Thistle

If good fences make good neighbors, then good thistle control makes an excellent neighbor according to Tim Schnakenberg, agronomy specialist with University of Missouri Extension.

The only difference is that thistle control is not only neighborly, it is also the law.

"Controlling seed production and spread is critical to getting thistle populations under control. Once the plants start to send up seed stalks about the first of May, control becomes very difficult," said Schnakenberg. "And worst of all, the weed can spread to your neighbors."

According Schnakenberg, early spring is the best time of year to control musk thistle.

For musk thistle, herbicides can be used prior to the time buds begin to show a little pink. The product 2, 4-D can be applied in the early spring before the head shoot begins to grow.

For best results, daytime temperatures should be in the mid 60's or higher and nighttime temperatures no lower than the mid 40's.

"After the head shoot begins to grow, other herbicides such as Ally, Banvel, Grazon, Remedy or Tordon can be used. Ally would not be the best choice for fescue fields especially if it is intended for seed production," said Schnakenberg.

Weevils at work

When to spray musk thistle and whether or not weevils are at work is a common question. According to Schnakenberg, musk thistle and weevils both run biological cycles.

"About three to four years ago we had an increase in musk thistle. The weevil populations have increased during that time so weevil populations are high and we should expect a decrease in musk thistle numbers over the next few years," said Schnakenberg.

Then, as musk thistle numbers go down, so will the weevil population.

"There will come another time when we have another explosion in the musk thistle population like we did in 2006," said Schnakenberg.

From the time a little pink can be seen in the bud through the end of July, control should be left to the musk thistle weevil.

"When the buds begin to open, the adult weevil comes out of hibernation and lays its eggs on the flower head," said Schnakenberg.

The eggs hatch; the larvae bore into the head and destroy the seed. Next the larvae pupate and later emerge as adults and go into hibernation until the next year. If this process is allowed to happen, weevil numbers increase each year until they are high enough to control the thistle.

"If we kill or destroy the musk thistle during this period of time, we reduce the weevil population for the following year and lose the long-term control," said Schnakenberg.

More information

Missouri has 11 weed species that have been designated by state law as being noxious (injurious to health). This noxious weed list includes common teasel, cutleaf teasel, kudzu, musk thistle, Scotch thistle, Canada thistle, Johnsongrass, field bindweed, multiflora rose, marijuana and purple loosestrife. These are weeds that are required to be controlled on public properties, in road rights-of-way and, in many cases, on private land.

For more information on the musk thistle weevil, contact the nearest University of Missouri Extension Center and ask for guide sheet 4867, "Integrated control of musk thistle using weevils." An informational brochure is also available online at http://extension.missouri.edu/greene.

David Burton

University Of Missouri-Extension

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