Overview & Lesson Sequence

Focus on Standards & Assessment

Materials Needed, Preparation & Planning, Management Strategies

Background Info

Intro Activity
Why Study Predator - Prey interactions?

Activity 1
Observation of a predator-prey interaction

Activity 2
What makes a predator successful?

Research Project
Designing a predator - prey experiment

Optional Activity
Biological Control vs. Pesticides

Case Study
Rabbits in Australia

References

Glossary

Suppliers

Resource Sheets

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Optional Activity : Biological Control vs. Pesticides

This activity has students collecting data in a control-treatment study over the course of two weeks. They compare and contrast biological control methods with pesticides and investigate relationships among predators and prey in a more natural environment.

Class time required
Approximately 2 weeks of 15 minutes per day for observation and data collection

1 hour each for data analysis, presentations, and class discussion

Materials needed
Per group:

• host plant seeds
• One gallon cage (see One Gallon Cage instructions)
• ladybird beetle adults and larvae
• lacewing larvae
• praying mantids (optional)
• big-eyed bugs (optional)
• soft forceps, small paint brush or eyelash brushes (see Eyelash Brush instructions)
• dissecting microscope or other strong magnifier
• a plastic pot or one gallon plastic milk jug for planting host plant
• Predators in the Environment data sheets
• Graph paper for data analysis and presentation
• Guide to insect identification (optional)
• General garden insecticide

**CAUTION**
Do not use garden pesticides indoors.  For indoor-only experiments use Safer Soap spray or home recipes such as a soap and water or a chili oil and water mixture.  Be sure to use protective gear for pesticide application (long sleeve shirt, pants, gloves, goggles.)
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Preparation
Either construct yourself or have students construct several (one per team) one gallon cages (see One Gallon Cage instructions).  You may want to have students who are finished with their research projects build the cages.

This lesson requires the use of vigorous host plants which are several inches tall and have sufficient leaf surface to support a colony of plant-feeding aphids for a minimum of one week. All plants used in the experiment must be approximately the same size and age and kept free of all insects until you are ready to infest them with the pest insect.

Although we use pea plants and pea aphids as our model here, you may use plants and insect pests from your local area and which suit your environmental and seasonal conditions. For example, you could use cabbage aphids or aphids on rose bushes.  If you decide to use pea plants, start them from seed in one-gallon containers eight-to-ten-weeks before you plan to do this activity. Refer to the “preparation and planning” section at the beginning of this lesson for instructions on how to grow pea plants and infest them with aphids.  There should be at least 25-50 aphids on each plant before beginning the experiments.

Predators will also be needed for this investigation. Any species or combination of species will be fine as long as the predators are well suited to the pest insect and host plant.  For example, if you choose aphids as your pest species, be sure that the predators you choose will actually eat aphids.

Procedure
Begin by asking students: What does it means to have a “control” in an experiment?  Why are "controls" important?  What is an experimental "treatment"?  How might you set up an experiment to determine the relative effects of predators and insecticides on controlling an aphid population?  What would be your treatment(s)?  What would be your control?  Treatments could include a plant with aphids and predators versus a plant with aphids treated with an insecticide.  In order to determine what might happen if there was no treatment, the “control” would have to be a similarly infested plant with no predators or insecticide. You may want to remind students that the aphid population may reach its carrying capacity (see Great Gravidity lesson) and diminish in size without any treatment OR have them (re)discover this concept as part of their investigation.

Ask students how they plan to keep the "control" plants free of predators. They may have to use netting or another device. Keep in mind that data can best be compared when as many variables are controlled as possible.  For example all plants should experience the same environmental conditions (temperature, day-length, humidity, etc.)

There are several possible ways to conduct the control-treatment experiments.  Students can be divided into teams and assigned one of several "treatments" decided on by the class or students can choose a "treatment" they would like to test in their individual groups.  Whatever method is chosen, be sure that the students divide the work evenly  i.e. all students should get a chance to participate in data collection, analysis, and presentation of results.

