7 Things to know about samurai wasps, a natural enemy of brown marmorated stink bugs

The brown marmorated stink bug (BMSB) is controlled in its home region of Asia by samurai wasps. The wasps are now found in the United States and could help control BMSB in Michigan.

samurai wasp on stink bug eggs
The tiny samurai wasp parasitizes eggs of brown marmorated stink bug. Photo by Chris Hedstrom, Oregon State University.

The samurai wasp with the scientific name of Trissolcus japonicus was found in Michigan for the first time in 2018. This tiny parasitic wasp has great potential to provide effective biological control of brown marmorated stink bugs (BMSB) that have invaded Michigan orchards, crops and homes. In 2019, Michigan State University researchers started a rearing and release program of samurai wasps at Michigan State University. We are boosting population sizes and increasing its distribution across Michigan to speed up control of BMSB. This program has generated a lot of interest in the media and online recently, and with that there are some common misconceptions about what these developments mean. I wanted to answer some frequently asked questions about the samurai wasp and the brown marmorated stink bug.

1. How did these wasps get to Michigan? Where else can they be found?

The samurai wasp is native to northeast Asia where BMSB comes from. It was not intentionally released in North America, but got introduced accidentally, likely the same way as BMSB by being stowed away in shipping containers, planes or any other means of transportation. It was first found in Maryland in 2014, and in 2015 in Washington in the western United States. Based on its patterns of discovery, the wasp appears to follow BMSB distribution and has been moving west from Maryland. In 2017, it was found in Ohio and the following year we found it in Michigan. Now, the samurai wasp has been found in 12 states (Pennsylvania, New York, New Jersey, Maryland, Delaware, West Virginia, Virginia, Ohio, California, Michigan, Utah and Washington), the District of Columbia and in British Columbia, Canada. The densities of samurai wasps are very low in most places and this is why several states, for example, New York, Ohio and Washington, have started similar rearing and redistribution programs of samurai wasps as is Michigan.

2. What is an invasive species and is the samurai wasp invasive?

The samurai wasp is a non-native species and not considered an invasive species. Invasive species are those that reach high densities locally or regionally and result in economic damage or other harm to the environment.

3. Are they going to attack any species other than BMSB?

The samurai wasp is adapted to parasitize stink bugs only. It cannot attack any other species. However, there is a concern it might attack stink bugs native to Michigan. In the laboratory, it was able to attack native stink bugs when given no other choice, however, it shows a strong preference for BMSB. Right now, BMSB is the most prolific stink bug species in most habitats, so it will likely be the primary target of the wasp. It is well-known that no-choice laboratory tests often overestimate impact that will actually occur in the field. In nature, the behavior of parasitoids is influenced by many things that cannot be replicated in a laboratory setting including climatic, biological and environmental factors.

Wasps use many different cues to locate hosts, such as chemicals emitted by stink bugs and the plants the stink bugs feed on. A recent study found when the wasps were exposed to these types of chemical cues, in every single case they chose to attack BMSB and not the native stink bug species. So, when given a choice in the field, it is likely they will attack BMSB, their primary host that they are adapted to and which is our most common stink bug.

4. Can I find samurai wasps in my orchard or garden? Will they sting people?

BMSB on apple
Brown marmorated stink bug feeds on an apple. BMSB are pests of many crops and are a nuisance in people’s homes. Photo by Bill Shane, MSU Extension.

These wasps are tiny, only 1/8 of an inch, about the size of a sesame seed. They cannot sting people and most people will never see them in nature. They track BMSB eggs flying around in tree canopies, hiding among leaves of crops or taking shelter under barks of trees. They are hard to find and see with the naked eye. Scientists use so-called sentinel eggs to capture them. The sentinel BMSB eggs are usually laid in lab colonies and then affixed to leaves and left for a few days in habitats where BMSB and thus the wasps likely reside. If samurai wasps find these sentinel eggs, they can parasitize them by laying their own eggs inside the stink bug eggs. The sentinel eggs are taken back to the lab and monitored to see if wasps emerge. This is how samurai wasps were detected in Michigan and elsewhere.

5. Why do we think the samurai wasp will be effective at controlling the brown marmorated stink bug?

The samurai wasp is the parasitoid that keeps BMSB populations down in Asia in its native range. There are other parasitoids attacking BMSB in Asia, but the samurai wasp is the one with the highest attack rates, parasitizing 60-90% of BMSB egg masses. Because of its effectiveness in Asia, we assume it will be a good biocontrol agent here. It is adapted to BMSB so it will respond to the cues and chemical signatures that the stink bug leaves on the trees and crops and can find BMSB in many different habitats.

Another trait that usually characterizes successful biocontrol agents is the ability to have several generations during a growing season. BMSB has only one generation in Michigan, but lays eggs over an extended period of time, probably from June to August. Wasps can complete one generation in two weeks so they can keep attacking BMSB during the entire season as the new generations emerge.

Finally, because BMSB is a season-long pest and can feed on hundreds of plant species including many adjacent to crop fields, it is difficult to control with pesticides. Samurai wasps can follow stink bugs across the landscape into all the different habitats where they may feed and reproduce so they can be effective at suppressing their numbers over large areas.

6. Are there efforts to monitor impacts on native stink bugs?

My lab is taking steps to better understand the host choice of samurai wasps and their potential impact on native stink bugs. As part of our release and monitoring program, we placed over 180 sentinel egg masses of native stink bugs in BMSB habitats and in places where wasps were released and so far, none of them were attacked by samurai wasps.

7. Have we seen any results yet of the wasps controlling BMSB?

No. It is too early to see any impact samurai wasps may have on BMSB. Like with most biological control programs, it will take probably years until a newly arrived natural enemy such as the Samurai wasp builds up large enough populations to have a measurable impact on its target. This is the process we are trying to speed up a bit with our augmentative releases.

Also, the samurai wasp will not eradicate BMSB—that is not how biological control works. What is expected of a successful biocontrol agent is to bring down pest densities and reach a low equilibrium density where the wasps and BMSB co-exist, but BMSB would no longer create enough damage to be considered a pest.

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