Insects are getting faster at developing resistance to GM crops
Study finds a surge of insects fighting back against insecticidal crops, but says adding non-GM plants can delay the evolution of that resistance. Tim Wallace and Elizabeth Finkel report.
Insects are rapidly evolving resistance to crops genetically engineered to poison them.
That’s the conclusion from an analysis of global data covering the first two decades of the use of ‘Bt crops’ – plants genetically engineered to produce insecticidal proteins sourced from the bacterium Bacillus thuringiensis (Bt). The good news is that when farmers provided non-Bt plants near their Bt crops, a so-called refuge, the development of resistance was delayed.
“The primary lesson from the past two decades is that abundant refuges can delay pest resistance to transgenic crops,” conclude entomologists Bruce Tabashnik and Yves Carrière of the University of Arizona in Tucson. Their findings are published in the journal Nature Biotechnology.
Farmers have sprayed Bt on crops as an insecticide for decades. It kills caterpillar and beetle pests, and is safe and environmentally friendly – making it a common choice for organic farmers. Since 1996, farmers have also had access to plants genetically engineered to carry the instructions for making Bt proteins in their DNA. Not only does that reduce the labour and costs of spraying, it is far more effective for crops such as cotton, where caterpillars feeding inside cotton bolls can be impervious to sprays. In Australia, for instance, Bt cotton has reduced chemical pesticide use by 85%.
Globally in 2016, farmers planted more than 98 million hectares of Bt corn, cotton and soybeans.
From the beginning scientists expected insects to evolve resistance to Bt proteins, as they ultimately do to all insecticides. The recommended strategy to forestall it was to plant companion crops that did not produce the proteins. Insects with no resistance to Bt could multiply freely in these refuges; the idea being that these susceptible insects would mate with and dilute the rare resistant insects.
However, not all farmers complied with the recommendation to establish refuges. Indian farmers, for instance, were less likely to establish refuges than farmers in the southwestern United States; and even in the US the requirement for how large refuges need to be has been debated. Since 2007, the US Environmental Protection Agency has relaxed its requirements.
To investigate how refuges and other factors impacted the rate at which resistance evolved, Tabashnik and Carrière examined 36 case studies from 10 countries.
In the decade between 1996 and 2005, they found three cases of resistance. Over the next decade resistance surged, with another 13 cases occurring by 2016. On average, the rate at which insects developed resistance more than doubled in the second decade. It took eight years from the first introduction of Bt crops on a farm till resistance genes spread throughout an insect population. Between 2008 and 2014 that time shrank to an average of three years.
The main reason resistance evolved faster, says Tabashnik, is that even when new crops were introduced that carried novel toxic varieties of Bt, the insects could still muster some degree of cross-protection from their previous exposure.
However, he points out a silver lining "is that in 17 other cases, pests have not evolved resistance to Bt crops". The remaining three cases are classified as "early warning of resistance", where the resistance is not severe enough to have practical consequences.
The authors found that refuges have indeed helped to delay resistance. For example, resistance of pink bollworm to Bt cotton evolved after less than 10 years in India where refuges were scarce, but has not evolved after more than two decades in the US where refuges have been abundant.
"Same pest, same crop, same Bt proteins, but very different outcomes," says Tabashnik.
“We always expect the pests to adapt. However, if we can delay resistance from a few years to a few decades, that's a big win.”