Parasitic Wasps: A Gardener’s Best Friend

Invisible to the naked eye, harmless to humans, and a natural enemy of pest insects, parasitic wasps roam the landscape, looking for an unsuspecting host in which to lay eggs. While these parasitic wasps are important for pest control, they are also necessary as pollinators in agriculture and home gardens, and often rely on the same flowering plants that attract other pollinators like honey bees. While it may sound like science fiction, parasitic wasps are real, and many live right in your own back yard!

What are parasitic wasps?

We have all heard of parasites. Ticks, fleas, and mosquitoes are often what come to mind, because they parasitize humans and spread disease. Parasitic wasps do the same thing; however, they do not cause harm to humans. Instead, these wasps parasitize other insects, most of which can be pests in the garden. With almost 1,900 species in North America, parasitic wasps are an abundant and vital resource, which makes them very important as natural pest controls. Parasitic wasps are beneficial to home gardeners because they attack many insects like caterpillars, stink bugs, aphids, and scale insects that plague the home a garden. In terms of economics, parasitic wasps save farmers and homeowners millions of dollars in chemical applications, annually. Parasitic wasps belong to the same group of insects as predatory wasps, like yellow jackets and hornets; however, they are much smaller. Most species of parasitic wasps are microscopic and are invisible to the naked eye; one wasp, called the “fairy fly” is so small it depends on wind currents to fly, and is smaller than the head of a pin.

How do they control pests?

Like their predatory cousins, parasitic wasps use a stinger to paralyze their prey; however, instead of feeding on their prey, these wasps lay their eggs either on or inside a host insect. The venom of the wasp paralyzes the host, often causing it to stop feeding, which is good for your plants, but bad for the insect. Like something out of science fiction, parasitic wasps use their host as an incubation chamber. Larvae hatch inside its host, and mature, feeding on nonessential fluids of its prey. During the last stage of development, wasp larvae chew their way out of the host and spin a silken cocoon, where they will pupate. Almost one week later, adult wasps cut an escape hatch in the top of the cocoon and fly away, searching for another host in which they can lay their eggs. It is not until the last adult wasp flies away that the caterpillar will die.

Identifying wasps

While you may not have seen a parasitic wasp, you are likely to have seen the fluffy white egg cocoons on the backs of caterpillars. The tiny white cocoons on the backs of tobacco hornworms are an indication that it has been parasitized by a wasp. Hornworms with these white cocoons should be preserved on the plant, or moved to another area so that the wasps can finish maturing and continue to be beneficial in garden.

Attracting parasitic wasps with flowers

Although honey bees and butterflies get a lot of attention as pollinators, arguably the largest and most effective at pollinating flowers are parasitic wasps. Because they do not feed on caterpillars, wasps must supplement their diet with pollen and nectar, just like honey bees. If you want to attract pollinating wasps to your garden, consider adding plants that flower year-round. Many of the plants that are beneficial for pollinating bees and butterflies are also great at attracting parasitic wasps as well. Marigolds are great spring plants that attract wasps, as well as cowpea, and white clover, which is rich in high-quality sugars. If you grow herbs, consider adding fennel, rosemary, dill, or lavender to the garden, as these plants are highly nutritious to foraging wasps.

With all of the pests that plague the garden throughout the year, it is comforting to know that beneficial parasitic wasps are out in force. By planting and maintaining flowering plants that provide nectar and pollen, you can do your part to help maintain natural, biological pest controls.

Learn More!
For more information on parasitic wasps and other beneficial insects and spiders visit http://ces.ncsu.edu, where you can post your questions via the ‘Ask an Expert’ link, or contact your local extension office. If you live in Pender County, call 259-1235. In New Hanover County, call 798-7660. In Brunswick County call 253-2610.

Written By

Sam MarshallExtension Agent, Agriculture - Horticulture (910) 253-2610 (Office) Brunswick County, North Carolina

Posted on Apr 14, 2014

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