Parasitoid Wasps (Hymenoptera)

parasitized hornwormParasitized hornworm Picture of Close-up of Scoliid wasp. Scoliid wasp
Parasitic wasp emerging from aphid mummyParasitic wasp emerging from aphid mummy Ichneumonid waspIchneumonid wasp

There are many species of parasitoid wasps, but most are so tiny that they are rarely noticed. What they lack in size they make up in sheer numbers and efficiency, and as a group they may be the single most important biological control method gardeners have. Wasps belong to the order Hymenoptera, which includes more parasitoids than any other order of insects, with thousands of parasitic species in over 40 families. Parasitoid wasps are very diverse in appearance, ranging in size from as small as a fleck of pepper up to nearly 3” long, and from uniformly dark in color to brightly colored and patterned. These tiny agents of death may be ectoparasitoids or endoparasitoids, but the good news is, they do not sting people.

Important species in Maryland: The most numerous and important species of parasitoid wasps in this area are in two superfamilies, Chalcidoidea (Chalcids) and Ichneumonoidea (Ichneumonoids). Among the more important Chalcid wasps are species in the Aphelinidae (Aphelinids), Chalcididae (Chalcidids), Encyrtidae (Encyrtids), and Trichogrammatidae (Trichogramma) families. The more important Ichneumonoid wasps are species in one of two families, Braconidae (Braconids)and Ichneumonidae (Ichneumons) wasps.

Life stage(s) that feed on pests: Larvae. Adults usually feed on nectar, pollen, and honeydew, although a few may feed on host insects as well.

Pests fed on: In general, the eggs, larvae, and sometimes pupae of many insects, including aphids, caterpillars (larvae of butterflies and moths -Lepidoptera), sawflies, beetles, leafhoppers, true bugs, thrips, psyllids, and flies. Two species of wasps are very important aphid parasitoids: Aphelinidae and Aphidius (Braconid wasps). Trichogramma are endoparasitoids of the eggs of over 200 species of moths and butterflies, and are the most widely released biological control agents in North America. Encrytids are highly successful generalist feeders that attack a wide range of host insects. Ichneumons and Braconids are primarily parasitoids of dozens of different caterpillars (such as armyworms, cabbage looper, fall webworm, tent caterpillars, tomato fruitworm, redhumped caterpillar); Cotesia spp.  (Braconid wasps) are important parasitoids of tomato hornworm and imported cabbageworm.

Appearance: Parasitoid wasps are typically so small – most range from the size of a fleck of pepper to under 1/2” long - that they can only be reliably identified by an expert.

Eggs: Are rarely seen, as they are usually inserted within the eggs or bodies of host insects.
Larvae: Are typically not seen, although some may be glimpsed as a dark shape within the body or egg of a host insect.
Pupae/Cocoons: The pupae of some parasitoid wasps may be seen as small whitish/yellowish, rice-like cocoons on or near parasitized insects.
Adults: Chalcid wasps, in general, are tiny, dark-colored wasps (often metallic blue or green), with clear wings. Chalcids include the smallest of all known insects (Dicopomorpha echmepterygis) and the diminutive Mymaridae – fairyflies or fairy wasps – that are so tiny they can fly through the eye of a needle! Among the more important Chalcid wasps: Aphelinids are extremely small, usually 1/25” or less, yellowish-brown, somewhat stout; Chalcidids are tiny (1/10-1/3”) but robust, shiny dark bronze or copper colored, with an enlarged femur on the hind leg; Encyrtids are about 1/12” and typically develop  within the eggs or larvae of their hosts; Trichogramma wasps are almost microscopic (about the size of the period at the end of a sentence), non- metallic yellowish/brown color, often with wing hairs arranged in rows.  

Ichneumonoid wasps in general are larger than Chalcids, typically ranging from about 1/10” to 1 1/2” long, although one Ichneumon species (Megarhyssa) can be nearly 3” long including its very long tail-like ovipositor. Ichneumons are slender, wasplike, with an abdomen longer than the head and thorax combined, and females frequently have an ovipositor that is longer than their body. Ichneumons vary greatly in size and color, from uniformly yellowish to black, or brightly colored with black/brown or black/yellow markings. Braconids resemble ichneumons, but are usually smaller in size and darker in color, although some species have striking coloration.

Where to find:
Gardeners are more likely to see the results of parasitoids’ activities than the wasps themselves. Chalcid wasps can be found almost everywhere, particularly on flowers, foliage, and in leaf litter, but are rarely noticed because of their tiny size. They may be seen tapping leaf surfaces with their antennae in search of prey, and they leave sickly or dead hosts in their wake. Host eggs parasitized by Trichogramma may turn black as the wasp larva develops within.

Aphid parasitoids are typically found among aphid colonies and leave behind distinctively shaped and colored aphid mummies stuck to the plant; the mummies are swollen, hardened, and hollowed out, often with an exit hole chewed through the skin. Aphelinids leave blackish mummies behind, and Aphidius create tan or golden aphid mummies.

Parasitic Wasps Picture

Picture of Ichneumonid wasp drilling through bark to reach a wood boring beetle grub.
Braconid wasp
Ichneumonoid wasp drilling through bark to reach wood boring beetle grub

Braconids are mostly internal parasitoids, but many emerge to pupate in silken cocoons attached to the outside of their hosts, or separate but close-by. Perhaps the most widely recognized stage of any parasitoid wasp is the cluster of whitish/yellowish rice-like pupal cocoons of Cotesia spp. wasps (Braconids) found on tomato hornworms and cabbageworms. Ichneumon wasps are the largest family of wasps, with thousands of species, and are common almost everywhere. The dead hosts of many Ichneumon wasps shrink and become hard and brittle. The normally greenish or white cocoons of larval hosts infested by some Ichneumon wasps may darken as the wasp develops within. 

Scoliid Wasps
Scoliid wasps, also called digger wasps, are a type of ground nesting wasp. Scoliid wasps are about 5/8 inch long, blue black, with blackish purple wings. There are two yellow stripes, one on each side of the abdomen. Their bodies are fairly hairy and the back part of the abdomen is covered with reddish hairs. They are generally seen flying over the lawn during the day, leaving in early evening. Scoliids are beneficial wasps parasitic on grubs. They are not aggressive and generally do not attack people. They disappear at the end of the season and help control the beetle grubs. Adults are often seen visiting golden rod flowers in late summer.

How to attract and conserve:
Parasitoid wasps are very sensitive to insecticides, so avoid or limit the use of chemical sprays. Most adults feed on plant fluids and sugars, so provide flowering plants that provide 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. Plants with floral nectaries are also important sources of food, as are aphids and other honeydew producing sucking insects. Plants that provide shade on hot summer days are a big help to parasitoids. Trichogramma wasps and those that attack scale insects, filth flies, aphids, and other insects can be purchased commercially for release, but it’s important to procure the right species to control the pest you have.
For more on parasitoid wasps:

Contributors: Mike Raupp, Jon Traunfeld, and Chris Sargent
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IET Departmentof the College of Agriculture and Natural Resources. © 2015.