Biological Control Stops Dalmatian Toadflax Invasion Following Wildfires

Biological control is a solution for an exotic invasive weed invasion following a major disturbance caused by wildfires.

Across the western United States, wildfires burn with greater frequency and cover more area than in the past, largely due to shifts in land use patterns and climate change. Fires are a form of disturbance that may open the landscape for invasion by exotic plant species that can take advantage of reduced competition from native species and the release of nutrients that fires bring about.  

Some invasive plant species such as cheat grass (Bromus tectorum) and tamarisk (Tamarix ramosissima) are highly flammable and may increase fire frequency, bringing about more favorable conditions for their own spread. It’s not surprising then that natural resource managers are on alert for dramatic increases in invasive plant species following wildfires.

In the spring of 2012, the Hewlett and High Park fires combined to burn an area of about 39,000 hectares in the mountains northwest of Fort Collins, Colorado. By the spring of 2013, it was clear that fire-induced disturbance had enabled the expansion of exotic weeds, none more dramatic than the invasion of Dalmatian toadflax that turned thousands of hillside hectares bright yellow with showy blossoms. Dalmatian toadflax (Linaria dalmatica) was introduced from Europe into the United States in the 19th century for use as an ornamental. It’s easy to see why this plant would be prized in a flower garden since it reaches heights of nearly a meter with showy bright yellow flowers. Unfortunately, it escaped cultivation and became invasive across much of the western U.S. where it is a perennial that produces up to half a million seeds per plant every year, and also may expand via an extensive lateral root system.  

Dalmatian toadflax is not readily used as forage by livestock or wildlife and it may out-compete desirable native grasses and forbs, so in Colorado it has been declared a noxious weed and is the focus of control efforts by resource managers. Dalmatian toadflax is difficult to control using conventional methods since the waxy leaves resist penetration by herbicides and the extensive root system can survive mechanical or chemical control measures. The steep hillsides and remote location of the burned areas also would make control through herbicides or mechanical removal nearly impossible as well as extremely expensive.  

Biological control, which is the use of self-propagating and self-dispersing natural enemies against invasive species, is by nature very useful against widespread, dense and remote infestations of exotic weeds. The Colorado Department of Agriculture (CDA) formulated a biological control plan for Dalmatian toadflax as part of a regional effort to bring invasive species under control after the Hewlett and High Park fires. The biological control agent used was the toadflax stem boring weevil Mecinus janthiniformis (Coleoptera: Curculionidae), a small cylindrical beetle native to Europe, that feeds within Dalmatian toadflax stems as a larva and on the foliage as an adult. It is host-specific, which means that it feeds exclusively on Dalmatian toadflax and will even avoid yellow toadflax, a Dalmatian toadflax relative also introduced from Europe.

Release of Dalmatian toadflax weevils at the Hewlett and High Park burn area. Image Credit: Dan Bean

Reuniting Dalmatian toadflax with a highly coevolved natural enemy from the native range of the plant makes it less invasive in the new introduced range. The Biological Pest Control program within the CDA battles exotic plant species by reuniting natural enemies from the native range with introduced invasive species found in Colorado and the western U.S. Introduced natural enemies are host-specific, and before release they are evaluated for safety through a lengthy process that takes five to 10 years and involves intensive host choice tests.

The CDA began releasing weevils at sites scattered around a 400-hectare area above Seaman Reservoir, which serves as a municipal water supply for Greeley, Colorado, and cannot easily be treated with herbicides. The weevils were obtained from the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service, an agency that helped support the post-fire remediation efforts. Four monitoring plots were set up in the release site, each including a 4-by-4 meter square around the weevil release point within which all Dalmatian toadflax stems were counted annually. The weevils established, meaning that self-propagating populations survived and expanded from one growing season to the next.

During the 2014 and 2015 growing seasons, plants showed feeding damage to the leaves and beetle larvae were found within the stems. By 2015, adult weevils were abundant throughout the burned areas and in 2016 the Dalmatian toadflax density had plummeted and remaining plants were heavily infested with weevils. The results were visually stunning as it was difficult to find Dalmatian toadflax plants in areas where they had been abundant following the fires. The stem density decline also was quantified with stem counts showing a steep decline between 2015 and 2016. This case demonstrates how biological control provides a safe and inexpensive way to combat a post-disturbance invasion of exotic Dalmatian toadflax.

Dan Bean is the director of biological pest control for the Colorado Department of Agriculture.

Mike Racette is a biological control specialist for the Colorado Department of Agriculture.

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