All gardeners have had insect problems at one time or another. Either caterpillars eating away at your prized plants or aphids sucking the life out of them. We all have tried various ways to control these pests, sometimes with more success than others.

But we are not alone in this fight with insect pests. When we are in our gardens and yards, friends are all around us — allies ready to help us in this fight. But we probably never see them. And who are these friends? Small parasitoid wasps.

All wasps belong to the insect order Hymenoptera. There are many different species of wasps but in general, wasps are either hunters or parasitoids. Hunter wasps are the ones we are familiar with. These include yellow jackets, paper wasps, hornets and others. Although they can sting us if we disturb them, they are beneficial insects that prey on other harmful insects.

The other group, parasitoid wasps, are more numerous than hunter wasps. The Xerces Society — an organization dedicated to protecting invertebrates and their habitats — estimates that over 70% of wasps are parasitoid. They also control harmful insects, but, as we’ll see, in another way.

Most parasitoid wasps are very small — less than a millimeter in size. In fact, the world’s smallest insect, the fairyfly wasp – about the width of a human hair, is a parasitoid wasp. Their small size is what makes them almost invisible to us. If seen at all, the adults are usually observed feeding on flowers. You are more likely to see them in their other life stages but may not recognize them as wasps.

The female parasitoid uses her ovipositor to lay her eggs in or on a host, which is usually another insect, its eggs or a caterpillar. Most parasitoid wasps are specific to a single host. When the wasp eggs hatch the wasp larvae begin to feed on the host’s bodily fluids or nonvital body parts.

The host remains alive during the feeding. The larva wants to keep the host alive as long as possible to preserve its food source and use the host’s body as protection from predators. When the larva completes its development, the host dies and a new generation of adult wasps emerges from the host to start the cycle over again. It is when they are about to emerge from a host that we may spot them. A caterpillar with parasitoid wasp pupae about to emerge looks like it has grains of rice stuck to its body.

There is a difference between a parasitoid and a parasite. A parasite is an organism that lives in or on a host and benefits at the host’s expense — but typically does not kill the host. An example is a flea. A parasitoid lives off its host to complete its development and kills the host when its development is finished. For example, the monster in the old sci-fi movie Alien was a parasitoid.

The parasitoid wasp’s life cycle may seem gruesome to us, but it is how they evolved to survive. And their life cycle helps control the numbers of harmful garden insect pests.

To encourage more of these gardening allies, the University of Minnesota recommends having a variety of continuously blooming plants and limiting the use of pesticides.

Sprague is an Ohio State University Extension Master Gardener Volunteer in Mahoning County.