Detection and Eradication

Prevention is the first line of defense, but it is not perfect. When weeds defeat prevention measures, successful management relies on detecting new weeds early when eradication is most successful. Early detection in the form of surveys typically focuses on locating invaders in high-risk sites, such as areas near seed sources. In weed-free areas remote from seed sources, high-risk sites include weed pathways, areas where at-risk material was used, and human-modified sites, such as developed areas, campgrounds, fishing-access sites, and disturbed areas. Cost-effective monitoring requires that sampling be directed to these high-risk sites, since the prospect of detecting additional weeds away from these areas may become too low. High-risk sites are initially inventoried and rechecked periodically, and significant portions of low-risk land are annually surveyed to confirm weed-free status.

Detection is the highest level of management intensity performed in Weed prevention Areas (WPAs). Seasonal weed scouts help inventory WPAs and assist landowners with monitoring and eradication. Weed scouts usually survey for new weeds on horseback or all-terrain vehicles (see Figure 3). Transect intervals vary with site characteristics that depend on physical terrain features and vegetation cover. On average, weed scouts survey grasslands by following transects at 100-foot intervals. They can usually survey rangelands at a rate of about 200 acres per hour at a cost of about $0.04 per acre, on average.

New weeds are mapped and eradication strategies are formulated. These strategies require 100 percent control where every plant is eliminated from an infestation to prevent re-invasion. Eradication treatments must be highly effective and cover sufficient area to locate any satellite plants away from the patch, since even recent invasions may have widely dispersed plants. The need to implement highly effective strategies over adequate areas may be augmented using trained invasive weed detector dogs to locate plants that are potentially missed by surveyors. Canines are an effective and broadly usable detection technology. Detector dogs are more effective than humans for locating rare weeds because they have a strong sensitivity to a target and can quickly and thoroughly cover large areas. A recent Montana State University study found that on average, trained canines were significantly more accurate in detecting new invasions of spotted knapweed (Centaurea maculosa) than human surveyors (92 percent versus 76 percent). These findings suggest detector dogs may help with invasive weed eradication efforts. The popularity of weed detector dogs may also make early intervention more visible.

Figure 3. Weed scouts provide on-site ranch visits to promote prevention and assist landowners with weed monitoring and eradication.

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Last Modified: 08/02/2007