Crops engineered to carry the insecticidal toxins of the bacterium Bacillus thuringiensis (Bt) have produced “some spectacular successes and disappointing failures” in their first 25 years of use, according to a global review.
A team led by Professor Bruce E. Tabashnik of the University of Arizona analysed 73 cases of continuous growing of Bt crops between the years 1995 and 2020. They found resistance to Bt toxins documented in 26, with 11 pest species resistant to nine different toxins, and in seven countries.
In 30 cases, there was no change in the susceptibility, while in the remaining 17 there were genetic signs of resistance that had not manifested in real-world decreases in efficacy.
Bt boom brings different field responses
The area of land planted with Bt crops rose from 1 million to 109 million hectares between 1996 and 2019, with them mostly used against challenges from moth and beetle pests. Bt corn and cotton are the most common crops, dominating the market among farmers in superpowers such as the US and China, but others such as soybean, potato, tobacco and aubergine are also authorised for use.
The crops’ huge market growth is testament in part to their success in reducing damage from their most serious pests. The big wins seen over the quarter-decade have included the sustained effectiveness of Bt cotton against the pink bollworm (Pectinophora gossypiella) and tobacco budworm (Chloridea virescens), and a similar lack of resistance in the European corn borer (Ostrinia nubilalis) to Bt corn, wrote Tabashnik in Journal of Economic Entomology.
However, the study also highlighted a sharp increase in cases of pest resistance since 2005, with three recorded cases rising to the 26 seen by 2020.
New toxins, IPM approaches needed
Using multiple Bt toxins in crops rather than a single one has helped slow down the development of resistance in a number of cases, Tabashnik noted, but more novel approaches will be needed to tackle those pests which have proved particularly prone to evolving tolerance.
“Although it will be difficult to implement new transgenic insecticidal crops that have safety and initial efficacy comparable to the most successful Bt crops deployed during the past quarter century, the potential rewards are great and prospects are emerging,” he said.
Such innovations might include combining RNA interference, a form of gene-silencing, with Bt toxins, or else incorporating non-Bt toxins that have shown good performance against troublesome pest insects, such as those derived from some soil bacteria and ferns.
Regardless of the toxins used or other possible molecular changes to Bt crops, the findings underlined the importance of integrated pest management approaches, he stressed. In particular, providing non-Bt crop refuges for pests near to where the transgenic crops are grown has shown significant value in delaying the development of resistance.
“As we move forward to the next generation of transgenic insecticidal crops, the challenge is to implement what has been learned to enhance their sustainability for the benefit of all,” he added.
