Resistance vs. Susceptibility

By Betty Elder -

The soil bacterium Bacillus thuringiensis (Bt) produces proteins toxic to some insect species. There are many strains of Bacillus thuringiensis each producing a different type and amount of toxin. Bt has been used in an insecticide formulation for over 50 years in the organic and home garden industries. In the early 1990’s, Bt genes were genetically engineered into food plants so that tissues of these plants would produce the Bt toxins. Bt provides engineered plant lines with the trait of being resistant to a target group of insects.

The European corn borer is an important corn pest throughout the corn growing regions of North America. European corn borer larvae damage corn by feeding on the leaf, stalk, and ear of the plant. Millions of dollars worth of corn is lost each year to the European corn borer. Traditional methods of dealing with corn borers usually include chemical insecticides. With the advent of genetically engineered crops, there are now Bt corn lines that are almost completely resistant to European corn borer.

Resistance is the evolved ability of a species to survive during a stressful event or set of circumstances. Susceptibility means that the individual or species will become ill or possibly die in the face of the stressful event or circumstances.

Within natural populations of European corn borer, the vast majority (>99.9%) of corn borers are naturally susceptible to Bt corn lines. In theory, high dose Bt corn will kill 99.9% of all individuals that carry the susceptible gene. The few remaining corn borer larvae are carriers of genes that make them resistant to the Bt toxins.

Resistance can be passed onto offspring only if both parents have the gene for resistance. However, if only one parent is resistant to Bt and mates with a non-carrier, their offspring will be carriers of the resistane genes but be susceptible and die if they eat Bt.

Bt is a powerful tool in controlling insect populations. However, selective pressure could result in populations of insects that are resistant to Bt toxins. To avoid creating populations of resistant European corn borers, the United States Environment Protection Agency and the Canadian Food Inspection Agency require producers to plant refuges of non-Bt corn in or near their fields. Refuges are areas of non-Bt corn that are a haven for susceptible corn borers. Refuges help ensure that a healthy population of susceptible corn borers is available to mate with resistant corn borers



The purpose of this lesson is to develop the student’s understanding of the concepts of resistance and susceptibility and how populations are impacted by a stressor. The concept of refuge and its impact on preserving susceptible populations is also a focus of this lesson plan.

Following completion of this lesson, the students will:
1. Understand the concepts of resistance and susceptibility.
2. Develop a refuge system to protect susceptible individuals
3. Increase their understanding of trait inheritance.

Poster board divided into grids containing 4 columns and 4 rows. Each square represents a hotel room/corn plant and needs to have a slot cut to accommodate the cards as the corn borers “check in”. The analogy of “hotel rooms” refers to the corn plants that the corn borer larvae will be feeding and growing on. The corn plant is a fully accommodated suite that the European corn borer use for food and protection.
2. Printed corn borer cards and plant type cards available here for print in pdf or Microsoft word document.
European corn borer cards:
1/3 of the insects are labeled susceptible and will die after eating Bt (SS)
1/3 of the insects are labeled carriers and will die after eating Bt (RS)
1/3 of the insects are labeled resistant and will survive to produce offspring (RR)
Corn pictures:
one of each type for every hotel room

Hotel rooms will be divided into a 4X4 grid. Cut two slots in each “room” to accommodate the cards with the corn borer and corn cards. The slots should be placed one above the other leaving 3 inches between each slot within each square (see diagram)

2. Divide the corn borer cards into a set of 16, one for each hotel room. The ratio of corn borer cards in this set should be SS=10, SR=4, RR=2. This ratio roughly mimics the distribution of traits in a natural population.

3. In each grid place a corn borer card in the top slot. This card should be turned around so that you cannot see the picture of the larvae. Distribute one in each hotel room as they will be checked in randomly.

4. In the first exercise, feed only Bt corn to insects. Place a Bt corn card in the bottom slot of each square.

5. Once each slot is filled, turn around all corn borer cards to determine the effect on the corn borers residing in the hotel room. SS and SR corn borers residing in the Bt corn hotel rooms will die. SS and SR corn borers residing in the refuge hotel rooms will survive. RR insects in all rooms will survive. These survivors will mate.

6. Create a new generation with surviving insects. Use the population chart to determine what traits the next generation will have. The population chart is found here in pdf format or Microsoft excel document.

7. Repopulate the field, as in step #3, with the next generation of corn borer larvae and observe the results. Obviously only resistant (RR) corn borers will exist the second year.

8. Discuss the results of the first all Bt corn planting.

9. Repeat the experiment providing an area of non-Bt refuge in your field. In the real world farmers are required by law to provide at least a 20% refuge to ensure susceptible corn borers. Have your students determine how much refuge they would need to plant (how many squares) to insure that resistance is kept at a minimum 2 per 16, or reduced to 0. You may not want to tell your students the correct number of hotel rooms to plant with refuge. Let them come up with the amount and experiment.

10. Repopulate the field as in steps 2 and 3

11. Flip cards over to determine what happens to the larvae.

12. Create new generation with surviving insects. Use the chart to determine the traits of the offspring.

13. Repeat the experiment with refuge, as many times as needed to get a correct number of susceptible SS corn borers.

14. Discuss the results.

Review the need to maintain susceptible populations of insects.

2. Discuss the merits and risks of selective mating of populations.

3. Develop a set of guidelines for planting Bt corn.

Important Notes
European corn borers mate in weedy field edges and pairing is thought to be random. Mixing of seed types is a concept that many students will wish to attempt. However, in real world applications refuge and Bt corn is planted in separate blocks or stripes in order to help preserve susceptible populations.

Teacher Background
Instructor information can be found in the guidelines from the EPA located at

Additional Articles for Review
Biotechnology in Food and Agriculture at the Food & Agriculture Organization of the United Nations.
Canadian Plant Inspection Agency, Plant Health and Production Division, Plant Biosafety Office.
February 8, 1999.
EPA, Office of Pesticide Programs.
Nebraska Cooperative Extension NF00-425 Revised November 2000.

This group activity can be based upon the results the students obtained in the exercise. Track the number of insects of each type through the successive generations and have students record and submit data. Each student should be able to develop a refuge that would maintain the insect susceptibility using the data as a guideline. Compare student recommendations with the guidelines from the EPA located at

© 2001, 2002 University of Nebraska - Lincoln