For non-aggressive wasps I've had great luck spraying nests with this Spectracide wasp remover in the evening. For a nest up high in an eave, soffit, or tree, this Gotcha pole adapter clamps onto the can so you can spray from the end of an extension pole and treat the nest from 10+ feet away instead of standing right under it. And for anything aggressive I wear this ridiculous-looking upper torso beekeeping suit and keep my distance. It seems silly, but trust me, I learned the hard way.
Most people picture a yellow jacket when they hear the word “wasp” — a stocky, yellow-and-black insect defending a papery nest under the eaves. But the overwhelming majority of wasp species on Earth look nothing like that and behave nothing like that. They are parasitic wasps: mostly small, mostly solitary, almost entirely uninterested in you, and collectively responsible for more insect pest control than every bottle of insecticide ever sold.
A parasitic wasp is a wasp whose larvae develop by feeding on another insect or spider. The adult female locates a suitable host, lays one or more eggs on or inside it, and her offspring consume the host as they grow. It is a grim way to make a living, and it is also the single most important natural check on plant-eating insects in gardens, farms, and forests worldwide.
If you garden, parasitic wasps are already working on your property right now, almost certainly without your noticing. This guide covers what they are, how to recognize them and the evidence they leave behind, whether they pose any risk to people or pets, and — most usefully — how to avoid accidentally killing the ones you have.
For a shorter introduction to this group, see our brief overview of parasitic wasps in the backyard. This page is the full treatment.
Nearly every insect commonly called a “parasitic wasp” is technically a parasitoid, and the difference is worth understanding because it explains their value as pest control.
This is why parasitoid wasps function so effectively as biological control agents. Every successful parasitoid larva represents one dead pest insect, and each adult female may parasitize dozens or hundreds of hosts over her lifetime. A predator like a ladybug eats many aphids but produces relatively few offspring; a parasitoid converts pests into more parasitoids at close to a one-to-one ratio.
You will see both terms used. “Parasitoid wasp” is the precise term entomologists prefer; “parasitic wasp” is the common usage and the one most gardeners search for. Throughout this guide they refer to the same insects.
Two groups get filed here incorrectly:
Gall wasps (Cynipidae) cause oaks and roses to grow the round balls and woody knots you have probably noticed on branches. Their larvae feed on plant tissue, not on other insects — they are plant parasites, not parasitoids. Ironically, galls are a major hunting ground for true parasitic wasps, which drill into them to reach the larva inside. See our gall wasp guide for the full story.
Fig wasps (Agaonidae) are pollinators locked into an ancient mutualism with fig trees. Some related fig-associated wasps are parasitoids, but the pollinating species are not. Our fig wasp guide explains that relationship.
Hunting wasps — mud daubers, spider wasps, cicada killers, and tarantula hawks — occupy a middle ground. They paralyze a single prey animal and their larva consumes it, which is functionally parasitoid-like. The difference is that they carry the prey to a nest they built rather than leaving the egg where the host lives. Most references call these predatory or provisioning wasps rather than true parasitoids.
Female parasitoids are extraordinary searchers. They locate hosts using a combination of chemical cues — volatile compounds released by damaged plants, the scent of host frass (droppings), pheromone traces — and, in wood-boring specialists, vibration detection through the substrate. A female giant ichneumon wasp can find a horntail larva burrowing two inches inside solid heartwood by tapping the bark with her antennae.
Many plants actively recruit them. When a caterpillar chews a leaf, the plant releases volatile compounds that parasitic wasps have evolved to home in on. Your tomato plants are, in a real sense, calling for help.
The female uses her ovipositor — a slender egg-laying tube at the rear of the abdomen — to place eggs on or inside the host. Ovipositor length varies enormously, from microscopic to the four-inch filament of a Megarhyssa. This structure is the source of most homeowner alarm; see our guide to wasps with long tails if that is what brought you here.
A solitary parasitoid produces one adult per host. A gregarious parasitoid produces many — this is why you sometimes see dozens of tiny cocoons on a single caterpillar. And hyperparasitoids parasitize other parasitoids, adding a layer of complexity to the system that also, in agricultural settings, occasionally undermines biological control.
