Invading weeds are met by an offense of plant-eating insects

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What is the best course of action when an invading noxious weed threatens to attack crop yields and assault grazing land? Invite a friend to dinner. In this case, the friend is a plant-eating insect—the stem-mining weevil.

The current issue of the journal Rangelands presents a study of in sect biocontrol agents in North Dakota. Fighting weeds with herbicide, tillage, and mowing can be costly and labor intensive, while the weevil is a natural enemy.

The primary invader across North Dakota’s 13.5 million acres of rangeland and 22 million acres of cropland is the Canada Thistle. It can displace native plants, lower crop yields, and negatively affect the quality of grazing land. Its creeping root system has already allowed it to infest about 1 million acres.

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.

Full text of “Home on the Range: Establishment of a Canada Thistle Biocontrol Agent,” Rangelands, Vol. 34, No. 5, October 2012, is available at 

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