Samurai wasp
The samurai wasp is a tiny, shiny black parasitoid, 1–2 mm long, that lays its eggs inside the eggs of the brown marmorated stink bug. StopBMSB describes it as stingerless. In Asia it kills 60–90% of the pest's eggs by StopBMSB's figure, or up to 80% by Cornell's, usually attacking every egg in a mass. It reached the US on its own, was first found in 2014, and by 2021 was known from at least 13 states, mostly in the Northeast and on the West Coast by Cornell's account. You cannot buy it: Cornell's fact sheet answers "Commercially available? No." Deliberate movement between states has been restricted, so any redistribution is done by researchers within their own state. The gardener's job is to conserve it. Leave dark grey or black stink bug egg masses on the leaf, and avoid broad-spectrum sprays.
At a glance
- Kind
- Wild — not sold; conserve, do not buy
- Adult
- 1–2 mm; shiny black, legs black and yellow-brown
- Attacks
- Stink bug eggs, usually the whole mass
- Egg to adult
- About 10–14 days inside the host egg
- Kills in Asia
- Up to 80% (Cornell) or 60–90% (StopBMSB) of BMSB eggs
- In the US since
- Found 2014; at least 13 states by 2021
How it works
A female samurai wasp finds a brown marmorated stink bug egg mass, usually on the underside of a leaf, and lays one egg in each stink bug egg. The wasp larva kills the developing bug before it can hatch, and a new adult wasp emerges after about 10–14 days. In a typical mass of 14–28 eggs, one to three of the wasps that emerge are males. They come out first and wait to mate with the females, which then go looking for more egg masses.
USDA scientists found it in Asia and were testing it in quarantine for a deliberate release. Before approval came, surveys in 2014–2015 found it already living in the US, in populations genetically different from the quarantine colony. USU counted at least 13 states by 2021, with finds in Canada and Europe as well. Within some of those states, researchers have reared and released it to spread it further. Michigan released 7,200 wasps at 16 sites in 2019–2020 and recovered only 49 afterwards; Virginia detected it at or near 7 of 8 release sites by 2022.
In laboratory tests it parasitised some native stink bugs too, so it is not strictly specific. In a Washington field test, only 0.4–8% of native stink bug eggs produced wasps, against 77% of brown marmorated stink bug eggs, and the authors judged the impact on the native species probably minimal. Research on how much it cuts brown marmorated stink bug numbers in North America is still under way.
What it is confused with
The adults are too small to watch for; what a gardener sees is the egg mass. Knowing a parasitised mass from a healthy one is the useful skill.
- PestHealthy brown marmorated stink bug eggs — Light green eggs in a cluster under a leaf, hatching in 5–7 days. As healthy eggs develop, each shows a black triangle on its cap. Parasitised eggs turn dark grey, dark brown or black with no triangle, and wasps rather than nymphs come out about two weeks later.
- Also goodAnastatus wasps — Generalist egg parasitoids, 3–5 mm, larger than a single stink bug egg and ant-like at first glance; females often have a white band on the wings. Trissolcus wasps are no bigger than one egg. Both kill stink bug eggs, so leave both.
- Also goodNative Trissolcus and Telenomus wasps — Also tiny and all black; USU says the species can only be told apart under a microscope. USU rates the native ones moderately successful at best and unlikely to control the pest.
- Also goodGnats and small ants — USU notes these stink bug parasitoids are easily mistaken for small gnats or ants. USU's advice for a wasp seen on an egg mass is to treat it as beneficial and leave it.
How to keep it around
Nobody can order samurai wasps into a garden. Where they already occur, these steps help them survive and breed:
- Leave dark grey or black stink bug egg masses on the plant; USU prefers leaving any egg mass with a wasp on it rather than destroying it
- Avoid broad-spectrum insecticides. Cornell calls the wasp very susceptible, and bifenthrin residues cut female survival in orchard tests
- Keep wooded, weedy or undisturbed edges nearby, which Cornell says may give nectar and winter shelter
- Grow buckwheat, dill or cilantro: in lab tests they extended median wasp survival by 15, 17.5 and 3.5 days over water. Sweet alyssum, marigold, crimson clover, yellow mustard and phacelia did not
Pesticide sensitivity
Cornell's fact sheet calls the samurai wasp very susceptible to insecticides and generally incompatible with them. In USDA-ARS apple orchard tests, bifenthrin residues significantly lowered female survival, and bifenthrin, thiamethoxam + lambda-cyhalothrin and methomyl reduced emergence in some treated areas. Open the compatibility matrix →
When to choose something else
Conserving the samurai wasp is long-term help, not a fix this season. Look elsewhere when:
- Stink bugs are coming into the house in autumn — seal cracks and gaps before they arrive; see brown marmorated stink bug
- Nymphs and adults are feeding on tomatoes or peppers now — use row covers and hand-pick into soapy water; the wasp only kills eggs
- You want to buy something — no commercial release of the samurai wasp exists; the spined soldier bug (Podisus maculiventris) is sold, but it is a generalist
Samurai wasp by target
What it eats and when it turns up differ by pest. Each page below is the picture for one target.