Delaying Resistance

Refuge

The use of refuge in managing Bt resistance is an important concept and one that is essential to effective resistance management. When an entire field is planted with Bt crops, all insects are exposed to the toxin. Some of these insects (usually a very small percentage of the population) naturally have the genetics to be resistance to Bt toxins. When an entire field is planted to Bt corn, there is a greater chance that the targeted insects (corn rootworms in this example) have the genetic predisposition to survive on the Bt crop. Their offspring may also have this genetic predisposition. The concept behind refuge is that non-Bt plants are planted in the field as well. These non-Bt corn plants allow a susceptible (those that have the genetic predisposition to die while eating a Bt toxin) to survive. The result is that a mix of resistant and susceptible corn rootworms (those hat fed and developed on non-Bt corn) will survive. The Bt toxin susceptible corn rootworms mate with Bt toxin resistance corn rootworms. They will then pass on their genes so that most of the offspring are still susceptible to the Bt toxin. An effectively designed refuge is key strategy to preventing the development of resistance in corn feeding insects.

By providing a refuge area more corn rootworms survive (usually a higher percentage of Bt susceptible, than Bt resistance gene carriers). Refuge provides a place for corn rootworms that carry Bt susceptible genes to develop, and as adults mate with both susceptible and Bt resistance corn rootworms. As the genes that cause resistance are recessive, the susceptible trait (killed by Bt toxins) tends to dominate corn rootworm populations. If there are far more corn rootworms that are susceptible and killed by particular Bt crops in a region, the corn trait (crop type) continues to be effective. Refuge is an essential tool in managing corn rootworm resistance.

In the past producers were charged with setting up a refuge area within their fields. As compliance to planting refuge varied by farmer and by farm site, many seed companies are moving to "refuge in a bag" where non-Bt seeds are generally mixed in with Bt seeds and planted all together.

Toxin Expression

The level of expression of the Bt protein is another significant factor for managing resistance. One approach to managing resistance is to have a high level of expression of the Bt protein in the plants. This produces a large quantity of the toxin to ensure that the corn rootworms are killed. This is analogous to making a concentrated chemical application. With higher mortality rates fewer corn rootworms survive to pass on their genes to the next generation. This is an effective tool in managing resistance, especially when combined with a well-designed refuge. A limitation to this approach is that the level of expression in the Bt proteins for corn rootworm is lower (thus a higher percentage of corn rootworms may survive), especially when compared to those used to control the European corn borer. So the same management strategies don’t work equally for all corn pests, with all Bt toxins events.

Agronomic Practices

Agronomic practices play an important role in the management of corn rootworm resistance as well. They are the most important ones to the producer since they are the factors that are actually under their control. Good agronomic practices to control corn rootworms will go a long way in delaying the development of resistance. Simple practices such as control of volunteer corn to crop rotation can be used as effective tools in resistance management.

Extended Learning