Just as humans and other multicellular animals use chemical messengers to regulate their metabolism and the activity of their organs; so do insects and plants. The principles are basically the same; specialized tissues respond to stimuli to produce hormones that are transported to target tissues that alter their activity in response to these hormones. In the case of insects and plants, some of these hormones have had important practical uses for the control of insect pest and the production of food crops.
Learning Objectives: The successful student will be able to ...
- give examples of and describe the stages of development of insects that have incomplete and complete metamorphosis.
- list the major hormones involved in molting, pupation, and metamorphosis and describe their tissues of origin and their action.
- outline the classic experiments that discovered the presence and functions of these hormones.
- discuss how agricultural scientists can use our knowledge of these developmental hormones to develop safer insecticides to protect crops.
- list the major hormones involved in plant growth, maturation, and responses to external stimuli and describe their tissues of origin and action.
- outline the classic experiments that discovered the presence and functions of these hormones.
- describe examples of the chemical defenses used by plants to protect them from predation by insects.
Lesson One: Insect Development.
Members of the phylum Arthropoda (insects, crustaceans, spiders, etc.) have a unique problem as they grow and develop; their exoskeleton must be removed and replaced as the animal grows in size (molting). The life cycle of most insects follows one of two patterns. Insects such as crickets and grasshoppers hatch from the egg looking like miniature versions of the adult (minus the reproductive organs) and through a series of molts grow larger and larger until the final molt to the reproductive adult (incomplete metamorphosis). Butterflies, mosquitoes, and beetles hatch from the egg as larvae (caterpillar, maggot, grub, etc) that looks nothing like the adult. After a series of molts the larval form produces a pupa (chrysalis) from which the reproductive adult emerges (complete metamorphosis). These changes are regulated by the action of several hormones and the insect's nervous system.
Read the Kimball pages that outline the role of the three major growth and development hormones. Be sure you understand the interplay between juvenile hormone (JH) and ecdysone and the different stage of development of the butterfly. These two hormones are controlled by cells in the brain which in turn are responsive to environmental stimuli (temperature, light, food, etc.).
Read this article that demonstrates how scientists are trying to use the properties of these hormones to control insect pests.
Homework, Due . Email the answers to the following questions using your Fontbonne account.
- The caterpillars of some moths and butterflies form their pupae in the late fall and overwinter in this stage of development. Describe what you think is happening to the levels of juvenile hormone and ecdysone in these caterpillars as the time for pupation approaches.
- Suggest at least two different environmental stimuli that would trigger pupation in the moths described in question 1.
- Do some research and describe two different ways in which scientists are trying to use their knowledge of insect hormones to control insect pests.
Caterpillar of Puss Moth by Anthony Rowell at pixdaus.com
Lesson Two: Plant Hormones.
Although plants don't have circulatory system in the same sense that animals do, they do circulate water and nutrients by means of their vascular tissues (xylem and phloem). These vascular tissues carry several important hormones. Most of them influence bud, stem, and root growth, and the development and ripening of fruit. Go to the Kimball pages and read the sections on auxin, ethylene, gibberellans, and abscisic acid (you'll need to follow links at the bottom of this page). Keep in mind that most of these names refer to families of related hormones (e.g. there a many different but related "auxins").
For each of these hormones you should be able to name their tissues of origin and their function. Be sure you are familiar with the classic experiments involved in the discovery of auxin's role in tropism and growth.
Plants have evolved a number of defenses again insects and other herbivores. Many of these defense mechanisms involve the production of hormones and other chemicals. Go to Science Daily and read about some examples here and here. A more extensive list of chemical defenses used by plants can be found here.
Homework, Due . Email the answers to the following questions using your Fontbonne account.
- Outline the steps of a controlled experiment that would test the relationship between the concentration of auxin in the tip of a growing seedling and the intensity of light shinning of the plant (e.g. you hypothesis might be that plants exposed to more intense light require less auxin to stimulate the phototropism response).
- Auxins are used as herbicides to kill weeds. Explain how a naturally occurring substance could be used to harm the plants that make it.
- Do some research and find three more chemical produced by plants that help protect them from insect predation. List their names and describe their action.
- People sometimes put apples or tomatoes in a paper bag to speed up their ripening. Using your knowledge of plant hormones, explain how this technique is effective.
