Monsanto, whose stock fell on Monday, after The Wall Street Journal reported on the implications of Dr Gasmann’s discovery, denied the revelation was a problem.

Begemann said, "We've known about this for years. The key is that it has not been getting worse.” He recommended corn farmers use a newer seed developed by Monsanto that contains two root-worm resistant toxins, one of which is the Cry3Bb1 protein of Dr Gassmann’s study.