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ine insects leave tell-tale signs. Eggs of the thistle head weevil, only 1/16-inch long, are deposited on the underside of the plant heads in early May. The eggs are covered with brownish plant material as protection against sun and predators and appear as little brown specks.
In an infested thistle, as the flower begins to dry, the center portion that goes to seed develops a tuft or protrusion that extends beyond the rest of the flower head. If the flower heads are pulled open, larvae feeding on the seeds are visible inside.
A field with just a few thistles showing weevil signs should be bypassed for a few years to allow the insect to become firmly established. But infested flower heads can be cut from sites thick with tuft protruding thistles.
Mr. Bywaters suggests asking the Virginia Tech Extension office for help in obtaining weevils.
Lab specialist Tom McAvoy from Virginia Tech’s quarantine lab at Price’s Fork reported the weevils are sold at $1 apiece.
Adult thistle head weevils, Rhinocyllus conicus, should be released in May in time to breed and lay eggs. They are placed by hand, 10 at a time, on the seed head of the thistle. A single release should include approximately 70 females and 30 males.
The rosette weevil, Trichosirocalus horridus, is released in mass when the plants have reached the rosette stage.
Recently, Virginia Tech shipped weevils to Colorado, Montana, Kansas and Argentina. Land owners interested in getting adult weevils for a release should contact Dr. Loke Kok, Virginia Tech entomologist and weevil researcher.
Mr. McAvoy emphasized that weevils by themselves aren’t always the solution to a thistle problem. “Don’s overgraze. Thistles love overgrazed pastures. But a good stand of grass will give thistles stiff competition.”
Not every farmer is satisfied with the job weevils do as thistle controllers.
Nic Kortlandt of Rectortown tried a weevil experiment nine years ago. he released 25 in a pasture bordering a neighbor’s thistle-ridden fields, hoping the insects would disregard property lines.
Asked how the experiment is progressing, Mr. Kortlandt said he’s disappointed. His population of weevils hasn’t increased enough to keep down the thistle population.
He noted, however, that landowners on either side of his pastures aren’t doing anything to control thistle infestation. Seeds from their thistles blow into Mr. Kortlandt’s fields while his weevils fly into theirs to dine on thick patches of the noxious weed.
“I’ve had trouble keeping the weevils established,” MR. Kortlandt said. But he noted the release on his land nine years ago included only the head feeding weevil, adding he may have better success with a two-pronged attack, including the rosette weevil, in his arsenal.
Although weevils haven’t been a complete success yet in halting the spread of thistles, Mr. Kortlandt is sticking with biological controls.
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