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Wyoming Weed and Pest Council introduces biological control project in Fremont County

by Wyoming Livestock Roundup

What some people may think is a beautiful white flower is actually an aggressively invasive weed. Hoary cress, also known as whitetop, is a noxious weed causing many problems throughout Wyoming. To mitigate the spread of this noxious weed, the Wyoming Weed and Pest Council (WWPC) has begun biological control with a tiny mite.

Whitetop completely takes over and impacts recreation, irrigation, farming and the economy. It costs millions of dollars to control it and the costs increase due to inflation and economic downturn.

“Whitetop has affected every single county in Wyoming for a long time,” said WWPC President Larry Smith. “While we’re dedicated to managing and controlling the spread of this invasive weed, it is difficult to contain. We implement cultural practices, herbicides and grazing every year to slow the spread, but it still heavily impacts Wyoming. That’s why we need biological control to help over time.”

Because of the severe repercussions whitetop has had on Wyoming, WWPC has been contributing to biological control research about hoary cress since 2001. After years of research and development, the first whitetop biological control agent was approved for release in the U.S. This project was a joint effort between Montana State University and the U.S. Department of Agriculture Agricultural Research Service – European Biological Control Laboratory.

Researchers found a tiny, plant-feeding gall mite, Aceria drabae, can feed and develop on whitetop without threatening native plants and wildlife. These mites, originally from northern Greece, are incredibly small – nearly invisible to the naked eye. They are dispersed on the invasive weed by the wind.

The mite harms whitetop plants by causing a “gall.” Galls are abnormal growths which can develop primarily on plants’ flowers, flower buds and sometimes the stems. 

Usually, they grow in reaction to the mites feeding on the plant. Galls divert energy from the plant, preventing the formation of seeds and potentially stunting growth.

After over two decades, Wyoming can finally start releasing the mite in Fremont County. This project will take time and isn’t expected to eradicate whitetop in Wyoming. As the mites’ population grows, the weed’s seed production will be limited. 

It won’t be able to spread as quickly. And since it’s just starting, the species will need to be tested more over time. Hopefully, this project will help save thousands of dollars and improve invasive species control.

“This biocontrol agent won’t be the cure of whitetop, but we are excited to have another tool in our toolbox,” said Wyoming Biological Control Steering Committee Chairman Aaron Foster. “We hope once the mites are established in Fremont County, we’ll be able to share with other counties and eventually have it everywhere in the state.”

To learn more about this biological control project and others throughout Wyoming, visit For more information, visit or follow Wyoming Weed and Pest Council on Facebook and Twitter.

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