Parasitoid Wasps: A Beneficial Insect in the Garden

Written By Lara Wadsworth

There are estimated to be around one million different species of parasitic wasps worldwide. In fact, most wasps are parasitic, which means they live on or in a host at the host's expense. For common garden pests like aphids, whiteflies, and leafminers, the presence of parasitic wasps means death. Attracting these beneficial parasitic wasps to your garden can improve overall garden health without the need for harsh pesticides, as nature takes its course with natural checks and balances.

Do Parasitic Wasps Sting?

Believe it or not, most wasps don’t sting. The stinging wasp you are thinking of is likely the Yellowjacket, the Paper Wasp, or the Baldfaced Hornet. Yellowjackets are parasitic wasps and are an important predator of flies. Paper Wasps can effectively control caterpillar infestations; however, nobody will blame you for not wanting them around. They are aggressive and can even sting more than once. Some of the most important and beneficial species of parasitic wasps often go completely unnoticed. They are harmless to humans and are often very small.

Why Are Wasps Beneficial to the Home Garden?

Parasitic wasps are wonderful friends in the garden because they help keep pest populations in check. Almost all parasitic wasps have a specific species that they target, and they usually leave all other species alone (including humans!). For example, one wasp called The Insidious Flower Bug (Orius insidiosus) is a special predator of aphids. These tiny wasps are even smaller than a house fly or mosquito and are hard to spot if you don’t know what to look for. They fly around and lay their eggs inside aphids. The aphid then turns into a mummy (yes, that’s the scientific term for it) and then hatches the new wasp, and the cycle starts all over again. They are highly effective at helping to control aphid populations.

In addition, the adult wasps typically feed on pollen and can be helpful pollinators! Gavin Broad, a researcher at the Natural History Museum, said, “Eating other arthropods alive is their way of life. I don’t want to down butterflies, but they just stand there with their gangly legs, sticking their tongue down the flower, and probably transfer very little pollen. Whereas wasps are in there, sticking their heads right in, and they come out covered in pollen.”

How to Identify Beneficial Parasitic Wasps?

This is a bit of a tricky question because there are so many species of parasitic wasps in the world. My biggest piece of advice is to contact your local extension office and find out which species are native to your area and how to identify them. With the exception of the incredibly beneficial hoverfly, helpful wasps that do not sting humans do not have the yellow patterning on them that yellowjackets and hornets do. Beneficial parasitic wasps are small flying insects that can be identified by their long antennae and brown or black bodies, with some variation in coloring. Some species can be over 4 inches long, while others are as small as 1/25 inch. You might see them feeding on or near flowers or flying near plants to lay eggs. In most cases, it takes an expert to identify a parasitic wasp properly. Most of the time, their eggs are easier to spot than the wasps themselves.

Should I Buy Parasitic Wasps for My Garden?

You may have heard of greenhouses or other large growers buying parasitic wasps from a biological control retailer to bulk up the populations in their area. For the typical home gardener, this would not be very cost-effective. Mail-order parasitic wasps must be released immediately and have a high death rate in small numbers. Additionally, they aren’t good at staying in one spot because they fly around looking for pollen. You are much better off planting native and attractive plants that beneficial parasitic wasps are seeking out in order to draw them into your garden. Consider growing an assortment of flowers to attract beneficial insects to your garden.

How to Attract Beneficial Parasitic Wasps to the Garden

The best way to attract beneficial parasitic wasps is to plant native flowers that provide food sources for the adult wasps. According to the University of Maryland, “Most adults feed on plant fluids and sugars, so provide flowering plants that provide good nectar sources. The best nectar sources are flowers with wide or shallow corollas where the wasps can easily reach nectar, such as members of the carrot (Umbelliferae) and cabbage (Cruciferae) families.” In addition, parasitic wasps are harmed by harsh chemicals such as insecticides. Avoid using this if at all possible. Providing an environment conducive to parasitic wasp life will naturally reduce the presence of pests that you might want to use insecticides on. It takes a bit of faith to take a step in this direction, but you won’t regret it once it is working!

Some suggestions of plants to grow to attract beneficial insects include: sunflowers, salvia, goldenrod, milkweed, lavender, phlox, bee balm, purple coneflower, and other native flowers to your region.

Regional Native Flower Lists and Other Resources:

• 10 Natives Of The Southwest USA For Pest Control
• Northeastern Natives For Attracting Beneficial Insects
• Native Plants For Pest Control In The Pacific Northwest
• 10 Native Plants To Attract Beneficial Insects In The Midwest
• 10 Native Plants For Attracting Beneficial Insects In Southeastern USA
• 10 Native Plants For Pest Management In The Rockies

How Can I Learn More About Beneficial Insects?

One of my all-time favorite resources for learning about beneficial insects and deciphering which insects are good or not is a document put out by Washington State University Extension titled “Beneficial Insects, Spiders, and Other Mini-Creatures in Your Garden.” It can be downloaded for free! While it is focused on the environment of the Pacific Northwest, it contains relevant information for anyone wanting to know about beneficial insects in North America. Again, I encourage you to contact your local extension office or visit their website for specific information about your region.

Lara Wadsworth, True Leaf Market Writer

I am a native of Southwestern Michigan, where I also reside, and I love all things plants! I got a Bachelor's Degree in Horticulture and found the first work-from-home job I could get. Now, I spend my days writing for TLM, playing with my dog, eating delicious food with my husband, and plotting my next landscape or gardening move. I believe everyone should get down and dirty in the soil now and then. Happy Gardening!