Learning Objectives –
• Visualize how resistance can develop in populations through natural selection.
• Understand the correct use of refuge can manipulate the gene pool to delay resistance.
Introduction –
This activity is designed to provide a visual representation of how Bt resistance happens in corn rootworm populations in the field. This activity simulates the interaction between planted refuge areas and corn rootworm populations as well as the effect it has on corn rootworm populations. Although this activity is very simplified from what occurs in the corn rootworm system, it is meant to provide insight into the basics of concept of natural selection and its application to refuge.
Procedures –
1. Find 20 pieces of the same type of small household materials – we suggest low-value coins like pennies
2. Find another 20 items that can easily be distinguished from the first 20 such as a different colored coin or paper clips. These objects must be easily distinguishable from each other.
• The first 20 represent a proportion of the susceptible members of the population.
• The second 20 objects represent a proportion of the resistant members of the population.
3. Find a piece of copy paper (this paper represents the field).
Example 1 – (No Refuge)
4. Randomly throw or spread your objects on the paper.
5. All the field has Bt traited corn planted.
6. For each susceptible one that survive, add two more like it. (i.e. 10 unmarked, so add 20 unmarked). For each marked one that survived add one more like it. All of these together represent the next generation.
7. Repeat steps 4 through 6 two or three more times until the pattern or trend becomes apparent.
Example 2 – (10% Refuge)
Now the same exercise can be done again, but with refuge corn planted in blocks.
8. Randomly mark the paper with 1-inch squares to designate refuge and Bt corn plantings. If are using a copy sized piece of paper; nine 1 in2 squares represents about 10% refuge.
9. Randomly throw or spread your objects on the paper.
10. Any objects that land inside a marked area eat the refuge corn and survive. Those that landed outside of the marked area ate Bt corn and died, except those objects (rootworms) with resistant genes. These survive regardless of the corn type they eat.
11. For each susceptible one that survive, add two more like it. (i.e. 10 unmarked, so add 20 unmarked). For each marked one that survived add one more like it. All of these together represent the next generation.
12. Repeat steps 8 through 11 two or three more times until the pattern or trend becomes apparent.
Example 3 – (20% Refuge)
13. Randomly mark the paper with 1-inch squares to designate refuge and Bt corn plantings. If using a copy sized piece of paper; place eighteen 1 x 1 squares represents about 20% refuge.
14. Randomly throw or spread your objects on the paper.
15. Any objects that land inside a marked area eat the refuge corn and survive. Those that landed outside of the marked area ate Bt corn and died, except those objects (rootworms) with resistant genes. These survive regardless of the corn type they eat.
16. For each susceptible one that survive, add two more like it. (i.e. 10 unmarked, so add 20 unmarked). For each marked one that survived add one more like it. All of these together represent the next generation.
17. Repeat steps 13 through 16 two or three more times until the pattern or trend becomes apparent.
Example 4 – (Resistant rootworms from a neighbor)
18. Use the paper representing the field from the last example for this one.
19. Randomly throw or spread your objects on the paper. With each year or time it is done add 2 extra objects representing the resistant rootworms. These would represent others migrating in from a neighbors field who has not managed it properly.
20. Any objects that land inside a marked area eat the refuge corn and survive. Those that landed outside of the marked area ate Bt corn and died. Except those objects (rootworms) with resistant genes. These survive regardless of the corn type they eat.
21. For each susceptible one that survive, add two more like it. (i.e. 10 unmarked, so add 20 unmarked). For each marked one that survived add one more like it. All of these together represent the next generation.
22. Repeat steps 19 through 21 two or three more times until the pattern or trend becomes apparent.
Now try your own example
From the examples above you can create your own example to get an idea of what happens in different situations. It is important to remember that this represents a simplified version of what can occur in the field.
Questions for thought –
How would this be different if there were no refuge?
What would happen if those around your field are not managing populations properly?
What benefits does the refuge provide?
How would this change with a crop rotation?