The following is an example of one possible way to conduct the control-treatment experiments:

1) Divide class into ten teams of three students each. Each team will be responsible for one host plant(s) in the experiment.

2) Assign any one of the following treatments to each team.  You may want to have more than one team testing each treatment:

1. netting-enclosed plant, aphids, no predators
2. netting-enclosed plant, aphids and predators
3. netting-enclosed plant, aphids and insecticide
4. netting-enclosed plant, insecticide
5. open plant, aphids, no predators
6. open plant, aphids and predators
7. open plant, aphids and insecticide
8. open plant, insecticide

Have teams set up the various treatments, measure plant health (height, number of true leaves, etc.), count initial aphid populations, fasten netting, and place the prepared plants in an outdoor location or greenhouse where they will not be disturbed by people or other vertebrates.  Next, transfer predators or apply insecticide as necessary.  The plants should receive an adequate amount of sunlight, be away from heat reflecting walls and a minimum of five feet apart.

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USE CAUTION HANDLING ALL INSECTICIDES AND FOLLOW ALL MANUFACTURER’S DIRECTIONS.

• read all the information on the label of the insecticide container. 
• put on all protective equipment recommended in the insecticide label.
• apply insecticide to test plant following exact manufacturers directions and using all suggested precautions.
• clean and/or dispose of all equipment used as directed in insecticide label.
  

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You may want to have students draw a diagram of the experimental site on grid or graph paper.  Be on the lookout for features that might affect the experiments such as nearby weedy vegetation, location of the sun at different times of day, or other factors that might affect the local environment and thus the results of the investigation.

 Teams should check the plants daily and water as necessary.  Record all changes in insect populations and plant condition on the Predators in the Environment or other data sheet.  Students may also want to note weather conditions including temperature and precipitation if available.

Approximately one to two weeks after the start of the experiment, have teams bring the plants back into the classroom.  Record the final day of data. Students should inspect each plant for insects and record and/or estimate the populations of both pests and predators. Be sure to include insects immediately around the host plant since some may have dropped off during transport indoors or when the netting was removed.  See the Sampling Methods sheet for ideas on how to estimate large populations that are too difficult to count.  Note that some insects from the local environment may have established themselves on the plants.

Data Analysis
Have each student group examine and analyze their data.  Use the following questions to help students organize their thoughts about their data:

• What happened to the aphid population? 
• What happened to the predator population?
• What is the condition of the plant? 
• Are the leaves wilted, green, smaller or larger?
• Did the plant grow during the experiment?
• How close did your predictions come to reality?
• If a treatment group failed, why?

Have each group present their findings to the class using graphs, photos or other means of displaying their data.  Once all the groups have presented, facilitate a class discussion with the following questions:

• What patterns or trends do you see in the data?
• How effective were the predators or insectide in controlling the aphid population?
• What are some of the other advantages of using a predator? (less harmful to the environment) Disadvantages? (expensive on a large scale; some predators may not be native to the environment and could be disruptive, for example, some people are concerned that the introduced Chinese Mantid may be displacing native mantids in the United States.)
• What are some of the other advantages of using an insecticide?  (easy to apply, family tradition) Disadvantages? (kills beneficial insects, pollutes the environment, causes health problems in humans and other animals.)
• If you were a farmer trying to control a pest, which method would you use and why?

You may want the class to work together to develop a formal presentation and/or poster showing the combined results of their experiments.  Pretend the students will be presenting their findings to a professional society.

Assessment
Over the course of this activity use the Observation Check sheet to record students’ ability to design and perform a control-treatment experiment and assess the affects of predators and insecticides on a population of insect pests. Review students’ journal entries, data, and/or posters to determine what levels of conceptual understanding they are developing.  Use a rubric (see Rubric for Group Presentations) to assess the student presentations.  You may want to have students construct their own rubric and use it to assess one another’s presentations.