Birds are not the only natural enemies of garden pests. Predatory insects also play an important role in pest control without chemicals. But while gardeners usually welcome voracious aphid-eating lacewings and ladybugs, predatory wasps are met with fear. That is because wasps such as yellowjackets are first and foremost associated with painful stings; they are not the type of insects you want to have in or near your garden, let alone in large numbers. Not all wasps, however, are the same. Parasitic wasps are generally harmless, do not sting and are not a threat to humans.
What Are Predatory Wasps
Many of these beneficial wasps lay their eggs within the eggs or inside the bodies of their hosts—they parasitize them, that’s why they are also called parasitic or parasitoid wasps. It is usually during their larval stage that these wasps do their job of biological pest control. They eat the eggs, larvae, and pupae of their hosts, which keeps populations down. The adult wasps mostly consume nectar, pollen, and honeydew and only occasionally prey on other insects.
There are two basic types, ectoparasitoid and endoparasitoid wasps. The first type lives outside the hosts and the second type enters the host’s body. Some species are both ectoparasitic and endoparatisic. Cotesia congretata wasps start off as internal parasitoids inside the body of the tomato hornworm but then leave the caterpillar’s body to pupate. The probably most well-known image of a parasitic wasp in action is the tomato hornworm being covered with clusters of rice-like wasp cocoons.
Parasitic wasps range considerably in size and color, many of them are so tiny that they go unnoticed or aren’t even visible to the naked eye. You will know they are there because of their activities.
Types of Predatory Wasps
There are thousands of parasitic wasp species in over 40 families. The insects they prey on varies from species to species and include many common garden pests.
Here are some of the most important predatory wasp families:
Aphelinids (Aphelinidae) include about 1,000 species, mostly solitary wasps whose adults are less than 1 millimeter long. They attack aphids, mealy bugs, psyllids, scales, and whiteflies by laying eggs on them. They are widely used in biological pest control. The species Encarsia formosa is used to control the greenhouse whitefly.
Braconid wasps (Braconidae) are a family of parasitic wasps with also more than 1,000 species attacking different hosts, including the larvae of beetles, caterpillars (tomato hornworm, imported cabbageworm, gypsy moth), flies, sawflies, and several types of aphids (green peach aphid, melon aphid). The adults are less than 13 millimeters long.
Digger wasps (Sphecidae) include species such as the great golden digger wasp (Sphex ichneumoneus) whose colorful female is more than 1 inch long. These solitary wasps dig vertical tunnels in the ground, hence their name. They paralyze their prey—grasshoppers, katydids, and crickets—by stinging it, then drag it into their tunnels where they lay an egg on it so when the egg hatches, it can feed on it right away.
Scelionid wasps (Scelionidae) are tiny wasps, usually less than 1 millimeter long but highly varied in their body form. They are internal parasites of various insect and spider eggs. The Trissolcus basalis wasp is used widely for the biological control of the southern green stinkbug.
Scoliid wasps (Scoliidae) include about 20 species in North America. Ranging from ¾ inch to 2 inches in size, the adults are hairy, and often with interesting color patterns or stripes. Scoliid wasps go after Japanese beetles and June bugs. They can often be seen flying over lawns searching for beetle grubs. When a female wasps find a beetle, she burrows into the soil, stings the grub, and lays an egg in it.
Tiphiid wasps (Tiphiidae) are black with red, yellow, or white markings. There is about a dozen species in North America, some of them are small but others are more than 1 inch long. They are solitary hunting wasps that parasitize beetle larvae, mostly scarab beetles. The female deposits her egg on a beetle larva in the ground.
Trichogrammatids (Trichogrammatidae) are tiny wasps ranging from 0.3 to 1.1 millimeters in length. Because they attack the eggs of over 200 species of moths and butterflies, they are widely used in biological pest control, including in orchards.
Do Predatory Wasps Sting Humans?
The likelihood of wasps stinging is much higher with social predatory wasps that live in a nest with a queen, such as yellowjackets and bald-faced hornets. These wasps sting when they feel threatened, especially when their nest is being disturbed. Solitary predatory wasps such as cicada killer wasps and mud daubers are much less aggressive and usually don’t sting unless you get into direct physical contact with them. Parasitoid wasps, on the other hand, do not sting.
How to Attract Parasitic Wasps to Your Garden
There are two things you can do to make parasitic wasps feel at home in your garden.
As most species are highly sensitive to insecticides, do not use any chemicals, and if you do need to spray, use an organic insecticide, and apply it in a very targeted way. Never use a broad-spectrum insecticide that indiscriminately kills all the insects within reach, including valuable pollinators.
Secondly, plant flowers plants that provide nectar to the adult parasitic wasps. Flowers whose corolla is wide, short, or shallow makes it usually easiest for wasps to access the nectar. Two plant families in particular have suitable flowers: members of the carrot family (fennel, masterwort, sweet cicely, angelica, and Queen Anne’s lace) as well as members of the cabbage family (after the harvest, just let a crop plant such as radishes or kale flower).
There are also companies that sell parasitic wasps but it’s important to that you know exactly which pest you need to control in order to pick the right beneficial insect population, as well as closely follow the release and care instructions.
Parasitic Wasps. North Carolina State University Extension.
Parasitic Wasps. University of California Integrated Pest Management Program.
Parasitoid Wasps. University of Maryland Extension.