Special 4-H Project


Raising Parasitoids From Their Hosts

Project Skill:Learning about parasitoids
Life Skills:Learning to learn
Planning and organizing
Applying science and technology
What To Do:Collect and prepare a display of parasitoids

Parasitoids (so called to distinguish them from commonly known animal "parasites" such as fleas and tapeworms) are one type of natural enemy that live in or on a single host insect. They tend to have a more complex life cycle than predators do. The free-living adult seeks out the host and then deposits eggs in, on, or it. The egg hatches to the larval stage, which often is maggot-like in appearance. The larva feeds attached to or inside the body of the host, eventually consuming most of the body contents and killing it. A parasitoid kills only one host during its development.

Most plant-feeding insects (as well as many other types of insects) are attacked by one or more species of parasitoid. Most of these parasitoids are small and nondescript, and it is often difficult to distinguish them from other small insects unless you know what the host was. There are two major groups of parasitoids. First is a specific group of flies, called the tachinid flies, which look similar to house flies. Second are the parasitic wasps, which are quite variable in size ranging from less than 1/32" to over 3" and color. Parasitic wasps are incapable of stinging; unlike their larger relatives such as hornets and yellowjackets they use their "stinger" (actually an "ovipositor" or "egg layer") only to insert eggs into their hosts. Most parasitoids are completely harmless to humans. There are thousands of types of parasitic flies and wasps, and they attack a great diversity of other types of insects. The following are just a few examples of the types of insects from which to raise parasitoids.

Caterpillars are generally easy to raise in containers. Caterpillars on cabbage (or related plants) may be one of the best to use in this project. The three common species of cabbage caterpillars imported cabbageworm, diamondback moth, and cabbage looper tend to be numerous on unsprayed plants, are frequently heavily parasitized, and the plant material they eat keeps well when detached from the plant. Cotesia glomerata is a very common parasitoid of imported cabbageworm caterpillars. Several wasp larvae develop inside each caterpillar, and they emerge together from the caterpillar's body to spin their yellow to orange cocoons in a group. The diamondback moth is parasitized by several ichneumonid and braconid wasps which spin cocoons outside the caterpillar when it dies. Copidosoma floridanum kills cabbage looper larvae shortly after they form a cocoon. The bloated caterpillars curl into a S-shape, and hundreds of tiny black wasps emerge from the mummified cabbage looper. Maggots of the tachinid fly Voria ruralis poke breathing holes in cabbage loopers, which appear as a small dark spot on the back of the caterpillar. They form typical brown fly pupae when the looper dies. Caterpillars on soybeans, small grains, or even ornamental plants are other good choices, but it may be more difficult to keep food plant leaves fresh. In addition to caterpillars, Colorado potato beetle, alfalfa weevil larvae, or other plant-feeding beetles can also be used. Aphids are also often heavily parasitized, but are more difficult to handle and need to be fed on potted plants. However, any insect you can raise in containers can be used in this project.

Galls on oak trees are another possibility for raising parasitoids. Galls are formed on the oak leaves, twigs, and stems by tiny cynipid wasps that cause the plant tissue to proliferate around the developing wasp larvae. However, many of these plant-feeding wasps are parasitized by other wasp species. Collect well-developed fresh galls and hold them individually in covered containers until something emerges. Because all of the insects emerging will be tiny it may be difficult to distinguish between the cynipid wasp adults and the parasitic wasp adults. One group of parasitoids of gall wasps are called torymid wasps. These are often bright metallic blue or green, and the females have a very long ovipositor.

PROJECT DESCRIPTION:

Level 1: Collect several of one type of plant-feeding insect and raise them until adult hosts or parasitoids emerge. Create a display of the host and parasitoid(s) you raised.

Level 2: Collect all plant-feeding species for a single plant/crop.

Level 3: Allow new hosts to be parasitized by the parasitoids raised.

EQUIPMENT NEEDED:

LEADER DOES:

Locate host insects and food plants. The best time in the Midwest to find hosts is mid-June through mid-August. Make sure the plants have not been sprayed with any pesticides. Find some books for identifying the preserved specimens (see RESOURCES; also check your local bookstore or nature store).

MEMBERS DO:

Prepare containers to hold the insects. Cut a piece of paper towel to fit in the bottom of each container. Poke several pin holes in the lid to allow some airflow, but avoid making large holes as the parasitoids are often small and might escape.

Collect as many individuals of one species as you have containers. Try to collect the largest ones so they won't have to be raised as long. Place one insect in each container with some plant leaves and put on the lid.

Check the containers daily and replace the leaves as necessary with fresh leaves. Because the insects are eating, they are also producing lots of "frass" (waste material). Change the paper towel when the container gets too dirty with frass.

Raise the caterpillars or other insects until they pupate and/or some adult insect emerges.

If you wish to make a display of the parasitoids and hosts, leave the adult parasitoids in the container until they die or place in a tightly sealed jar in the freezer for a day. Then pin, mount, spread or otherwise preserve the specimens. (Refer to Discovering the World of Insects, University of Wisconsin-Extension Publication 4-H 338 or equivalent.)

Include in the display photographs of the insects raised, drawings, or pinned specimens of the different parasitoid species that emerged from the host insects.


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