The Problems Driving Resistance to Bt Crops—and Some Proposed Solutions
By Dominic Reisig, Ph.D.
Bt crops—those genetically engineered to produce an insecticidal toxin from the bacterium Bacillus thuringiensis—are special due to their benefits: reducing foliar insecticide applications, which increase populations of beneficial insects and minimize environmental harm; reducing pest populations throughout the landscape; and preserving yield, to name a few. Therefore, preventing resistance to Bt crops is important and is usually formalized in a set of Insecticide Resistance Management (IRM) practices. Because bollworm (Helicoverpa zea,also commonly known as corn earworm and tomato fruitworm) is now resistant to two Bt toxin families (Cry1A and Cry2A) in cotton, IRM practices may have to change to slow resistance to other Bt toxins.
Although bollworm field resistance to single-toxin Bt has been known for many years, cotton farmers are now experiencing problems in the field due to resistance, spraying more insecticides forbollworm in two-Bt-toxin cotton. However, the implications of this resistance for IRM are bigger than just an increase in foliar insecticide sprays. In a forum article published in Environmental Entomology this month, cotton industry researcher Ryan Kurtz and I sought to detail many of these implications for the U.S. production system from our field-based perspective, listing current challenges and providing suggestions to meet those challenges.
We documented several challenges to insecticide resistance management for bollworm in Bt crops:
1. Availability of non-Bt refuge hosts. One of the ways that resistance is delayed is by allowing bollworm to reproduce in non-Bt crops. This reduces selection pressure on the population by allowing non-selected (and hopefully still Bt-susceptible) bollworms to mate with any individuals that survived in Bt crops (that may have been selected for resistance). Corn growers in cotton-growing regions of the U.S. are required to plant 20 percent of their total corn acres to non-Bt corn to assist in this effort. Corn is a very important host crop for bollworm, allowing insects to survive and reproduce. Nevertheless, bollworm rarely causes yield loss in timely planted corn. Therefore, growers that plant non-Bt corn in this region could help delay bollworm resistance to Bt without sacrificing yield.
Previous surveys, however, have indicated that most corn growers in these areas do not plant non-Bt corn refuge. In our review, we accessed corn seed catalogs from three major seed companies that were used to market hybrids to growers. We found that non-Bt hybrids listed in these catalogs declined 1-5 percent annually. Two out of the three companies had zero non-Bt hybrids listed in their 2018 catalog. Additionally, there were no non-Bt corn hybrid entries from 2013 to 2017 in the North Carolina corn grain Official Variety Testing program, a program intended to provide unbiased results to assist grower selection of crop varieties. Therefore, we concluded that the lack of marketing and availability of non-Bt corn seed in this region is contributing to non-Bt refuge compliance.
2. Pyramiding toxins. Another way that resistance is delayed is by using multiple Bt toxins in the same plant. New corn hybrids and cotton varieties are being introduced that contain three Bt toxins. The problem is that bollworm is now resistant to two out of three of those toxins (Cry1A and Cry2A). This exerts enormous selection pressure on the single toxin (Vip3Aa) that bollworm is not resistant to, leaving these pyramids to stand on a single point, rather than three.
3. High dose Bt toxins. Resistance can also be delayed using toxins that are high dose. This allows fewer chances for individuals to survive on sublethal doses and to pass resistance genes to the next generation. We documented several field studies that indicate that bollworm is likely to survive on both corn and cotton that produces Vip3Aa, indicating that this toxin may not be a high-dose toxin.
We list several ways that IRM can be improved and challenges can be met:
1. Collect data on current resistance in the field to inform future resistance models. IRM programs rely, in part, on computer simulation models that predict how quickly resistance may occur in different scenarios. Since Bt crops have been commercialized for over 20 years, there are ample studies that have linked model performance to field outcomes. We suggest that collecting field-based information currently unknown for bollworm, such as fitness costs due to resistance, and the dominance and frequency of resistance genes, would allow future models to be more predictive. This would allow more informed future action to delay resistance to other toxins, such as Vip3Aa.
2. Adjust the way that Bt toxins and non-Bt refuge are deployed in the field. Resistance should be delayed as non-Bt refuge is increased. This can be accomplished by many means. Two of these are by blending refuge (mixing non-Bt plants and Bt plants in the same field) and by increasing the amount of non-Bt available (by increasing the proportion of non-Bt plants). While many studies have demonstrated that blended refuge can produce almost as many bollworm individuals as a structured refuge (a refuge that is pure non-Bt), there is some evidence that individuals produced in a blended refuge could carry more resistance alleles than those from a structured refuge and hasten resistance.
As an alternative, can we increase the proportion of structured non-Bt refuge? We detailed earlier that many growers are not planting non-Bt refuge corn. One reason is because insecticide susceptibility is a common-pool resource. In order for non-Bt corn to be an effective resistance management strategy, most growers have to participate. We highlight experimental evidence from the social sciences that suggest that community-based regional programs can be an effective mechanism to manage common pool resources. Therefore, it may be possible for corn growers, the seed industry, and regulators to work together to establish a new strategy to incentivize non-Bt refuge compliance.
Another potential solution could be to restrict effective Bt toxins, such as Vip3Aa, in certain crops. In the case of bollworm, which rarely causes yield loss in timely planted corn but can cause large yield losses in cotton, Vip3Aa could be restricted from corn in the cotton growing regions. The use of Vip3Aa could be permitted in corn in other regions of the U.S., such as the Great Lakes region of the U.S. Corn Belt, where it effectively controls a major pest in that region, western bean cutworm (Striacosta albicosta).
3. Incentivize corn growers to plant non-Bt structured refuge. We provide experimental evidence that growers can be incentivized to plant non-Bt corn refuge using moral suasion. We also cite evidence from the social sciences suggesting that extension personnel should emphasize communicator credibility, trust, and membership in the community to increase non-Bt refuge among growers.
4. Offer corn growers hybrids with high-yielding genetic potential at a reasonable price. Growers must be offered non-Bt corn hybrids that are not cost prohibitive and have the same genetic yield potential as Bt hybrids for non-Bt refuge compliance to increase.
Dominic Reisig, Ph.D., is an associate professor and extension specialist of entomology at North Carolina State University. Email: firstname.lastname@example.org.