Tom Turpin
Professor of
Purdue University


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Unusual Caterpillar Walks on the Wild Side

wasp pupae on hornworm
Wasp pupae on hornworm

In biology, a parasite is defined as a plant or animal that lives in or on another living organism. In addition the parasite lives at the expense of the other living organism, called a host. The host provides food or shelter. In other words the relationship between a parasite and a host is a one-way street - the parasite benefits and the host does not!

Way back in history a parasite was defined relative to humans as one who eats at the table of another, repaying him with flattery. In biology the repayment thing seldom exists. In fact, the relationship is often detrimental to the host.

For instance consider the cowbird, a well-known avian parasite. These birds are called cowbirds because they hang around on the ground where cattle are grazing. The cattle flush insects that become food for these birds. No problem here. The bird is a parasite because it lays eggs in the nests of other species. That means that other birds feed the developing young of the cowbird. To make things worse the young cowbirds frequently grow faster than the young of the host and even sometimes kick the other babies from the nest.

Some species of bacteria could be considered parasites when they cause the host harm. Examples of human diseases caused by bacteria include bubonic plague, tuberculosis, some types of meningitis and Lyme disease. Some fungi are also parasitic. Athlete's foot is caused by a fungal parasite.

Some plants are also considered parasites. One of the most recognizable of the plant parasites is the Christmas decoration called mistletoe. Dodder is another parasitic plant.

Many, many insects and other arthropods are parasites. Some we consider pests because they parasitize our domestic animals or us. Sometimes both. For instance, fleas. Fleas survive by taking a blood meal from mammals. Donating blood to fleas is considered detrimental to the host. At a minimum such feeding causes a host reaction. The same can be said for lice and ticks.

Some insects are parasites on mammals because the immature stage of the insect lives in the mammal. The bot flies do this. This type of fly lays an egg on the host, and the larvae hatch and feed within the host during that stage of their life cycle. A bot fly that infests cattle is known as the cattle grub. The human bot fly, as the name suggests, feeds on humans and is the only bot fly to do so. The human bot fly is found from northern Mexico to the lower third of South America. So travelers to this part of the world sometimes become a host to a bot fly. Not, I am told, the most pleasant of situations!

Most insect parasites use other insects as their host animals. In this case, if the host is an insect that is considered a pest, we humans call the parasite a beneficial insect. An example of such an insect is a tiny wasp that is a parasite of the tomato hornworm.

black wasp on green leaf
Apanteles wasp

The hornworm parasite is called a Braconid wasp and belongs to of the genus Apanteles. The adult wasp inserts eggs into the hornworm caterpillar with her ovipositor. The wasp larvae develop within the body of the caterpillar and then chew through the skin of the caterpillar and spin a cocoon attached to the host. After a few days in the cocoon an adult wasp emerges to complete the life cycle.

Many insects are parasitized by other insects. Such parasitism helps keep populations of insects from growing to extremely high levels. It is part of what is known as the balance of nature. When the parasitism is associated with a host insect that is a pest to humans it is considered a part of what is called a biological control approach to insect management. Many insect parasites have been introduced into North America just for that reason.

So as the gardening season gets underway this year, remember that many tiny insects flying around might be beneficial because they are parasites. One way to help the beneficial parasites is to plant flowers in the vegetable garden, because many adult parasitic insects feed on nectar. And don't smash hornworms with little white things on them! Remember the old adage: the enemy of your enemy is your friend. Hooray for insect parasites, tiny though they may be!


Writer: Tom Turpin
Editor: Olivia Maddox