Upon its accidental arrival in the U.S., as with most other invasive species, brown marmorated stink bug (BMSB) numbers expanded quickly in the absence of natural enemies that it left behind in its native Asia. For a number of years, we have studied how our native natural enemies attack BMSB and found that they are not effective in keeping it from damaging crops. But that came to an end in 2014 when Trissolcus japonicas, often referred to by researchers as just Tj, wild populations were found in Maryland. Like BMSB, it had found its way to the U.S. accidentally. Researchers working with Tj have given this wasp the informal common name of ‘samurai wasp.’ Despite its small size of 1 to 2 mm, Trissolcus japonicas, or Tj, is an important natural enemy of the BMSB in Asia.
Tj is an egg parasitoid of stink bugs. A female inserts a single egg inside a stink bug egg, then the Tj larvae hatches and kills the stink bug egg. Since BMSB lays eggs in groups of 28 to 35, Tj must place an egg into each of the stink bug eggs to control egg hatch of the entire mass. A Tj female will typically have 42 eggs at any one time, more than enough for a BMSB egg mass. Since its original detection in 2014, Tj has been found in New York, Pennsylvania, Ohio, Virginia, New Jersey, Michigan, California, Washington, Oregon, Delaware, and Utah. Since 2007, Tj has been studied in quarantine laboratories in several locations in the U.S. to determine if it would be suitable for release and control of BMSB. But before approval was granted for release, Tj was detected in the field. Genetic matching studies have found that these wasps were from a different population than those in quarantine.
In Asia, Tj is able to parasitize up to 60% to 80% of BMSB eggs. If that level is realized in the U.S., Tj will provide significant control of BMSB. However, studies done in quarantine found that besides BMSB, Tj will also lay eggs in the eggs of spined soldier bug, but to a much lesser extent. Spined soldier bug is one of our predatory stink bugs and is considered a beneficial insect.
While we have not been able to detect Tj in Kentucky, a UK graduate student, Lauren Fann, has been trying since 2017. While USDA APHIS regulations prohibit us from bringing Tj across state lines without approval, once it arrives here on its own we will be able to conduct studies and potentially release it within the state. Ms. Fann’s studies with BMSB eggs masses have found that with our existing natural enemies, only 3% to 4% of BMSB eggs are parasitized and about 40% are eaten by predators. Having another enemy of BMSB will help to moderate its populations for years to come.
Ric Bessin, Extension Entomologist