Scientists have learned that initially rare genetic mutations that grant resistance to Bt toxins are becoming more common as a growing number of pests adapt to Bt crops. Bruce Tabashnik, head of the department of entomology at the University of Arizona, who co-authored a study on genetic mutations, considers the findings an early warning to farmers, regulatory agencies and the biotech industry.
Biotechnology giant Monsanto admitted that there is evidence that genetic mutations for resistance commonly exist in most pest populations, but at very low levels that will remain low if properly managed. “The presence of resistance mutations is not surprising and they are believed to always exist at low levels in any population due to random mutations. However, there have been no reports of failures of biotech cotton crops grown in China,” said Willie Wessels, regulatory affairs manager at Monsanto.
He added that Monsanto is not aware of any specific mutations which have arisen in South African populations of cotton bollworm, but it is reasonable to assume that some exist at very low levels, as this is the normal situation. Wessels explained that most mutations to Bt crops involve changes in Bt receptors in the insect gut which mediate the toxic effects. “Fortunately there are multiple types of Bt proteins with different specific receptor targets in pests so we can continuously improve and bolster efficacy and resistance management.”
Farmers have been encouraged to plant refuge areas with non-Bt crops to maintain populations of susceptible insects. These susceptible insects then mate with rare resistant ones that may survive in a biotech crop. This way the resistant mutations are continually diluted in the population.
But Tabashnik said the dominant mutations discovered in China throw a wrench in the refuge strategy because resistant offspring arise from mating between susceptible and resistant insects. Wessels countered that biotech crops have been grown on millions of hectares in SA since 1996 with only one documented resistance issue, namely Busseola in maize.
What are Bt toxins?
To decrease sprays of broad-spectrum insecticides, cotton and maize have been genetically engineered to produce toxins derived from the bacterium Bacillus thuringiensis, or Bt. Bt toxins are environmentally friendly and kill certain insect pests, but are harmless to most other creatures, including people.
