Parasitic wasp life cycle

This Parasite Wasp Uses Saw-Like Spikes to Cut Its Host Open From The Inside

It’s only the size of a sesame seed, but a newly discovered parasitoid wasp has enormous saw-like spines that could be used for a grisly purpose. Dendrocerus scutellaris could be looking for mates – or tearing its way out of a host’s body from the inside.

We don’t actually know for sure, because the Costa Rican insect hasn’t been observed by scientists in the wild – it’s only known from preserved specimens in London’s Natural History Museum collected in 1985.

But a research team led by PhD candidate Carolyn Trietsch of the Frost Entomological Museum at Penn State was able to piece together details about how the wasp lives based on other species of the Dendrocerus genus, and its morphology, even with specimens over 30 years old.

Because other Dendrocerus wasps are parasitoids, it’s reasonable to conclude that D. scutellaris is also a parasitoid. Not to be confused with parasitic, which means to live in a host without killing it, parasitoids live free of a host, but provide their eggs with one – often the young larvae will feast on the body of their host before they move on.

There are two kinds of parasitoids. Ectoparasitoids will lay their eggs on or near a host, so that the larvae can latch onto the host once they have hatched, and feed on it from the outside.

Endoparasitoids, on the other hand, lay their eggs directly inside the host. The larvae hatch, and devour the host from the inside out. They’ll stay safely tucked inside the host’s body until big enough to face the world, then tear their way open, leaving a dead husk behind them.

It’s difficult to tell which insects D. scutellaris would lay its eggs in, but based on its morphology, it is an endoparasitoid.

In fact, another member of the Dendrocerus genus, D. carpenteri, is what is known as a hyperparasite – it lays its eggs inside another wasp, which itself parasites aphids. So it’s possible that D. scutellaris is also a hyperparasite, the researchers noted.

But there are noticeable differences between D. scutellaris and other Dendrocerus wasps; namely the shape of its mandible, which is straight instead of pointed, and a saw-like comb of spikes on its back.

D. scutellaris; arrow shows spike location (Carolyn Trietsch)

There could be a few different uses for this comb of spikes; the researchers believe it could be used to rub against the inside of the host’s body, using them like a saw to effect an escape.

This is reinforced by the mandible shape, suggesting that the insect could not use its mandible to pierce a hole and escape its gizzardy prison that way.

It sounds like a horrible nightmare, but, according to the researchers, parasitoid wasps can be useful.

“While their lives may sound gruesome, parasitoid wasps are harmless to humans and can even be helpful,” they said.

“Depending on the host they parasitise, parasitoids can benefit agriculture by controlling pest insects like aphids that damage crops.”

Now that they know the wasp exists, entomologists may be able to track it down in the wild to learn more about its lifestyle.

The research has been published in the Biodiversity Data Journal.

Parasitoid wasps may be the most diverse animal group

Estimates about the number of parasitoid species vary widely: most are tiny, so there are many undiscovered species. Some put the cap at 2 million.

“There are probably more species of them than any other kind of animal on Earth,” says Andrew Forbes of the University of Iowa in Iowa City. “If you stop and think about the sheer number of animals on Earth that make a living by laying eggs in other animals and doing all sorts of horrible things to them before consuming them alive, it can give one pause.”

The ancestral parasitoid wasp was probably similar to modern sawflies, which feed on dead wood that has been digested by symbiotic fungi. It may be that one species lost the fungi, so started killing other species that did.

The first order of business for an adult parasitoid is to find a suitable host. She first identifies its preferred habitat, which has a distinct look and smell. Then it’s a matter of laying an egg appropriately.

The wasp may stab the caterpillar in this way a dozen times

To pull this off, most parasitoid wasps have a needle-like organ with which to stab their hosts. For instance, Iseropus wasps thrust their eggs into the larvae of Hemerocampa moths. In a 1929 lecture, the entomologist Robert Cushman described the attack in detail.

Iseropus attacks the Hemerocampa caterpillar late in its life, when it has already built its cocoon in an elm tree. The wasp lands on the tree and feels around for the cocoon.

“Eagerly she tests it all over until, apparently satisfied that it suits her purpose, she takes a firm hold with her feet, arches her abdomen until the stiletto points perpendicularly toward the cocoon, then thrusts it through the meshes,” according to Cushman. The wasp may stab the caterpillar in this way a dozen times.

Other species have to work far harder. Their hosts can fight back.

The startling, metallic Lasiochalcidia pubescens (sometimes L. igiliensis) lays her eggs on the larvae of antlions. That sounds like a monumentally bad idea, because antlion larvae are ferocious predators.

She provokes the antlion to attack her legs

They dig small pits into sand, then bury themselves at the bottom. When a luckless ant enters the pit it slides down the side, and the antlion grabs it in its powerful jaws.

Despite the risks, L. pubescens has evolved to lay her eggs in the antlion’s throat.

She provokes the antlion to attack her legs with its mandibles. When the antlion grabs on, she holds its jaws apart using her muscular legs, and carefully injects an egg into the membrane of the antlion’s exposed throat.

Other parasitoids are far more cautious. They don’t even approach their hosts, and instead leave eggs for them to find.

One family, the Eucharitids, use ant larvae as hosts. They lay their eggs on plants near ant nests. When the wasp larvae emerge, they sit around until they encounter an ant heading back to its nest.

Other parasitoids must venture into truly dangerous territory

Once inside the ant nest, the parasitoid larva attaches to an ant larva. It emits a chemical bouquet so similar to its ant hosts that the ants accept its presence.

Even when it becomes an adult wasp, the ants treat it as one of their own, grooming and feeding it. Before the ant-mimicking odour wears off, the wasp emerges from the nest and leaves.

Other parasitoids must venture into truly dangerous territory to find their hosts. One Japanese species has learned to scuba-dive.

Agriotypus gracilis lays her eggs in the pupae of a caddisfly called Goera japonica. Like all caddisflies, these larvae build protective cases out of silk and sand grains. They also live 6-15 inches underwater.

Larvae that develop inside a host’s body need to obtain some air

To reach one, the female wasp crawls down a plant stem or the side of an exposed stone. She can survive about 14 minutes underwater. Her thick hair forms a sort of bubble that allows her to breathe.

After laying an egg, the female floats to the surface to look for another pupa. Once the larvae hatch, they usually eat up their host.

Larvae like these have it easy. They are outside the host’s body, so can breathe normally. However, larvae that develop inside a host’s body need to obtain some air.

Encyrtus infidus is a parasitoid on the scale insect Lecanium kunoensis (sometimes called Eulecanium kunoense). Many parasitoid larvae develop on one scale larva, and use it as a buffet.

They turn their hosts into their personal bodyguards

At first, the larvae remain attached to the egg from which they hatched by a stalk. This helps them get their air. Later, the scale’s innards get crowded, the larvae start competing for space, and the stalk gets cut. But the wasp larva has a solution.

The scale larva has a network of tubes that supply air throughout its body, called trachea. Each trachea ends in an opening called a spiracle, through which the scale larva exchanges air with its surroundings.

When the parasitoid larva loses connection with its egg, it fuses its spiracles with the tracheal system of the scale and “steals” air until it pupates.

Other parasitoids do more than steal their hosts’ air. They turn their hosts into their personal bodyguards.

Among the leafy undergrowth of Brazil, the parasitoid Glyptapanteles seeks out caterpillars of the moth Thyrinteina leucoceraea and deposits up to 80 eggs. The host caterpillar continues feeding even after the larvae hatch from the eggs.

It swings its head violently from side to side to keep predators at bay

The parasitoids feed on the caterpillar’s insides until they are ready to pupate. Then, almost all of them eat their way out of the still-living caterpillar, and spin a cocoon on a nearby twig or leaf.

However, a few of them stay inside the caterpillar. Their job is to control the caterpillar and make it guard their pupating brothers and sisters.

The beleaguered caterpillar stops eating. It uses its body, which by this point is riddled with holes, as a tent to protect the pupae. It also swings its head violently from side to side to keep predators at bay. Once the wasps emerge, the caterpillar dies.

After pupation, the adult has to emerge from its host’s body. This is the particularly gruesome bit, and resembles nothing so much as the famous chest-burster scene from Alien.

The wasp comes forth very largely covered with body fluids and fragments of tissue of the host

Writing in 1932, Curtis Clausen explained that the adult wasp “must first effect a break in the puparium which surrounds it and then scrape or bite away a varying amount of host viscera or tissue, and finally cut a hole in the heavily chitinised integument…”

All this biting and cutting creates an almighty mess, and “the wasp comes forth very largely covered with body fluids and fragments of tissue of the host.” While the wasp “is readily and quickly cleaned”, the luckless host “dies immediately as a result of this gross mutilation”.

Once this is over with, the adult wasps have one task left in order to complete the cycle. They must mate.

The males do nothing towards the care of the eggs, so their only task is to fertilise females.

In some parasitoids, males fly around searching for chemical signals secreted by receptive females. But sometimes the roles are reversed. In some species of the genus Melittobia, which infects the larvae of solitary bees and wasps, males produce odours that attract females in droves.

Melittobia lays eggs in its hosts just before they pupate. The female stings the host into submission, then lays a cluster of eggs on the outer surface. This simple act launches a bizarre sexual drama.

The larvae feed through the host’s skin, pupate and become wasps. Almost all of them are female. If the host is large enough, the eggs develop rapidly into short-winged females. These lay even more eggs, draining the host completely.

Brothers fight one another for access to their emerging sisters

Eggs laid later develop into long-winged females, which chew through the host cocoon and fly out to find more victims.

Meanwhile, within the cocoon the few blind males begin courting the females. They raise and lower their legs, stroke the females with their legs and antennae, and flutter their wings, according to a 2008 study by Robert Matthews of the University of Georgia in Athens.

Competition among the males is fierce. “Brothers fight one another for access to their emerging sisters,” according to Matthews. Sometimes, all the males wind up dead.

The females are then left without mates. So they make some more.

A female wasp finds a new host and lays a few eggs, usually less than ten. All these eggs develop into males.

She stays around, stroking her sons with her antennae, watching them grow into pupae and then emerge as adults. While most female parasitoids abandon their offspring as soon as the legs are laid, Melittobia females can be positively maternal.

When the first adult male emerges, the female mates with him. Having been fertilised by her own son, she lays a full clutch of eggs on the same host.

If that strikes you as a rather peculiar way to reproduce, brace yourself, because Copidosoma floridanum has gone one better. “These wasps invented genetic cloning long before humans even had an inkling of the idea,” says John Werren of the University of Rochester in New York.

A female C. floridanum lays a single egg in the egg of a noctuid moth. The host egg hatches and the host larva grows, evidently unaware of the 1500-2000 parasitoid larvae inside it. Once the host reaches its final larval stage, the parasitoid larvae start eating it and develop rapidly.

By killing their half-brothers, they make food for their identical sisters

“As the embryos begin to develop, they divide, and divide, each egg making identical genetic copies of itself,” says Werren. “This is twinning gone berserk.”

Among the genetically identical daughters, a subset become “soldiers”. They develop faster than their sisters and defend the host. If another wasp approaches, they attack her and eat up any eggs she may manage to deposit.

The soldier females also do something that seems counter-productive: they seek out their brothers and kill them. They do it because they are genetically identical to their sisters, but only share half their DNA with their brothers. By killing their half-brothers, they make food for their identical sisters.

A few males escape the slaughter. They eventually mate with the fertile females that emerge later, and the cycle goes on.

You may feel that parasitoids are irredeemably horrible. But the odd thing is, they are rather useful, largely because each parasitoid is so specific in its choice of host.

Each year, one-fifth of the world’s crops are eaten by insect pests: for instance, about 25% of India’s rice is lost to insects. Parasitoids can control these pests, without the need for damaging insecticides.

We often don’t understand the intricate relationships between parasitoids and their hosts

Back in 1888, the cottony cushion scale insect was decimating the citrus fields of California. So farmers brought in a “now legendary predator”, the Australian parasitoid vedalia beetle. It quickly suppressed the cushion scales, which even now are still kept in check.

By the end of the 20th century, the world had seen more than 3600 purposeful introductions of parasitoids against more than 500 pests in almost 200 countries and islands.

