Biological control of knapweed studied

Tim Schnakenberg, an agronomy specialist with University of Missouri Extension, has spent time recently releasing knapweed weevils along Missouri roadsides.

Schnakenberg, who along with fellow MU Extension specialists in southwest Missouri has been tracking the advance of knapweed for several years, says the release is necessary to slow the advance of the week that has found its way into Texas County.

Q: Spotted knapweed has gotten some attention in the last couple of years for being a potential invasive weed in Missouri. What makes it a concern in southwest Missouri?

A: Spotted knapweed has been a difficult weed to keep under control in the western states and has caused tremendous economic damage for farmers and municipalities when it invades private and public lands. “Our Missouri legislature added it to the noxious weed list last year hoping to raise awareness about the problem as it moves into Missouri,” said Schnakenberg.

Q: How serious has the problem become in Missouri?

A: In the last eight to 10 years it has quickly invaded roadsides in southwest and south central Missouri and has started to move into farm fields. “This is a concern particularly if mature knapweed is baled up for hay and fed elsewhere. That is our most serious concern with its spread on farms,” said Schnakenberg.

Q: I understand that the University of Missouri has worked to find a biological control for this weed. What can you tell us about that?

A: Several agencies around the state have worked together to find a solution to the problem. The Missouri Department of Transportation, Missouri Department of Conservation, USDA and MU Extension started last year releasing knapweed weevils to help combat the problem and MU Extension is currently releasing flower head weevils, thanks to a grant from MU’s Plant Protection Program. “These weevils have been successful in the western states over a period of about 30 years to lower the incidence of this weed. The good news is that over 30 years of time they have not found it to invade desirable plant species so we have no reason to think that it will become a pest to us,” said Schnakenberg.

Q: How does this knapweed weevil work?

A: There are two kinds of weevils. One works on roots and crowns of the young plant over the winter by feeding on it and weakening the plant and the other feeds on the flower heads, minimizing the number of viable seeds the plant produces. They both work like the thistle weevils that were released here in the 1970s and 1980s. “We are realistic enough to know that the weevils will not eliminate the weed over time but like the thistle weevils, that have now spread to every county in the state, it will make a significant dent in the knapweed population. We plan to evaluate the release sites over several years,” said Schnakenberg.

Q: Are there other weeds where biological control could be beneficial?

A: “Currently poison hemlock is probably even a bigger threat economically than knapweed. It continues to get thicker each year on farms. There is a moth that attacks this weed and we are investigating its feasibility for controlling the weed,” said Schnakenberg.

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