We’ve covered, in the magazine and on our website, various methods of managing vegetation, from mowing roadside areas to applying herbicides to controlled burning. One method that’s getting a lot of attention lately-although it’s arguably been around far longer than any other-is biological control: bringing in grazing animals or insects to eat the plants you’re trying to remove.
Biological controls for weeds and invasive plants can be an economical and ecologically friendly alternative to mechanical and chemical controls. Often, though, it’s an invasive animal that’s used to fight an invasive plant, and there are plenty of cautionary tales of how things can go wrong. In the 1880s, for example, the water hyacinth was introduced to the US from Japan at the Cotton States Exposition in New Orleans. The plant spread rapidly throughout the Southeast, clogging navigational waterways, and it proved almost impossible to kill. Someone eventually hit upon the idea of bringing in nutria, very large South American rodents once prized for their fur. Nutria were released throughout Louisiana, where they not only thrived on the water hyacinth but, by the mid-1900s, were destroying rice and sugarcane fields, damaging marshes, and weakening earthen levees by stripping them of vegetation.
A more recent example involves the tamarisk, or salt cedar, a shrub native to central Asia. Brought to the US in the 1800s and used to create windbreaks and to control erosion, the tamarisk spread and crowded out native plants like cottonwood and willow. Several years ago, the US Department of Agriculture began releasing salt cedar beetles-Diorhabda elongata-from Kazakhstan along streams in Colorado to control the tamarisk without the use of herbicides. It worked-but some scientists believe the beetle killed the shrub in areas it wasn’t intended to reach, destroying the habitat of endangered birds.
When things go right, however, the results can be gratifying. For more than a decade, the Southern Idaho Regional Bio-Control program has been using various insects to destroy invasive weeds like knapweed, leafy spurge, and purple loosestrife, making room for native plants to reestablish themselves. Some insects attack the plants’ seed heads, while others feed on the root crown or root system. Groups of middle school and high school students-“Bug Crews”-help rear the insects, release them in designated spots, and monitor the sites.
Larger animals work well, too. Owners of a solar array in California are using miniature sheep-short enough to fit neatly underneath the 22-inch-high solar panels-to keep plants from growing too high and blocking the sun. In Arizona’s Tonto National Forest, goats have been used to munch on up to 1,600 acres of underbrush, reducing the risk of wildfires. A few private companies have recently gotten into the business in a big way, renting out herds of goats to clear invasive plants and weeds in several Pacific Northwest states.
The state of California is currently trying a biological control for the water hyacinth-not nutria this time, but rather a South American insect known as the water hyacinth plant hopper. Scientists have exposed the insects to 90 species of plants native to the area in which they’ll be released, and they are satisfied that the plant hopper will eat water hyacinth and nothing else.
Has your state-or your own vegetation management program-used biological controls? How successful has it been? E-mail me at mailto:[email protected] or leave a comment at http://www.erosioncontrol.com.