A 15-year study has shown that maintaining nearby ‘refuges’ of non-genetically modified crops can help preserve the effectiveness of Bt cotton, the crop which is widely grown around the world to protect against a group of voracious moth pests.
Bt cotton is engineered to express the insecticidal proteins of the bacteria Bacillus thuringiensis. Its use can reduce the need for pesticide applications and improve profitability for farmers, and it now makes up most of the area planted with cotton worldwide. However, over the last two decades there has been a significant increase in cases of pests developing resistance to these novel crops.
In the major study carried out in northern China looking at the cotton bollworm, Helicoverpa armigera, researchers found that the frequency of a genetic mutation which brought about dominant resistance, known as T92C, surged 100-fold in populations of the insect between 2006 and 2016. However, this did not continue to rise in the period from 2016 to 2020.
Supporting susceptible pest populations
Refuges can be used as part of integrated pest management approaches. They work by providing habitat for pests that are susceptible to a toxin to reproduce and then mix with resistant individuals, reducing the prevalence of resistance overall.
The researchers noted that the percentage of non-Bt cotton and other non-Bt plants that can host the pest rose in the cotton-growing areas had grown from 66% to 85% between 2007 and 2019. Using statistical modelling, they calculated that the growth in these non-Bt refuges was sufficient to delay the evolution of resistance after 2016.
Previously, it had been thought that refuges of non-Bt crops were only effective against rare and recessive resistance genes in the moth. This is likely the first time that evidence has been presented of refuges’ potential to delay the development of dominant resistance.
Market forces push shift to refuge crops
The growth in potential refuge crops for H. armigera observed in the study was not by design; farmers in China had shifted to non-Bt corn and other crops for economic reasons, based on relatively high labour costs of cotton production, decreased market prices and government policies promoting corn cultivation.
The shift has been considerable, moving from a 3.7 ratio of corn to cotton in 2010 to 21 in 2019. Meanwhile, other research in China has confirmed that large percentages of the pest moth feed on crops other than cotton as larvae, likely contributing to the slowdown in resistance.
In the US, populations of the related pest Helicoverpa zea have developed resistance to all Bt toxins inserted into these genetically engineered crops. This is likely to be related to the fact that Bt crops makes up much of both cotton and corn production, leaving few refuge crops, the scientists said.
“The results…suggest that refuges of non-Bt host plant species other than the Bt crop are likely to be essential for achieving effective refuge percentages high enough to substantially delay evolution of non-recessive resistance to Bt crops in polyphagous pests,” the scientists wrote in the journal iScience.
“In particular, the potential for corn to delay evolution of resistance to Bt cotton by pests, such as H. armigera and H. zea has important implications for resistance management strategies in China, the United States, and elsewhere.”