How would using different Bt events change this?
Additional Examples and Discussion
Additional Scenario 1:
Imagine one of your fields that usually produces high yield with limited corn rootworm infestation. However, over-time it has become infested with rootorms that threatens to cause significant economic damage to the field. Many of the crops are killed and yield is reduced causing economic damage.
To prevent this from happening the following year the entire field is planted to a Bt traited corn genetically engineered to kill corn rootworm. This corn seed does an excellent job of reducing pest populations and protecting the crops. Minimal damage to the field is noticed and it seems like the problem has been solved.
However, a small population of corn rootworm survives despite the new trait in the crop. Though it was a small population and not enough survived to cause damage they are the only ones who will now pass on their genes and those are the genes that make it so they can survive on this crop.
The next year, there is evidence of some corn rootworm damage. The next year, even more evidence of damage including lodging. Thd following year, there is evidence that there is a serious problem indicative of inneffective corn rootworm control - significant economic damage effecting both quality and yield.
This is where the proper use of refuge can make a big difference.
In this example a randomized refuge or refuge in a bag shows how it can work. This time, the crop is infested as usual. However, most of the population is controlled for with the genetically engineered trait. However, a few survive to maintain susceptibility so that not all of the next generation will be resistant. By allowing them to survive and pass on their genes it is slowing the process of resistance development. It doesn’t completely reduce the Bt resistance corn rootworm populations to 0% but it is a technique for managing lower levels of resistant corn rootworm populations.
In the blocked refuge example, the same can be seen. By incorporating some type of refuge the resistant populations can be controlled to keep resistant populations at more manageable levels. This allows for both short-term and long-term considerations by lowering economic impact now, as well as prolonging the life of the technology being used.
Different types of refuge can be more appropriate in different situations. This will often depend on the biology of the pest, thus it is important to understand the biology of the pest in order to use refuge properly. Western Corn Rootworm Biology will be the topic of a future module.
Factors not considered in the activity
First of all, it is much more complex than this:
Assumptions –
• Simple recessive inheritance
This example fits well if the trait for resistance in the pest is what is referred to as simple recessive inheritance. This means that it is something that has to be passed on to offspring by both parents to show up or manifest itself in the offspring. A simple example of this is hair or eye color in humans. Blonde and blue eyes are both recessive, so if someone has blonde hair or blue eyes they had to get a gene for those traits from each parent. That is why they tend to be more rare.
Many studies have found that this is not likely the case in Western Corn Rootworm and it is much more complicated. Meaning a simplistic model such as this would not hold up as well, requiring more refuge as well as other control measures to adequately manage the Western Corn Rootworm.
• What about neighbors who don’t manage properly?
Another important consideration is that you may be taking the proper precautions, but your neighbor isn’t. If your neighbor isn’t taking the proper precautions, then resistant populations could move into your fields. Thus it is important to manage insects properly for your own sake as well as your neighbors.
• The Bt used for corn rootworm control is not a high-level expression event.
With Western Corn Rootworm, it is not a high-level expression event, so more of the less susceptible survive. One approach to managing resistance has been to have a high level of expression of the Bt protein in the plants or having the plant produce a large quantity of the toxin to ensure that the corn rootworms are killed. This would be analogous to making a concentrated chemical application. With higher mortality rates fewer survive to pass on their genes to the next generation. High Bt protein expression is an effective tool in managing resistance, especially when combined with a well-designed refuge. One potential problem with this approach is that the level of expression in the Bt proteins for corn rootworm is lower, especially when compared to those used to control the European corn borer. With the European corn borer most are killed, even those with some level of resistance. With the Bt used for corn rootworm it doesn’t express at the same level so those with lower levels of resistance are still able to survive. So the same management strategies don’t work for both cases and it is necessary to make modifications.
Refuge is one of many useful tools in managing resistance as well as pest populations themselves. Therefore important to understand how to implement it properly and then make sure it is implemented properly.
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