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Spraying thistle now reduces beneficial weevils

Musk thistle, a noxious weed, has grown in profusion this year. But, now is the worst time to spray herbicides on the prickly pests with purple flowers, if you hope for control.

A seed-eating insect is at work in the mature flower heads. The insect, called the musk thistle seed head weevil, will greatly reduce the thistle population next year, said an Extension biological control specialist at the University of Missouri.

Spraying after plants have bloomed is ineffective in killing the plant. Spraying now also hurts the buildup of natural populations of the thistle-eating insect. The weevil larvae eat the developing seed, keeping the plant from reseeding for next year.

“Don't spray,” is the advice of Ben Puttler, who has worked on biological control of pests for 30 years. He helped establish and spread the seed-head weevil in Missouri. The biological-control insect was brought from Europe, the original home of musk thistle. Both the thistle and the weevil are now found throughout Missouri.

“If you have a patch of musk thistle, you have the weevil,” Puttler said. “The thistle population goes in cycles. When the weevil is successful, the thistle population is greatly reduced. As the food supply, the thistle seed, disappears, the weevil population declines.”

Enough thistle seeds survive to maintain the plant species. That is also necessary to maintain the weevil population. “They wouldn't want to be 100 percent effective,” Puttler said.

“As the thistle population rebuilds, the weevil population rebuilds,” Puttler said. “Some years, such as this year, the weevils lag. But now they are catching up with the thistles.”

Spraying only interferes with the natural cycle. Besides, the thistle dies after the seeds mature.

Spraying nearly mature thistle is an almost futile undertaking. “They look terrible after they are sprayed. It appears you have achieved vengeance,” Puttler said.

“People are calling, wanting to buy the weevils for control,” Puttler said. “But, if they will break open the maturing seed heads, they'll find the weevil is already there.”

When the weevil larvae burrow into the seed, the seed is killed.

There are actually two different weevils at work in the thistle patch; one feeds on the newly emerged leaves while the other eats seed.

The musk rosette weevil eats the young thistle plants, called rosettes. The thistle, a biennial, produces a rosette of leaves close to the ground the first season. The second year it puts up a tall flowering plant.

Together, the weevils work both years of the thistle two-year life cycle.

Thistles are most vulnerable to herbicide spray in the rosette stage. “If you feel you must spray thistle, only spray the rosettes,” Puttler said.

“But, do it only in September and October, when the weevils are in hibernation for the winter.”

For Puttler, the biological control specialist, when it comes to spray, less is better. Spraying reduces the competition from other plants. “Thistle seed must have bare ground to germinate,” he said. “If you have thistle in your pasture, it's a sign of pasture mismanagement.”

A good defense against thistle is a strong stand of grass. He'd leave the thistle rosettes to the weevils.

Duane Dailey is an Extension and ag information senior writer for the University of Missouri.

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