The Bt 'pest' refuge that isn't

December 2014

Early scientific wisdom on crops genetically transformed to produce their own insecticides, such as 'Bt' proteins, highlighted a number of fundamental weaknesses.

First, the ubiquitous presence of a single insecticide in a monoculture will result in the rapid evolution of resistant pests.

Second, the presence of a single insecticide throughout the growing season will result in the rapid evolution of resistant pests.

Third, the presence of a single insecticide which isn't at a high enough concentration to kill will result in the rapid evolution of resistant pests.

'Bt' crops are designed to fulfil the first two weaknesses, and, being living organisms, will be variable and have a natural tendency to fulfil the third depending on the circumstances.

Note. Contrast the GM Bt strategy with conventional spraying of pesticides which, if used judiciously, can be applied in a targetted, time-limited and varied manner.

To get round biotech demands to convert US agriculture to GM despite its obvious limitations, farmers are obliged to plant a non-Bt refuge for the pests. The idea is that the refuge will maintain a supply of wild-type pests to breed with any Bt-resistant pests emerging in the Bt crop, so that any newly evolving resistance genes will be diluted and take longer to become a problem.

Scientists, however, recommended that a 50:50 planting of Bt to non-Bt plants would be necessary to stave off evolution. This wasn't attractive to farmers or the biotech industry, and the requirement was watered-down to a 20% refuge.

When even 20% proved onerous, and the biotech industry responded by creating crops containing multiple varieties of Bt genes, persuading the regulators that a tiny 5% non-Bt refuge would now suffice, and selling farmers Bt seed pre-mixed with 5% refuge seeds. Such refuge-in-a-bag Bt maize has been available since 2010.

Two years after commercialisation, an experiment was carried out to see if refuge-in-a-bag actually works.

It doesn't. At least, not in the case of pests which attack the ears of maize.

Since the refuge plants don't produce 'Bt' and much of the plant material which forms the ears is parent material, only those seeds which have been cross-pollinated by GM plants will produce any insecticide. Refuge-in-a-bag is based on the assumption that the hybrid seed arising would be insufficient in number, in the variety of Bt toxins they had inherited, and in the amounts of toxin produced to cause a problem.

These assumptions were wrong.

The experiment showed that Bt/non-Bt hybrid kernels on refuge plants generate sufficient Bt toxin to harm ear-feeding pests, but not to kill them. A perfect scenario to produce Bt-resistant pests at maximum speed.

Refuge-in-a-bag makes this pest problem worse, faster.


Refuge-in-a-bag GM maize is being sold to farmers all over the world. It's shocking that a simple experiment to check whether it works wasn't carried out until after commercialisation.

Note that, most meaningful safety testing of GM foods, also, only started years after we were expected to eat it. See

Note also how easily regulators are persuaded to base the rules on biotech-industry-led assumptions in lieu of science.

  • Fei Yang, et al., 2014, a Challenge for the Seed Mixture Refuge Strategy in Bt Maize: Impact of Cross-Pollination on an Ear-Feeding Pest, Corn Earworm, PLOS ONE, 9:11, November 2014

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