Observational Study of Tobacco Hornworms

Developers:

Dean O. Roberts
Marlborough Elementary
Upper Perkiomen Schools

Richard Slawecki
Agricultural Exploratory Research
Rohm and Haas Company
Spring House, PA

Grade
Levels:

Fourth Grade

Discipline:

Entomology

Goals:

The students will raise tobacco hornworms

Objectives:

• The students will observe the growth and development of tobacco hornworms from the egg, larval, pupal and adult stages
• The students will record the weight and length changes in the larvae during the 3rd, 4th and 5th instars.
• The students will graph the data collected on weight and size changes.

Background:

Most insects change in form during development and the stage larva and adult are not all alike. This change is called metamorphosis. There are two variations of metamorphosis in insects, simple metamorphosis and complete metamorphosis. The primary difference between simple and complete metamorphosis is that during complete metamorphosis, any changes that occur in the larva are internal and there is a pupal, or resting stage, prior to the last molt.

The tobacco hornworm, Manduca sexta, has become the choice experimental insect of many studies because it has a rapid rate of development and can reach weights of up to 10 grams as a full fed 5th instar larva.

Vocabulary:

instar - stage of development during larval period; stage of development between molts

molting - shedding of the outer exoskeleton; characterized by a cessation of feeding prior to molting

Materials:

• Tobacco hornworm eggs (available from Carolina Biological Supply)
• Artificial diet (Available from Bioserve Industries or Carolina Biological)
• 5 clear containers with loose fitting lids, at least 5" x 5" x 2"
• small watercolor brushes, cleaned
• 4-oz containers with lids, 1/worm
• shredded tissue, plain (for use during pupal stage)
• metric scale/balance and mm rulers

Procedures:

1. Upon arrival, place a few eggs in each of the clear containers. Within each container place a few small slabs of artificial diet surrounding the eggs. Eggs hatch in about 5 days and hatchlings will immediately need food.
2. After hatching, isolate worms into groups of about 8-10 with food. Use paintbrushes to gently move. Avoid handling larva at this stage.
3. When larva have reached third instar (approximately 9-10 days after hatching) isolate in 4-oz. jars with food
4. Clean jars every 2-3 days and replace food when it has turned dark brown in color.
5. At end of fourth instar, remove worms to quart jars with food. They may be handled at this time.
6. At end of the fifth instar, worms will again cease eating and the dorsal aorta will be seen as a dark line pulsing along the worm's back. Larvae are ready to begin pupating.
7. Remove all food and loosely stuff slightly shredded tissues into jar. Larva will burrow into the material and begin to pupate. Loosely replace the lid.
8. After pupation is complete, the larva is encased in a dark brown sheathing. At this time remove tissues and place a strip of cardboard against the side. This is for the emerging adult to climb up on to pump up its wings.
9. Moths will live 4-5 days. No food is necessary.
10. It is a good idea not to release the larva or moths into the wild, however accidents do happen. Although the tobacco hornworm is native to many areas, it would be wise to check with the county agent for the Department of Agriculture for further information.

Extensions:

Try feeding some of the larvae a steady diet of tomato leaves. What changes occur after 4-5 days?

Teacher's Notes:

1. Keep all artificial diet in the refrigerator until ready for use.
2. Occasionally empty out excess fecal matter from the containers.
3. Use inexpensive paper plates on "cage" cleaning days.
4. There will be some odor associated with the worms. This is due primarily to their diet and not the worms themselves.
5. Keep the worms out of direct sunlight and away from heating and air conditioning vents.
6. Tomato leaves are another favorite of the hornworms. They will revert to their natural green color on this diet. The artificial diet makes them a bluish color.
7. It is best to wait until the fifteenth day or so to begin any weighing or measuring of the worms. Any growth up to this point would be difficult for elementary children to identify.