Tamarisk leaf beetles along the Colorado River

Earlier this summer, much to my surprise, I got to witness one non-native species successfully controlling another. The introduced tamarisk leaf beetles were eating and killing non-native, invasive salt cedar – also known as tamarisk – along the Colorado River near Moab, Utah.

My family was on a camping trip. Our first stop was along the Colorado River outside of Moab. Beyond the towering red walls surrounding the campground, one of the first things I noticed were the dead tamarisk plants lining the river. Where green plants used to line the river – providing a nice contrast in the rocky landscape – dead plants now blended in with the red walls and brown river. Possible reasons filtered through my brain: fire, disease, insects. Turns out that insects were the culprit and they were there to kill the tamarisk.

Tamarisks were originally introduced to the American West in the 1800s from Asia and Mediterranean countries. They were used as a windbreak, ornamental plant and for erosion control. Tamarisks are adapted to hot, dry climates and alkaline soils. They now cover up to 1.5 million acres of land in the western United States. As they spread, tamarisks wiped out the willows and cottonwood and other riparian plants that use to line western rivers. Along with the plants went wildlife that depended on those species. Not only do tamarisks impact wildlife and native plants, but they choke out access to rivers for recreational purposes and increase wildfire potential.

Since the 1960s land managers have been fighting against the spread of tamarisk. Current means of removal include use of herbicides and heavy equipment. After 20 years of research, biologists made the decision to introduce the tamarisk leaf beetle Diorhabda spp. as another line of assault against the tamarisk.

The tamarisk leaf beetle, from the same area of the world as the tamarisk plant, feeds almost exclusively on tamarisk. (Some studies suggest they could also feed on the native species Frankenia but prefer tamarisk.) Both larvae and adults feed on the foliage of the tamarisk. Over a period of three to five years, the beetles kill the tamarisk plant. Information on a board at our campground – provided by the Bureau of Land Management – explained that once the beetles kill the tamarisk, land managers will use prescribed burns and heavy equipment to remove the dead plants. Native vegetation will be planted in the area if it doesn’t move in on its own. Complete restoration of the area is expected to take years – and lots of planning.

This form of biological control is not without controversy. An endangered bird species, southwest willow flycatcher, now uses tamarisk for nesting habitat. Biologists are concerned about losing tamarisk habitat without gaining willow habitat. One study (pdf) suggests that having some tamarisk would be beneficial to wildlife. That will probably happen anyway. Most managers agree that total removal of tamarisk will be impossible.

I had heard of these beetles many years ago when I worked in wetland and riparian habitats. That was around the time experts were considering introducing the beetles to kill tamarisk. I had been concerned about introducing a non-native insect to take care of a problem. We have many examples of an introduction gone wrong, including the tamarisk. The scientists, however, carefully studied these tamarisk leaf beetles to make sure they weren’t unleashing another disaster. Still, the beetles are adapting to longer days and are moving further south than the scientists had predicted.

I was amazed to see how well the beetles are working at killing off the tamarisk. I would like to see what a native riparian zone looks like, with native vegetation, but I know it will take years and lots of cooperation between groups and agencies. I am looking forward to seeing how these little tamarisk leaf beetles conquer tamarisk.

For more information:
Tamarisk Coalition

Biocontrol of Tamarisk Using the Tamarisk Leaf Beetle, Diorhabda elongata
By: Dr. Dan Bean of the Colorado Department of Agriculture’s Palisade Insectary

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