The Western corn rootworm (Diabrotica virgifera virgifera) is often considered the most damaging pest to U.S. corn. While the adults can cause issues in pollination through clipping silks, the most destructive life stage is the larvae, who live below ground and feed on corn roots. Larval feeding injury can result in corn stunting and lodging, negatively impacting ear production and harvestability. In Illinois, this pest is a well-known issue in corn throughout the state, with fields planted with continuous corn experiencing the most pressure.
Bacillus thuringiensis (Bt) is a vital component of integrated pest management (IPM) in modern crop systems. In Illinois, Bt corn protects against multiple pests, including western corn rootworm (WCR) in corn fields. Unfortunately, the efficacy of Bt against WCR is beginning to diminish, threatening the future value of Bt technology and leaving growers with fewer effective options for WCR control. Currently, WCR populations in Illinois have developed low-level resistance to all commercially used Bt toxins targeting the beetle larva. Despite low-level Bt resistance, WCR populations are down across Illinois, allowing growers to reduce reliance on Bt toxins and implement alternative control strategies. Cycling through other integrated control strategies will allow Bt susceptible WCR to increase their percentage within the population, easing the rate of Bt resistance development (To learn more about the intricacies of insecticide resistance, check out this article from our last issue).
Planting refuge
Using non-Bt refuges is a crucial strategy for delaying Bt resistance in pest populations like WCR, but current refuge requirements have limitations in effectively preventing resistance. Bt corn requires a mandatory refuge of either 20% non-Bt corn in a structured refuge or 5% non-Bt corn in a mixed refuge or “refuge in a bag” when planting pyramided Bt corn varieties (50% non-Bt structured or 20% non-Bt mixed in single toxin Bt corn). Both refuge strategies present challenges for WCR resistance management. In a structured refuge, WCR adult emergence from Bt and non-Bt areas typically occurs asynchronously, and often adults do not travel between refuge and non-refuge areas, reducing the likelihood of susceptible and resistant beetles interbreeding. In mixed refuges, adults are likely to interbred but the 5% non-Bt is often considered insufficient to offset the development of resistance. Therefore, to effectively manage Bt resistance in WCR, growers must implement additional control strategies alongside current refuge requirements.
Bt pyramids
As stated above, the required refuge size differs depending on whether a hybrid produces one Bt toxin or is pyramided with multiple Bt toxins. The difference exists due to the ability of insects to become cross-resistant to multiple Bt toxins after repeated exposure to just a singular Bt toxin. Bt pyramids prevent cross-resistance as it is more difficult for an insect, such as WCR, to build up resistance to multiple toxins at the same time. Hybrid pyramids offer better control of the target pest than single trait hybrids and are considered more robust to resistance overall. Therefore, planting a hybrid with a Bt pyramid will reduce the resistance rate while increasing the control level. Since not all Bt toxins target WCR, reference materials like the Handy Bt Trait Table can help growers determine which traits and pyramids are most effective against corn rootworms so that the proper hybrids are selected.
Crop rotation
One of the most common resistance prevention tactics involves the concept of rotation. For WCR, a well-established cultural control tactic is rotating corn with soybean, as WCR larvae cannot survive by feeding on soybean roots. The variant WCR has adapted to lay eggs in soybean fields in anticipation of corn the following year. Another group of WCR has developed an extended diapause where eggs remain in the soil for an extra year to avoid a potential soybean crop. However, the densities of this variant and extended diapause populations are currently low and are unlikely to cause substantial pressure on corn planted following soybeans. Beyond being an effective cultural control method, corn-soybean rotations indirectly weaken resistant WCR populations, as rotating does not select susceptible individuals but lowers resistant populations naturally. Corn and soybean rotations are a well-known practice, especially in central Illinois, but in some regions, it may not always make the most economic sense. Still, the strategy remains valuable in an IPM program for managing WCR and slowing Bt resistance development.
Seed treatments
Insecticidal seed treatments act to control subterranean pests like WCR; however, combining insecticidal seed treatment with a Bt-hybrid provides limited additional benefits while increasing seed costs. Recent studies have shown that adding a seed treatment, such as a neonicotinoid, to a Bt-corn hybrid is unlikely to increase WCR control. Even the non-Bt plants perform sufficiently well in fields planted with a 5% mixed non-Bt refuge without an insecticidal seed treatment. When seed treatments are used on non-Bt-hybrids and rotated with Bt-hybrids without an insecticidal seed treatment, both control seed types can effectively control WCR larvae. Therefore, rotating Bt-hybrids with non-Bt-hybrids treated with a seed treatment across growing seasons could be an effective strategy for preserving both Bt toxin efficacy and the effectiveness of insecticidal seed treatments. This approach not only delays resistance development but also helps grower input costs by limiting seed prices by reducing the number of control technologies incorporated in the seed. By incorporating this approach into an IPM program, growers can help delay resistance development while maintaining reliable WCR management and profitability.
Final thoughts
Since its introduction in the 1990s, Bt technology has been one of corn growers' most important crop protection tools. Across the United States, Bt hybrids help prevent damage from various pests, including WCR, the country's most economically damaging pest of corn. Illinois is the second-largest corn producer in the U.S. and heavily relies on Bt toxins to protect yields against WCR infestations. However, if the current Bt toxins continue to decline in efficacy, it is unlikely to be replaced with a similar technology soon, as developers are running out of Bt toxins to incorporate and are struggling to develop alternatives. Preserving the efficacy of current Bt toxins is critical for maintaining corn production throughout the state.
This spring, as planting decisions are being made, please remember these key points in preventing Bt-resistance:
- Follow proper refuge requirements
- Plant pyramided Bt varieties
- Consider rotating crops if populations reach the economic threshold
- Rotate Bt-hybrids with non-Bt-hybrids with an insecticide seed treatment
References
Colgrove, A., Decker, A., Das, S., Estes, K., Ford, T., Machecha, E., Plewa, D., Preza Fontes, G., Seiter, N., & Spencer, J. 2024, February. 2023 applied research results field crop disease and insect management. https://extension.illinois.edu/sites/default/files/2024-01/2023_field_crop_applied_research.pdf
Environmental Protection Agency. 2024, December 29. Insect Resistance Management for Bt Plant-Incorporated Protectants. EPA. https://www.epa.gov/regulation-biotechnology-under-tsca-and-fifra/framework-delay-corn-rootworm-resistance#q1
McCulloch, John B, and Aaron J Gassmann. “Effects of combining soil-applied insecticide and BT corn for integrated pest management and resistance management of western corn rootworm (Coleoptera: Chrysomelidae).” Journal of Economic Entomology, vol. 117, no. 5, 10 July 2024, pp. 1884–1891, https://doi.org/10.1093/jee/toae149.
Picture: Scott Bauer, USDA Agricultural Research Service, Bugwood.org