Sentinel Plots: A New Way to Monitor Earworm Resistance to Bt Toxins

written by: Darsy Smith & Veronica Yurchak
 
Dr. Galen Dively, a Professor Emeritus in the Department of Entomology at the University of Maryland, has become a leading figure in the effort to monitor and mitigate resistance development to genetically modified Bt corn, primarily addressing the very destructive corn earworm. At this week’s colloquium, Dr. Dively presented on his work designing a new approach for monitoring insect resistance in Bt corn, as well as how this approach is changing the way genetically modified crop technologies are regulated by the EPA.  

​Bt toxins are insecticidal crystalline (Cry) and vegetative (Vip) proteins produced naturally by the soil bacterium Bacillus thuringiensis. Pesticide sprays containing the Cry proteins are known as Bt pesticides and have been used for pest control in organic agriculture and ornamental landscapes for nearly 90 years. However, the use of Bt sprays in conventional agriculture has been limited due to inconsistent field performance caused by its tendency to break down quickly in UV light. As a solution, scientists and seed companies created genetically modified (GM) plants capable of expressing these insecticidal proteins within the tissue of the growing crop plant. Since their initial introduction in 1996, Bt crops have become one of the most rapidly adopted agricultural technologies, with the most common Bt crops being corn and cotton. The popularity of Bt corn in conventional agriculture is due to the fact that plants are protected from damage caused by several major corn pests including the corn rootworm, European corn borer, the corn earworm, the fall armyworm, and western bean cutworm from seedling emergence all the way through harvest. 
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​As is common with any pesticide product, insect resistance development is a major threat to the Bt crop technology. Scientists, regulators and industry personnel have been actively monitoring the development of resistance in a number of these target pests. Traditionally, resistance to and Bt corn is monitored using bioassays, or feeding trials where field collected insect pests are fed a diet containing the Cry proteins, and their development and survival is studied in a lab. Susceptible insects would be killed or made very sick, while those developing resistance to the Cry proteins would continue to grow and develop. After observing faults with this method, Dr. Dively decided there may be a more effective way to monitor for Bt resistance, particularly in caterpillar pests.

Figure 1. Sites within the sentinel plot monitoring network. Black dots indicate sites in locations where corn earworms overwinter, while white dots are locations where earworms must migrate to each season.

​Using paired sentinel plots of Bt sweet corn varieties and their corresponding non-Bt isolines, Dr. Dively is able to measure “practical resistance” by showing actual declines in the ability of these different varieties to control insect pests in the field. Dr. Dively has created a sentinel plot monitoring network spanning 23 US states and 3 Canadian provinces (Figure 1). By measuring the total damage per ear and other metrics, Dr. Dively has documented increasing resistance in the corn earworm population to a number of commonly expressed Cry proteins. Corn expressing the Cry1Ab protein, for example, contained around 81% less damage than non-Bt corn when first introduced. By 2020, however, corn expressing this protein contained only 6% less damage than the non-Bt isoline, indicating significant resistance development in the corn earworm population (Figure 2). Similar reductions in field efficacy were seen in other Cry proteins monitored as well. 
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Figure 2. Earworm damage in Bt corn expressing the Cry1Ab protein in 1996 (left) compared to damage to the non-Bt isoline (right).

​Based on these findings and the overall success of the sentinel plot monitoring network at detecting practical, field-evolved resistance, Dr. Dively and other researchers have recommended changes be made to the regulatory resistance monitoring process for Bt crops. Currently, for every new Bt variety introduced, an Insect Resistance Management (IRM) plan is required. These IRM plans have four basic components: 1) a high dose – refuge strategy in which the toxin expression is high enough to kill all target individuals except for the rare homozygous resistant ones, and where a certain percentage of non-Bt corn must also be planted in order to ensure the development of susceptible insects; 2) new varieties containing multiple proteins toxic to the target pest; 3) the development of resistance must be monitored; and 4) a mitigation action plan must be presented. As a direct result of Dr. Dively’s research, the EPA has proposed a number of changes to the IRM plans to slow the development of insect resistance by increasing the number of susceptible insects in the overall population as well as ensure only varieties expressing multiple Bt toxins are planted.
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​Due to the fact that Bt crop technology has become the foundation of corn and cotton pest management in the United States, it is extremely important ensure that the best resistance management practices are being implemented in order to extend the efficacy of this technology as long as possible. Dr. Dively’s research has identified a fantastic way to actively detect resistance development across the country and has spurred a number of proposed changes at the regulatory level, hopefully securing the effectiveness of these GM technologies for years to come.