The weevil, on the other hand, winters over in the soil and likes to get an early start in the spring. It feeds on Canada Thistle as soon as it emerges and females deposit eggs within the plant tissue. The larvae damage the plant by tunneling into its stems. While plants can compensate for this injury, they are weakened to other threats.
When introducing a biocontrol agent such as a weed-eating insect into an environment, being sure of its ability to survive and thrive is essential.
The North Dakota Department of Agriculture released the stem-mining weevil at 102 sites across 34 counties in 2004. The sites were assessed in 2008 and about half the sites were visited again in 2009. Twenty sites were lost to disturbances such as cultivation or flooding.
At the remaining sites, researchers split open thistle stems, looking for evidence of the weevil’s presence. In 2008, weevil activity was found at 45 sites and the infestation level ranged widely from 5 percent to 95 percent. The following year, 11 sites where weevils were not found before now showed signs of their activity.
Five years out, the weevil was documented at 73.3 percent of sites where it was successfully released. Despite the cold air and soil temperatures of North Dakota, the weevils have established a presence.
This study concluded that weevils alone cannot completely control these invading plants. But with the help of the weevils to weaken the plants, other weed control methods can deliver a knockout.
Click here to read more about the study. FG
—From Society for Range Management news release
