March 20, 2014
US study reveals pests'' resistance to GM corn''s ''deadly'' toxins
A team led by Aaron Grassmann, an entomologist at Iowa State University in Ames, has described the rise in some Iowa fields of a bug called the western corn rootworm (Diabrotica virgifera virgifera).
Genetically modified (GM) corn, which produces anti-pest toxin, is no longer as efficient at killing the bugs. The resistance arose quickly, due to some extent, to farmers avoiding the simple, but profit-cutting precaution of crop rotation.
The problem that may soon render null the economic and environmental benefits of GM corn, which produces the so-called Bacillus thurinigiensis (Bt) toxin, was highlighted in a study published online in this week''s Proceedings of the National Academy of Sciences.
The studied pest has developed resistance to two of three types of Bt toxins currently available on the market.
The vulnerability of GM corn could lead to disastrous consequences for farmers and the environment, Gassmann warned.
The US authorised use of Bt toxin-producing corn back in 2003. Since then the GM plants, which were believed not to require implementation of insecticides to combat rootworms and other pests, has dominated the market and currently accounts for three-quarters of the American corn crop.
However, the first cases of rootworm damage to GM plants were registered in 2009, with corn containing Cry3Bb1 Bt toxin being affected.
Two years later, the bugs started attacking the fields, at which the second toxin – mCry3A – was used by the farmers.
The lab tests performed by Gassmann and his team have revealed that a case of cross-resistance is in place as the pests that had become resistant to Cry3Bb1 were also resistant to mCry3A.
The most likely explanation to the phenomenon is that both toxins share structural similarities and some binding sites in the insect''s gut, the paper said. According to the research, the Bt corn simply doesn''t produce enough toxins to fully control the rootworms, which are known as really tough creatures.
Unlike the majority of other pests, which are almost completely defenceless against the toxins, over 2% of the rootworms can survive contact with Bt corn.
Resistance in the worms can develop very fast in fields where the same kind of corn is being grown every year. In Iowa, it showed up after an average of 3.6 years, the paper warns.
Agricultural biotechnology companies currently apply the so-called ''pyramiding'' technology to their seeds so that they can produce two different Bt toxins, with one of them being Cry34/35Ab1 that so far has not seen any resistance developed by the rootworms.
Gassmann confirmed that the pyramiding of toxins is an important way to delay the development of resistance, but added that the combination becomes less effective once resistance arises to one of the toxins.
So, while relying on technology, the farmers shouldn''t forget about such old-fashioned practices as crops rotation as it helps disrupt the pests life cycle.