Natural versus Synthetic: How do different types of pesticides compare against a pest?

From Experiment Central, published by U·X·L and available online in Science Resource Center.

In this experiment you will examine how biopesticides compare to a synthetic pesticide. The two natural pesticides are a spray made from chili peppers and one made from garlic. These are commonly used among gardeners as repellents. With chili, it is the hot chilies that make the most effective repellent. Garlic's strong odor can also act as a repellent. With the synthetic insecticides, look for one that works against general pests, such as aphids, caterpillars, beetles. Evidence of these pests can be seen in the holes they bore or bits of leaves that they have munched. Aphids will leave a sticky residue on the leaves.

Once you have made a spray of the natural substances, you can apply all the pesticides to the same type of plant and set outside. To measure the effectiveness of each pesticide you can examine the plant's general health, count holes in the leaves and pests on the plant, and feel the leaves.

Before you begin, make an educated guess about the outcome of this experiment based on your knowledge of synthetic pesticides and biopesticides. This educated guess, or prediction, is your hypothesis. It must be something you can test through further investigation. Your experiment will prove or disprove whether your hypothesis is correct. Here is one possible hypothesis for this experiment: "The synthetic pesticide product will better prevent pests from harming the plants than the biopesticides."

In this case, the variable you will change is the type of pesticide sprayed on the plant. The variable you will measure is the amount of damage to the plant caused by pests.

Conducting a control experiment will help you isolate each variable and measure the changes in the dependent variable. Only one variable will change between the control and your experiment. For your control in this experiment you will not apply any pesticide to a plant. At the end of the experiment you can compare the control plant to the experimental.

Level of Difficulty

Moderate.

Materials Needed

• four small plants of the same type, preferably broad leafed (coleus works well)
• one hot chili pepper (habañeros work well)
• one garlic bulb (five cloves) or crushed garlic
• spray bottle
• chemical pesticide (available from hardware store, drugstore, or greenhouse)
• outside area
• water
• two bowls
• marking pen
• chopping knife
• cheesecloth
• funnel
• rubber gloves
• several nice days

Approximate Budget

$15.

Timetable

45 minutes setup; overnight waiting; 10 minutes every 3 days for about 2 weeks.

How to Experiment Safely

Have an adult present for this experiment. Be careful when working with hot water and chili peppers. The pepper's seeds and juices can burn, so wear rubber gloves and avoid touching your face; never directly ingest the peppers or touch your eyes. Make sure you apply the chemical pesticide outside and follow the directions and warnings carefully. Wear a long-sleeved shirt and long pants when applying and wash your hands afterwards.

Step-by-Step Instructions

1. Label the plant containers: "Pepper," "Garlic," "Chemical," and "Control."
2. Prepare the chili pepper spray: Chop one chili and place the pieces in a bowl. Boil 1 cup (about 240 milliliters) of water and pour over the chopped peppers. Set aside overnight.
3. The next day, prepare the garlic spray: Finely chop about five cloves of garlic and add 1 cup (about 240 milliliters) of hot water. Set aside for 2 hours until cool.
4. When the solutions are ready, use the cheesecloth to strain out the garlic and the peppers. Use the funnel to pour one of the solutions into the spray bottle.
5. In an open area outside, spray the first solution on the plant labeled for that pesticide. After each application, set the plant in a distant area to keep each pesticide isolated from the other plants. Make sure to wash out the spray bottle thoroughly between the pepper and garlic spray (save each solution in a covered and labeled container). Repeat with the other two solutions. Do not spray anything on the control.
6. Set the four plants outside in the same general area, leaving enough room between the plants so they do not touch one another.
7. Every 3 days for the next 15 days observe the plants and note any pests or effects of pests. Reapply the sprays if it rains. If you reapply, again make sure to isolate each plant when you spray.

Summary of Results

Was your hypothesis correct? Look at your data and determine how the pesticides compared to one another. Was there one type of pest that was on one plant more than another? Some types of insects, such as aphids, gather on the underside of leaves. Note the relative amount of any different type of pests on each plant. How did the control plant compare to the experimental plants?

Troubleshooter's Guide

Problem: The natural sprays did not stick to the plant leaves.

Possible cause: The mixtures adhere to the leaves of some plants more than others. Try adding a drop of nondetergent dishwashing soap and mixing well, then reapply.

Problem: None of the plants had much evidence of pests.

Possible cause: This experiment works best when there are many insects around, often during the spring and summer months. Try to set your plants down in a wooded area or one that has a large quantity of plants and then continue your observations.