There are far more parasitic wasp species than there are species of birds, mammals, reptiles, amphibians, and fish combined. Estimates of the total run into the hundreds of thousands, with only a fraction formally described. Realistically, a homeowner will run into members of a handful of groups.
Roughly 25,000 described species worldwide, with credible estimates that the true total is several times higher. Ichneumons are typically slender, long-antennaed, and between a quarter-inch and two inches long, often orange, black, or banded. Hosts are usually caterpillars, beetle larvae, and sawfly larvae.
Some ichneumons are nocturnal and turn up at porch lights — the long-legged, orange, hunchbacked Ophion species are a classic “what is this thing in my bathroom?” insect. Full details in our ichneumon wasp guide.
Around 20,000 described species, generally smaller and stockier than ichneumons, most under a quarter-inch. Braconids are the workhorses of garden biological control. Two encounters are almost universal among vegetable gardeners:
Our braconid wasp guide covers this family in depth.
An enormous superfamily of very small wasps, most under a tenth of an inch, many metallic green, blue, or black. This group includes the wasps most likely to show up inside your home — usually because they emerged from stored-product pests in the pantry, from galls on firewood, or from insect eggs on cut flowers and houseplants.
Chalcidoids include the smallest insects known. Fairyflies (Mymaridae) parasitize insect eggs, and the smallest described male fairyfly measures well under a fifth of a millimetre — smaller than some single-celled organisms. See our chalcid wasp guide.
A handful of parasitoid families sit within Aculeata, the stinging wasps, because their ancestors’ ovipositors were modified into stingers. These are the exceptions to the “parasitic wasps are harmless” rule:
You are far more likely to notice the evidence than the wasp. Learn these four signs:
| What you see | What it means |
|---|---|
| Swollen tan, bronze, or black aphids mixed into a colony, each with a round hole | Aphid mummies — braconid or aphelinid parasitoids are established |
| A hornworm or other caterpillar carrying dozens of small white cocoons | Gregarious braconid cocoons. The caterpillar is done feeding |
| Whitefly nymph scales turned black on leaf undersides | Encarsia-type parasitism of whiteflies |
| Tiny round exit holes in galls, seed pods, cocoons, or dead insects | A parasitoid has completed development and left |
The single most common gardening mistake in this whole subject is destroying a cocoon-covered hornworm. It feels like removing a pest. It is actually removing a pest-control factory: the caterpillar has already stopped eating, and each of those cocoons is a wasp that will hunt down the hornworms you have not found yet.
For the overwhelming majority of species, no — and the reasons are structural, not behavioral.
They do not defend nests. Aggressive stinging in yellow jackets, hornets, and paper wasps is colony-defense behavior. Parasitic wasps are almost all solitary, build nothing, and have nothing to defend. There is no equivalent of stumbling onto a nest.
Most cannot meaningfully sting. In the big parasitoid families the ovipositor is a specialized egg-laying and drilling tool. It is not connected to a venom system capable of affecting a mammal. The giant ichneumon’s dramatic four-inch “stinger” cannot sting you at all.
A few can jab if handled. Some ichneumonids — Ophion and Netelia among them — will give a brief, mild prick if you close your hand around one. It is comparable to a pinprick and is not medically significant for people without insect-venom allergies. The aculeate parasitoids listed above are the real exception, and velvet ants in particular should simply be left alone.
They cannot parasitize humans, pets, or livestock. Parasitic wasps are host-specific to insects and spiders. No wasp anywhere lays eggs in mammals. The horror stories that circulate about insects developing under human skin involve botflies — which are flies, not wasps, and which are not a concern in most of the United States.
If you have been stung by something in the yard and need to work out what it was, our guide to wasp identification will narrow it down, and the odds are strongly against it having been a parasitoid.
Commercially reared parasitoids are a mature technology in greenhouse and field agriculture, and several are available to home gardeners:
Large-scale programs use the same principle. The samurai wasp (Trissolcus japonicus) is being studied and deployed against the invasive brown marmorated stink bug, and several tiny parasitoids are being released against emerald ash borer in North American forests.