But many haven’t been as successful as the vedalia beetle. Only 30% of the parasitoids introduced were able to establish themselves successfully, and of these, only 36% have completely controlled their target pests.

This isn’t a failure on their part, but rather on ours. In spite of more than a century of work, we often don’t understand the intricate relationships between parasitoids and their hosts. The wild diversity of these insects, it seems, is still beyond our grasp.

Parasitoid Wasps (Hymenoptera)

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Parasitic wasp emerging from aphid mummy


Hornworm parasitized by a braconid wasp


Ichneumonid wasp

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Parasitoid exit holes in scale covers


Scoliid wasp

The Importance of Parasitoid Wasps

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.

The Biology of Parasitoids

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.

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.
What do parasitoids feed 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 (see photo above)

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: Parasitoid wasps range in size from very tiny (some can fly through the eye of a needle) to about 1 ½ inch long. They are not interested in humans so therefore do not sting.

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.
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.

Beneficial Insect: Parasitic Wasp

These tiny, non-stinging wasps are some of the most beneficial insects in the garden and are known to parasitize over 200 species of pests. Though there are tons of different species of parasitic wasps, they all work by preying upon one or more pest insects.

Parasitic wasps generally measure between 1/32”-1/2”, though a few are larger. They have slender, elongated antennae and are found throughout the U.S.

Life Cycle

Most female parasitic wasps lay eggs inside or on host insects (though some may also lay eggs inside prey eggs). The wasp eggs hatch and consume the prey, beginning with non-essential tissue first, then proceeding to vital organs- eventually killing the host. Some species then pupate in external cocoons, which remain attached to the host’s body. Other species pupate within the prey and emerge as adults. Most species have a high reproductive capacity and develop rapidly.

Pests They Control

Depending on the species of parasitic wasp, they help rid your garden of: aphids, beetle larvae, bagworms, cabbage worms, Colorado potato beetle, corn ear worms, cucumber beetles, cutworms, gypsy moth caterpillars, Japanese beetles, leaf-miners, mealybugs, Mexican bean beetles, moth caterpillars, sawfly larvae, scale, squash vine borers, tent caterpillars, tobacco budworm, tomato hornworm and whiteflies. Wow!

How to Attract and Keep Them

  • Provide them a diversity of plants with single blossomed flowers and flowering herbs since they consume nectar and pollen. This will help welcome them to your garden. Choose: allium, alyssum, cosmos, dill, fennel, lemon balm, thyme, statice, yarrow and zinnia.
  • Purchase parasitic wasps and release them into your garden. Their eggs usually arrive on cards containing anywhere from several hundred to several thousand eggs. The cards are then placed in the garden or greenhouse, where they will hatch.

More About…

Technically speaking, ‘Parasitic Wasps’ are not actually parasites – they are parasitoids. This is because a true parasite is something that lives at the expense of its host but doesn’t actually kill it, whereas parasitoids nearly always kill their host. In general though most people still use the term ‘Parasitic Wasps’.

Sources

CCF staff
Good Bug, Bad Bug by Walliser 2008
The Organic Gardener’s Handbook by Ellis and Bradley 1996

Gardening Tips Pests beneficial insects bugs insects organic pesticides

A parasitic wasp (Hymenoptera: Braconidae), laying eggs in a cotton bollworm. Photo by W.Sterling.

Common Name: Parasitic wasp
Scientific Name: Varies
Order: Hymenoptera

Description: Adults of many species are very small (ranging from 1/100 to 3/4 inch long) and often go unnoticed. They vary in shape and coloration but usually have long, thread-like (filiform) antennae or they may appear elbowed, clear or colored wings with characteristic venation and a narrow “waist” between the thorax and abdomen. Females of many species have a spine-like egg-laying structure (ovipositor) at the tip of the abdomen. Larval stages are usually not observed unless they are dissected from hosts (internal parasites) or detected on the host (external parasites). They are usually cream colored, legless and tapered at both ends. Occasionally, caterpillars are observed with white silken cocoons of parasites (Braconidae) attached to their bodies. Stages of immature whiteflies parasitized with Encarsia formosa (Gahan) (Chalcidoidea, Encyrtidae) are darker or black when late in the parasite development compared to yellowish to creamy healthy ones. Aphids are hosts for species in the subfamily Aphidiinae (Braconidae) such as Aphidius spp. and others in the family Aphelinidae (Chalcidoidea). Parasitized aphids, called “aphid mummies”, appear puffed up, brown and hardened. The adult parasitic wasps chew a round hole in the abdomen to emerge.

