ACTIVITY 22

Plant Defenses--Plant-Insect Interactions (Without the Insect)*

Teaching Objectives:

1. Students will demonstrate the steps of the scientific method in experimentation.
2. Students will experiment to demonstrate butterfly tasting behavior.
3. Students will experiment to demonstrate a plant defense mechanism.

Materials Needed:

• Scalpel
• Poinsettia plants (live)

THINGS TO CONSIDER: This activity should be conducted during the time that poinsettias are available. The leaves are all that are necessary to conduct the activity, so it can be done up to two to three months past the time the plant has stopped blooming.

About the Plants:

Milkweeds are in the plant family Asclepiadaceae. Some of the more common species are:

• Asclepias incarnata-Swamp milkweed
• Asclepias syriaca-Common milkweed
• Asclepias tuberosa-Butterfly weed

Poinsettias are in the plant family Euphorbiaceae. The species which is sold in stores at Christmas is Euphorbia pulcherrima. The species with which we are working is Euphorbia pulcherrima.

Procedure:

Milkweeds, common in summer months in many parts of the United States, ooze a milky sap whenever a leaf is picked or nicked. Once the milky sap hits the air, it immediately begins to dry into a hard sticky mass, as would glue. Insects with chewing mouthparts have evolved certain feeding behaviors to get around this plant defense mechanism. These behaviors include trenching, nicking the mid-vein of the leaf, and girdling the petiole.

• Nicking the mid-vein--cutting directly into the large vein that runs up the center of the leaf
• Trenching--using a sharp object to make a semi-circular to circular area in the flat surface of the leaf without cutting entirely through the surface
• Girdling the petiole--cutting a circle around the slender stem that supports the flat part of the leaf

Poinsettia, a plant readily available through greenhouses during the winter, also exudes a milky sap when the leaf is damaged. With this milkweed-like response, could this milky sap be used by the poinsettia as an insect defense mechanism, and could insects work their way around it?

Discuss the following hypothesis with the students: Poinsettias do use milky sap to defend themselves against insect herbivores, and insects could circumvent this defense mechanism through a modified feeding behavior.

Cut a piece of leaf from a poinsettia and observe the milky sap that exudes from the wound. Does it become dry and sticky when exposed to the air?

Try using some of the milky sap to stick two small pieces of paper together. Does it work? If so, would this create a problem for insects trying to chew on a poinsettia leaf?

Choose another leaf and cut a nick in the mid-vein about half-way down the leaf. Wait 2 to 3 minutes. Now cut a piece of the same leaf off distal to the nick. Do you see the same amount of milky sap exuding from the wound as in step one?

Try another leaf and this time try trenching. Try cutting the leaf at the base of the petiole.

THINGS TO CONSIDER: Keep milky sap out of eyes and mouth. It may prove irritating and produce allergic reactions.

Conclusions:

From the observations made, do poinsettias use their milky sap as a defense against insect herbivores?

How could this be tested?

*Created by the Smithsonian Institution's National Museum of Natural History