Invisible to the naked eye, harmless to humans and a natural enemy of pest insects, parasitic wasps roam the landscape in search of an unsuspecting host in which to lay their eggs. In addition to being an important natural pest control, parasitic wasps are also necessary as pollinators in agriculture and home gardens, and often rely on the same flowering plants that attract honeybees and other pollinators. While it may sound like science fiction, parasitic wasps are real, and many are living right in your own back yard!
What are parasitic wasps?
We have all heard of parasites. Ticks, fleas, and mosquitoes likely 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 are 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 a natural source of pest control.
Parasitic wasps belong to the same group of insects as predatory wasps, such as 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 species, called the “fairy fly” is smaller than the head of a pin and depends on wind currents to fly.
Parasitic wasps are exceptionally beneficial to home gardeners because they attack many insects that plague our gardens, including caterpillars, stink bugs, aphids, and scale insects. In terms of economics, parasitic wasps save farmers and homeowners millions of dollars in chemical applications each year.
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. The larvae hatch and mature inside their host, 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, in which they 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 to lay their eggs. It is not until the last adult wasp flies away that the host caterpillar will die.
While you may not have seen a parasitic wasp, you have likely to have seen their fluffy white cocoons on the backs of caterpillars. Tiny white cocoons on the backs of hornworms are an indication they have been parasitized by a wasp. Hornworms with these white cocoons should be preserved on the plant, or moved to another area of the garden where the wasps can mature and continue to benefit the garden.
Attracting parasitic wasps with flowers
Although honeybees and butterflies get a lot of attention as pollinators, as a group, parasitic wasps are arguably the largest and most effective at pollinating flowers. 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 throughout the year. Many of the plants that are beneficial for bees and butterflies are also great at attracting parasitic wasps. Marigolds are a great summer annual that attract wasps, as do cowpeas, 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 control.
For more information on parasitic wasps and other beneficial insects and spiders, visit ces.ncsu.edu where you can submit questions via the ‘Ask an Expert’ link, or contact your local Cooperative Extension center by phone: If you live in Pender County, call 910-259-1238; in New Hanover County, call 910-798-7660; in Brunswick County call 910-253-2610.
Sam Marshall is the horticulture agent with the Brunswick County Cooperative Extension of NC State University, College of Agriculture & Life Sciences.