Not all species in these groups are parasitic. For instance, the tiny (1/32 inch) fig wasp, Blastophaga psenes Linnaeus (Chalcidoidea, Agaonidae) and related species are essential pollinators of certain fig varieties, with larvae that develop only inside a gall in wild Caprifig flowers.

Life Cycle: Biology and details of development vary with species. Adult wasps emerge from pupae and females seek suitable host insects into or on which to lay eggs singly or in clusters. Usually, a larva hatches from an egg and develops through several stages (instars) before forming a pupa. However, some parasitic wasps, such as Copidosoma spp. (Chalcidoidea, Encyrtidae), undergo a process called polyembryony, whereby an egg inserted into a host divides and gives rise to hundreds of larvae. Most parasitic species have high reproductive capacity and develop rapidly. Several generations can sometimes develop during a single generation of the host, although some species have only a single generation per year.

Habitat, Food Source(s), Damage: Mouthparts of larvae and adults are for chewing. Larval stages of parasitic wasps develop inside or outside of a single host during one or more of the host’s developmental stages (egg, larvae, pupae or adult). Those that kill their hosts are called parasitoids. Most insect groups (including aphids, beetles, caterpillars, flies, sawflies, scale insects and true bugs) are attacked by parasitic wasps. Many species are host specific, developing in one or a limited number of related host species. For example, the species of Trichogramma (Chalcidoidea, Trichogrammatidae) primarily parasitize the eggs of caterpillars. Adults of some parasitic wasps feed on other insects to obtain energy to seek other suitable hosts into or on which to lay eggs. This behavior is called host feeding and also helps reduce populations of host insects. Parasitic wasps may emerge from host insects if they are reared. A number of parasitic wasp species are commercially available from insectaries and are purchased and released in augmentative biological control programs. Other species have been imported from other countries from which pests have been accidentally introduced without their natural enemies and released to reintroduce the natural enemy with its host, a practice called importation, or “classical” biological control and which occasionally results in sustained suppression.

Pest Status: Numerous tiny wasps parasitize various stages of other insects; many are beneficial parasites (parasitoids) that kill other insect pests, while some are considered harmful because they parasitize parasites (a condition called hyperparasitism) or other beneficial insects; all are medically harmless.

For additional information, contact your local Texas A&M AgriLife Extension Service agent or search for other state Extension offices.

Literature: Arnett 1985; Borror et al. 1976; Metcalf et al. 1962; Swan & Papp 1972.

Parasitic Wasps

Description of parasitic wasps

Parasitic wasps, that is, wasps that live part of their lives as parasites inside other insects comprise one of the largest insect groups. There are several hundred species of these wasps found outdoors in the “real world.” This very large group is extremely variable in size and color, but most are small to medium-size and brown or black in color.

Life cycle of parasitic wasps

The parasitic wasps are not the stinging, social wasps more commonly encountered in and around houses (paper wasps, yellowjackets and hornets). Parasitic wasps generally do not sting and are not pests except for their occasional, annoying presence as accidental invaders in the house. They are harmless and beneficial because they provide some biological or natural control of other insects. Parasitic wasps use their stinger to lay eggs inside other insects and can sting if threatened or handled.

Damage caused by parasitic wasps

Parasitic wasps found indoors are accidental invaders that have crawled or flown into the house by accident or were carried in with firewood. The wasps may have been living inside the wood boring insects that were inside the firewood and emerged when the wood was warmed by the heat in the house.

Management of parasitic wasps

There is little that can or needs to be done to control parasitic wasps indoors. Carefully picking or vacuuming them up for removal is usually sufficient as is swatting the occasional invader. Parasitic wasps are seldom a persistent problem and special controls are usually not necessary.

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