Biological Control of Knotweed: The Knotweed Psyllid

The knotweed psyllid (Aphalara itadori) is a biological control agent introduced to help manage invasive knotweed species in North America and Europe. Originally identified in Japan, A. itadori was selected for its narrow host range and ability to suppress Japanese knotweed (Fallopia japonica), even at low population levels.

This effort is part of a Classical Biological Control approach, which involves introducing natural enemies from a pest's native range to reduce its impact in invaded areas. In this case, the target is Japanese knotweed and its close relatives, including Bohemian and giant knotweed. These aggressive plants were introduced to North America in the late 1800s for ornamental use and now pose a serious ecological threat, with F. japonica documented in at least 41 U.S. states and eight Canadian provinces.

Although A. itadori is not yet commercially available, experimental releases have expanded in recent years. The first field release in Canada occurred in 2014. Establishment has been slow, with low nymph survival likely due to native predators and the poor quality of older knotweed foliage. However, new research suggests survival improves when psyllids are released following cutting or herbicide treatment, when plants produce younger, more suitable regrowth.

In the summer of 2025, Pennsylvania joined this international effort. Technicians from the Bureau of Forestry's Division of Forest Health conducted the state's first-ever psyllid releases to target invasive knotweed. The releases focused on a specific strain of A. itadori chosen to control Bohemian knotweed, a hybrid of Japanese and giant knotweed species.

While these tiny insects feed by sucking sap from knotweed, they are not expected to kill plants outright. Instead, repeated feeding may reduce the vigor of knotweed stands, giving native species a better chance to establish and compete.

Monitoring efforts are underway at release sites—even those in less-than-ideal settings, like one unfortunately located near a combined sewage outflow after heavy rainfall. Despite the challenges, there is optimism that these initial efforts will lead to lasting biological suppression of knotweed in Pennsylvania.

Future releases may include additional strains of psyllids to target other knotweed species, supporting a broader integrated management approach. 

References

Dunn-Silver, A. 2021. Japanese knotweed psyllid. Cornell College of Agriculture and Life Sciences.

Jones, I.M., S.M. Smith, and Robert S. Bourchier. 2020. Establishment of the biological control agent Aphalara itadori is limited by native predators and foliage age. Journal of Applied Entomology. 144(8): 710-718

Shaw, R.H., S. Bryner, and R. Tanner. 2009. The life history and host range of the Japanese knotweed psyllid, Aphalara itadori Shinji: Potentially the first classical biological weed control agent for the European Union. Biological Control. 49 (2): 105-113.