Japanese knotweed psyllid

The knotweed psyllid (Aphalara itadori) became the first biocontrol agent approved for release against invasive knotweeds. It was first released in the UK in 2010 and later released in Canada in 2014. It was approved by USDA APHIS for release in the United States in 2020. A. itadori is a species of psyllid, commonly known as jumping plant lice, that is host specific to invasive knotweeds. The psyllid life cycle takes 4 to 6 weeks from the egg stage, through 5 juvenile stages, and then reaching adulthood. It feeds on the leaves and stems during its juvenile and adult life stages, using piercing mouthparts to ingest sap from the plant meristems. After feeding, the psyllids excrete crystallized honeydew for protection from predators. This substance is referred to as lerp (Grevstad et al 2017).  Adults may disperse to pine trees or other conifers for shelter during the winter months (Alford 2019). 

There are two biotypes of Aphalara itadori being released in the United States. Most releases are the Kyushu (southern) population because it has a bigger impact on Bohemian and Japanese knotweed. The Hokkaido (northern) psyllid feeds on Giant knotweed. Fewer Hokkaido psyllids have been released since giant knotweed is not as widespread as other invasive knotweeds (Grevstad et al 2017).

There are three species of invasive knotweeds found in the United States. They are Japanese knotweed (Fallopia japonica), Giant knotweed (Fallopia sachalinensis), and their hybrid, Bohemian knotweed (Fallopia x bohemica). Knotweeds are perennial, herbaceous shrubs with shoots that emerge from rhizomes early in the spring. They form dense stands growing up to 7 feet tall (up to 12 feet if giant knotweed). Because of their large leaves and overall height they are capable of completely shading out other vegetation. Very few plants are found beneath a thicket of knotweed and the only plants nearby are usually other invasives(VanDrieche et al 2002). 

Fallopia japonica is recognized as a major weed in the United States and Canada (VanDrieche et al 2002). Both Japanese and Bohemian knotweed occur over most of North America. Giant knotweed infests the Northeast and Pacific Northwest more abundantly (Grevstad et al. 2017). Although they can reproduce by seed, knotweeds spread primarily by fragmentation of rhizomes and roots. Rhizomes can regenerate when buried up to 1 meter deep which makes both mechanical and chemical control difficult. Shoots can grow through asphalt, municipal structures and cracks in home foundations, which causes significant economic losses. Both widespread distribution and difficulty with mechanical and chemical control make invasive knotweeds a good candidate for biological control (Grevstad et al 2013).