Resistance
vs. Susceptibility
By Betty Elder -
agbiosafety.unl.edu
Introduction
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
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
Purpose
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.
Objectives
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.
Materials
1. 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
Methods
1. 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.
Discussion
1. 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
http://agbiosafety.unl.edu/education/refugebuilder/refugehome.htm
http://64.26.172.90/unl2/esa.php?action=IRM
Additional
Articles for Review
Biotechnology in Food and Agriculture at the Food & Agriculture
Organization of the United Nations. http://www.fao.org/biotech/index_glossary.asp
Canadian Plant Inspection Agency, Plant Health and Production Division, Plant Biosafety Office.
http://www.inspection.gc.ca/english/plaveg/pbo/btcormai1e.shtml
EPA, Office of Pesticide Programs. http://www.epa.gov/pesticides/biopesticides/otherdocs/bt_position_paper_618.htm
Assessment
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 http://agbiosafety.unl.edu/education/refugebuilder/refugehome.htm
© 2001,
2002 University of Nebraska - Lincoln