Psyllids may help keep dangerous knotweed in check

by | Mar 11, 2022

A psyllid sits on a knotwood leaf

In the battle against invasive species, NCDA&CS has a tiny new ally.

The invader: Knotweed, a destructive plant which can grow roots systems more than 20 feet wide.

The ally: A two-millimeter bug known as a Knotweed psyllid.

Nancy Oderkirk, research specialist and quarantine officer with the NCDA&CS Beneficial Insects lab, is a one person expert to get the troops in line. A biologist with a background in botany and entomology, Oderkirk is is a member of the team that works on biological control of invasive plants and insects in North Carolina.

The Japanese Knotweed, and its relatives the Giant and Bohemian Knotweed, fall under her purview. Native – as one might expect – to Japan, the plant’s large roots systems can undermine concrete and pavement, wreaking havoc on sidewalks and roads. Both forms of the plant are established all over the state and has proven to be a difficult species to tackle due to how easily they can take root in new locations.

“All of these kinds of knotweed are very well-adapted to a variety of environments, and they have a very powerful root system which we call a rhizome. They grow very rapidly and forcefully, and they are extremely difficult to remove,” Oderkirk said. “They also spread easily. A small piece of either rhizome or stem can float down a stream and land on a bank or get carried off on the tailgate of your truck and land on the ground. Even those small pieces are very good at rooting and starting whole new colonies.”

Nancy Oderkirk, left, works with Greg Wiggins, biocontrol administrator, on a biocontrol project for Hemlock Woolly Adelgid.

Those rhizomes have grown so out of control in some areas that they disrupt or undermine highway pavement or even the foundations of buildings. The danger is serious enough – and the plant’s reputation bad enough – that in some parts of the world property cannot be sold if it has knotweed on the lot.

Enter the Knotweed Psyllid. Known officially as Aphalara itadori, the tiny bugs hail from Japan just like the knotweed they consume.

And consume knotweed they do – exclusively. The insects only feed and reproduce on that one specific plant, which makes them promising as a biological control tool to address the knotweed problem without causing knock-on effects elsewhere in the ecosystem.

Oderkirk maintains two populations of the psyllids at the Beneficial Insects Lab, one from northern Japan and another from the south. The two populations are actually the same species, but each shows a preference for a different flavor of knotweed.

“The whole idea of biocontrol is to find the natural enemy of an invasive or pest species that is part of it’s natural enemy complex where it came from. So we are just reuniting these psyllids with their host plants,” Oderkirk said. “We’re to the point now where we have tens of thousands of psyllids here in our lab.”

Introducing a new species into the environment of course comes with challenges, and it is important to make sure that doing so doesn’t result in creating a whole new invasive pest. That is why projects like the psyllid introduction go through heavy oversight before being put into full practice.

Knotweed growing in the wild

“There is actually an international code by which anyone who does biocontrol or proposes any studies will abide. In America, it requires oversight and eventual approval by multiple agencies, most notably the USDA Animal, Plant and Health Inspection Service.” Oderkirk said. “These agencies have strict protocols to test the would-be imported species against all possible hosts. You have to study the proposed host, then the most closely related ones, and then slightly less-related ones, and you test out further and further until you can show this organism will not accept any additional hosts.”

This process can take a decade or even longer. It’s something of a paradox, Oderkirk said; by the time a species is deemed invasive, a solution is generally needed quickly. That’s why work is already underway to test the effectiveness of the psyllids on knotweed growing here in North Carolina. While the program is still in the pilot phase, there are six sites throughout the state where the insects have been released onto knotweed populations. One of the major obstacles right now is determining what kinds of conditions are ideal for releasing the insects, which is what these sites will hopefully shed some light on.

There is still much work to be done before the psyllids can really be put to use, but Oderkirk has high hopes.

“These kinds of projects take decades typically, but I remain very optimistic. There has been even greater success than ours in the state of Oregon, which allows us to kind of hone in on what is causing that success and try to replicate it here,” she said. “Our department is very collaborative in nature and the stakeholders and cooperators really span the entire spectrum. From an apple grower to Duke Energy, lots of county parks and rec folks, we have such a broad base of cooperators. We’ve got great networks to take advantage of and people to work with us.”