Biocontrol for knotweed on the horizon?

What I call the “knotweed complex” (made up of Japanese, Giant, Himalayan and Bohemian species or their hybrids) has been proving itself to be the most intense invasive species to hit Western Oregon, particularly on the North Coast. Watersheds up and down the Coast Range are seeing infestations that grow from a few isolated patches into complete miles-long monocultures in a matter of seasons. Locally, members of the Tillamook County knotweed working group (now the Tillamook County Partnership for Regional Invasive Species Management)  has recorded a 300% increase in one such infestation along the Wilson River in Tillamook County.  Knotweed easily garners the tag “superweed” by its relentless growth, rapid reproduction and persistence in the face of a variety of control methods. The plants are so strong that they can ruin concrete and asphalt by growing through it and are so tenacious that they can persist in underground roots for decades.

Bohemian knotweed (Fallopia x bohemica). Photo by R. Emanuel.
Bohemian knotweed (Fallopia x bohemica). Photo by R. Emanuel.

Oregonians are not alone in dealing with the invader. The United Kingdom has been battling the knotweed since 1906 when it was first introduced as a garden ornamental. While England has made some strides in integrated control, their struggles look awfully similar to ours.  According to the British newspaper the Guardian, however, Britain might have a serious biological control on the horizon. Biocontrols are organisms that are native to the plant’s home ecosystem. Their feeding on the plant in that environment keep it in check.  Cinnibar moths, for example were introduced to control Tansy Ragwort with much success in Oregon.  If introduced to the new environment, they may work to balance the plants growth–but also pose serious risks to the new home as well. In another local example, Asian grass carp were introduced to control Eurasian watermilfloil, only to become invaders themselves by wreaking havoc on local ecosystems.

In this case, the Brits have studied a Japanese psyllid, an insect called Aphalara itadori that feeds on plant juices. Within about a year, the insect might be released into the countryside to see how it works to control the non-native knotweed.  I can’t tell you how many times I’ve been asked about biocontrols for knotweed.

So my answer to the persistent question is: yes there’s something on the horizon.  How long that horizon lasts, is another question I cannot answer. Likely, it will be measured in years, not months.  Recently, a Pacific Northwest Knotweed Biocontrol Consortium was formed to look at how insects like these might perform around here.  OSU is a partner in the effort with a quarantined facility to test the organisms.  In the meantime, let’s keep our fingers crossed and our eyes on the UK to see what happens when or if Aphalara itadori becomes its newest immigrant. Check out the full Guardian article here.

Another, more scientific analysis of the psyllid and its potential can be found in a very recent paper by Richard Shaw, Sarah Bryner and Rob Tanner published in Biological Control, Volume 49 (May 2009).  An abstract for that article can be found by clicking here.

Those needing local knotweed control information, contact me directly using the comment function below.


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