Parasitoids

Braconid wasp (Spathius sp.), a type of parasitoid. Photo: David Cappaert, Bugwood.org

Updated: August 16, 2023

What are parasitoids?

Parasitoids are small insects whose immature stages develop either within or attached to the outside of other insects, referred to as hosts.
Parasitoids eventually kill the host they feed on, as opposed to parasites like fleas and ticks, which typically feed upon hosts without killing them.
There are two general categories of parasitoids: endoparasitoids, which hatch within the host from eggs or larvae laid there by an adult female, and then feed and develop inside the host; and ectoparasitoids, which are fastened to the outside of the host and feed through the host skin, sucking out body fluids.
Most parasitoids are either wasps and bees (Hymenoptera) or flies (Diptera), although a few species of beetles, twisted wing insects, moths, and other insects have been identified as parasitoids.
Although some parasitoids are about the same size as their hosts, many are so small that they are easily overlooked by gardeners.
Parasitoid wasps are often as small as the period at the end of a sentence.
Adult female parasitoids attack all major orders of insects, laying their eggs in or on hosts, or on foliage where they will be consumed by a host.
Adult parasitoids fly freely about and feed primarily on honeydew, pollen or nectar.
The immature parasitoid stages, however, are the lethal ones, feeding directly on and killing their hosts. Young parasitoids may feed on non-vital tissues first, so that the host is not initially killed, but ultimately the host will die without a chance to produce the next generation of pests.
The host stages typically parasitized are eggs, larvae, nymphs and pupae.

Types of parasitoid insects

Wasps

Flies

Beneficial Nematodes

Why are parasitoids so important?

Among all natural enemies, perhaps, no other group is more important to maintaining pest insect populations below damaging levels than parasitoids.
Although individual predators tend to kill many prey, while each parasitoid kills only one at most, the sheer number and efficiency of parasitoids is so great that the cumulative impact often outweighs that of predators.
Their efficiency is due to several factors. Parasitoids tend to specialize, attacking only one type host, and this specificity makes them very effective against their target host.
Their life cycles are closely synchronized with that of their prey, so that parasitoids are out hunting when their hosts are also active in the garden. Parasitoids are efficient at finding hosts: they are very active, and adult female parasitoids often use host–related chemical cues to help them find prey, even when population densities are low.
Some parasitoids also get a head start on finding prey in early spring when they are in scarce supply by overwintering in their hosts’ bodies.

How do you find parasitoids?

Due to the small size of adult parasitoids, and because most early-stage parasitoids are hidden inside the host for much of their development, people rarely notice them.
An alert gardener may notice adults searching plants for hosts, examining potential insects with their antennae, mouthparts, or ovipositor.
Some parasitoid eggs or immature stages may be attached to the outside of a host, in ready view of the observant gardener. A common example is a parasitized tomato hornworm.
Many parasitoids emerge from the host body to pupate, and their cocoons may be found beneath host plants or on foliage near dead prey. But the most likely way to determine if your garden is home to parasitoids is to look for symptoms in the host insects.
Although parasitized hosts may appear to be healthy for a time, there are clues that their days are numbered if you know what to look for.

Photos of parasitized insects

hornworm larva covered with parasitic wasp cocoons

Parasitized hornworm (Braconids/Ichneumonoid wasps)

White tachinid fly eggs on Japanese beetle. Photo: Whitney Cranshaw, Colorado State University, Bugwood.org

Parasitized "aphid mummies". Photo: David Cappaert, Bugwood.org

Parasitoid exit holes in scale covers. Photo: John Davidson, University of Maryland

Characteristic symptoms of parasitized host insects

Hosts often change color because they are killed, normal development has been disrupted, or the immature parasitoid developing inside is visible through the host’s skin.
Parasitized eggs may also become much darker.
Some hosts may have the eggs, larvae, or pupae (cocoons) of ectoparasitoids visibly attached to the outside of their bodies.
“Mummies” of parasitized hosts may be found, especially aphids, caterpillars, and some scale insects. Mummified bodies become swollen with a hardened outer skin and hollowed-out interior, and remain attached to leaves. Aphid mummies often turn black or brown, and the mummies of soft scales are blackened.
Exit holes left by emerging new adult parasitoids are often seen in the bodies of dead hosts. Parasitoid emergence holes are usually round and relatively smooth-edged, unlike the ragged, irregular holes and tears left by predators.
Unusual host behavior may indicate parasitism: some hosts leave the plant part they normally feed on and move to another location, many simply stop feeding. Hosts continue to cling to plant leaves or stems, but barely move about or feed.

How can I help parasitoids flourish in my garden?

Include many different types of plants in the garden and landscape to attract parasitoids; low plant diversity leads to fewer parasitoids present.
Parasitoids are tiny and require shallow flower heads with easy access for their minuscule tongues to reach the nectar; provide for their needs by including flowering Umbelliferae plants (e.g., coriander, dill, parsley, wild carrot) in the garden.
Use local native flowering plants, trees, and shrubs in the landscape to best meet the needs of local parasitoids.
Plant an early-blooming cover crop in the fall in the garden to provide nectar and pollen in spring.
Choose plants with different flowering times to provide nectar and pollen all season.
Avoid using pesticides, especially when plants are in bloom.
Learn more about gardening for natural enemies.

Contributors: Mike Raupp, Jon Traunfeld, and Chris Sargent

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