A realistic expectation for a home garden: purchased parasitoids work best in enclosed spaces like greenhouses and hoop houses, where the wasps cannot simply disperse. In an open backyard, encouraging the parasitoids you already have is usually more effective and much cheaper than buying more.
Stop spraying broad-spectrum insecticides. This is the entire ballgame. Pyrethroids, carbaryl, and neonicotinoids kill parasitoids as readily as pests, and because parasitoid populations rebuild more slowly than aphid populations, spraying frequently produces a worse infestation two weeks later. If you must treat, use the most targeted product available, spot-treat rather than blanket-spray, and apply at dusk.
Plant small, shallow flowers. Adult parasitoids do not feed on hosts — they feed on nectar, pollen, and honeydew, and their short mouthparts cannot reach into deep tubular flowers. The best nectar sources are umbellifers and small composites: dill, fennel, cilantro, parsley left to bolt, yarrow, sweet alyssum, buckwheat, mountain mint, and goldenrod. A few square feet of flowering dill does more for parasitoid populations than any purchased release. Our guide to plants around the home covers the other side of that coin — which plants discourage the stinging social wasps you may not want near the patio.
Tolerate a low level of pests. Parasitoids need hosts. A garden scrubbed completely clean of aphids cannot support the wasps that keep aphids in check, which is why obsessive spraying tends to lock gardeners into a cycle of dependence.
Leave mummies, cocoons, and leaf litter alone. Many parasitoids overwinter in plant debris, in mummified hosts, or in the cocoons of their hosts. Aggressive fall cleanup removes next season’s workforce.
Finding small wasps at a window in winter usually has a mundane explanation. They have emerged indoors from something that was already in the house:
None of these wasps reproduce in your home, bite, sting, damage anything, or feed on household materials. They will die at a window within days. Vacuum them up if you want, but the productive response is to find and remove the host material.
Do parasitic wasps sting? The great majority cannot sting in any way that affects a person. A few ichneumonids will give a mild prick if handled, and the aculeate parasitoids — velvet ants especially — can sting painfully. None of them defend a nest or pursue people.
What are the white things on my hornworm? Braconid wasp cocoons, not eggs. The caterpillar has already been consumed from the inside and has stopped feeding. Leave it on the plant.
I killed the parasitized caterpillar. Did I ruin anything? You destroyed about fifty future wasps and gained nothing, since that caterpillar was no longer eating. Worth knowing for next time, not worth losing sleep over.
Are parasitic wasps good or bad? Overwhelmingly good from a homeowner’s perspective. They suppress caterpillars, aphids, whiteflies, scale insects, beetle grubs, leafminers, and flies without any input from you, and they carry essentially no sting risk. The main exception is that a few species parasitize honey bees’ and native bees’ nests, which matters to beekeepers more than to gardeners.
How long do they live? Adults of most species live from a few days to a few weeks — long enough to find hosts and lay eggs. Larval development inside the host takes anywhere from days to, in wood-boring specialists, more than a year. Our guide to wasp larvae covers the developmental stages in more detail.
Should I buy parasitic wasps for my garden? In a greenhouse or hoop house, often yes, with correct timing. In an open yard, usually no — the wasps disperse, and the money is better spent on nectar plants and on not spraying.
What eats parasitic wasps? Birds, spiders, dragonflies, robber flies, and hyperparasitoid wasps. See what eats wasps for the broader picture of wasp predators.
Parasitic wasps are the quiet majority of the wasp world. They are small, solitary, nest-less, host-specific, and — with a short list of exceptions you can learn in five minutes — incapable of hurting you. They are also the most effective insect pest control available to a home gardener, and they are free.
The practical takeaways are short. Learn to recognize aphid mummies and white cocoons on caterpillars, and leave both alone. Let some dill, fennel, and alyssum flower somewhere in the yard. Put down the broad-spectrum insecticide, which kills your allies faster than it kills your pests. And when a strange long-tailed insect turns up on a dead tree or at a porch light, take a look before reaching for a swatter — the odds are very good that it is on your side.
This page is the hub for parasitic and parasitoid wasps. For individual families and species:
Broader background: