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July 2004

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Mighty mites battle bindweed

By Barbara Maynard
Correspondent

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Colorado gardeners are familiar with field bindweed as a formidable foe. This noxious weed seems to respond to the endless pulling, spraying and cussing devoted to its eradication by smothering roses, tomato plants and slow-moving animals with twining green tendrils and triumphant pink flowers.

Now, more than 200 years after European colonists brought this pest to North America, its natural enemy from the Old World has been enlisted to rein it in.

Bindweed's growth habits have made it the bane of gardeners across the country. Its roots can descend 20 feet or more into the soil and produce lateral roots that break off to become independent plants. Seeds can last 60 years or more before germinating.

Despite its tremendous ability to survive and spread, bindweed is not considered such a nuisance in its native Eurasia, partially because the bindweed gall mite keeps it in check. The microscopic mites feed on the growing tips of bindweed plants, causing leaves to appear folded, distorted or even fuzzy. Eggs are laid in the galls, or swellings, produced along the stems, leaves and petioles. The mites overwinter underground on the roots or rhizomes, where they continue their damage by eating root buds. A sufficient infestation of mites will stunt growth, reduce seed production and eventually kill the bindweed.

Mites are available free of charge to Colorado residents, courtesy of the state insectary in Palisade. The insectary, a program of the Department of Agriculture, rears and releases natural enemies of introduced problem plants and insects in an approach known as biological control.

"We're applying a natural enemy to a plant that wasn't a weed in its native land," said Terri Locke, head of the bindweed gall mite program at the insectary.

The insectary ships out the mites as pieces of infected bindweed harvested from established populations. The recipients then introduce the mites to their property by placing the infested pieces in contact with living bindweed. As the original infested piece dries up, the mites will crawl onto the living plant to feed and breed. Wind can spread the mites to new patches of bindweed, but Locke recommends that people help them along by mowing or by picking infested pieces and placing them in contact with new bindweed plants. Because the mites are microscopic, infested bindweed is recognized by the symptomatic distorted leaves rather than seeing the mites themselves.

Locke ships out mites all summer long, from May through September on a first-come, first-served basis. This is the first year that the Insectary has actively advertised the program, so "we're going to just have to play it by ear," Locke said. "We're hoping that the availability is going to cover the demand."

The mites overwinter in the roots, so even if September shipments do not have a visible effect on the current stand of bindweed, they'll be ready for next year. "If they've got a few weeks to establish themselves before a freeze, they'll work their way down to the roots," Locke said.

Gardeners should have realistic expectations - bindweed will not disappear from yards overnight. "Bindweed will never be eliminated even if you had the best of bugs," Locke said. "We are hoping to diminish its numbers so it isn't such a pest for gardeners and farmers. It's a balance of nature that we're striving for."

Biological control programs run the risk of introducing another pest even worse than the first. For example, if the mites don't confine their ravenous appetites to field bindweed, then related species such as ornamental or native morning glories could be in trouble. So far, the mites have not shown signs of diversifying their gastronomic tastes.

"From what I and others have observed, I do not expect the bindweed mite to be a threat to cultivated or native morning glories," said Jerry Michels, an entomologist with the Texas Agricultural Experiment Station who has studied the mites. "However, I will say that in any biological control project, there is some aspect of risk. The risk that the bindweed mite will move to other morning glories must be weighed with the risk of doing nothing to control field bindweed. You cannot remove it from the ecosystem with herbicides without damaging other desirable plants, and there is no way that mechanical means will work. All in all, I see the bindweed mite as the most economically and environmentally feasible method to control this plant."

The insectary also has a smaller supply of a moth that eats bindweed leaves and flowers. The bindweed moth, however, does not attack the roots like the mites do. Locke said people can try either agent, or both, but their success can be influenced by the local habitat: "If it's a more moist row crop setting, I'd recommend the moth. If it's more dry land area, I would suggest the mites."

In addition to bindweed, the insectary also has biological control agents for leafy spurge, diffuse and spotted knapweed, Russian thistle, tall larkspur, puncturevine, musk thistle, yellow and Dalmatian toadflax, bull thistle, Canada thistle, purple loosestrife and poison hemlock, as well as a number of pest insects.

For more information about the insectary and to request mites or moths, visit http://www.ag.state.-co.us/DPI/insectary/Weed%20Programs.html or call 866-324-2963.


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