Visit any waterway around Moab, and you’re likely to stumble upon tamarisk, an invasive, feathery-looking shrub that now dominates many riverbanks on the Colorado Plateau.
This week, Science Moab speaks with local resident and scientist Tim Graham about the tamarisk beetle, a nonnative biocontrol released to thin tamarisk groves along the Colorado River and other southwestern waterways.
Science Moab: How was the beetle chosen as a biological control for tamarisk trees?
Graham: Biological control research on tamarisk has been going on in the government for decades. In 1984, they tested over 400 species of tamarisk feeders and settled on what they thought at the time was one species of beetle. The current beetle populations we have come from northern China and Kazakhstan at about 40 degrees latitude, which is an important part of the story because at different latitudes species have different life histories.
The beetles in Moab came from a release initially around Delta, Utah. That was one of the first free releases in the program. The Animal and Plant Health Inspection Service put out a call to county weed supervisors to come to get tamarisk beetles and turn them loose.
In 2004, the Grand County Weed Control Supervisor Tim Higgs got a bunch of these beetles and he did something that was pretty smart. He didn't just scatter them around. He put them in three different places in relatively high concentrations, which was good for a couple of reasons. That way you didn't have to try to track them all over the county right away, you could watch how they spread. Also, the beetle does best in high concentrations and high densities. They actually emit a chemical that attracts other beetles called an aggregation pheromone. And so when he put them out in high densities, they were able to mate and feed more effectively than if they had just been tossed randomly out of the truck.
Science Moab: What are the beetles actually doing to the tamarisk?
Graham: The way the beetles feed is that they scrape the epidermis, or skin, off the leaves and the small branches and then lap up the juice in the cells. They're actually killing the leaves of the tree, which are the photosynthetic organs, so the tree can't capture carbon and grow. Tamarisk trees put a lot of reserved photosynthetic product into their roots and then they use that to put out new growth in the spring and start to grow again. The beetles can actually girdle those small stems so they can kill more than just the biomass that they're consuming.
Science Moab: How has it been working? Do we see an overall reduction of tamarisk?
Graham: It's pretty significant. I think the tamarisk is no longer a dominant part of the system. If you look along the rivers, in particular, there's very little tamarisk up against the river anymore. The willows have taken over that shoreline. If you look at aerial photos of islands even as early as 2009, you can see these brown centers where the tamarisk stabilized the sandbar and since then the willows have taken over. As far as tamarisk control goes, the beetle has done an excellent job.
Science Moab: Do you think it's possible that we might see the complete loss of tamarisk?
Graham: I don't think the beetles will ever completely eliminate it; they don't in their native habitat. There was an incredible amount of research into the feeding habits of this beetle but not much about the ecology. What is it that allows the tamarisk and the beetle to coexist in Kazakhstan? I haven't seen a lot of literature on the beetle in its native habitat.
I'm really curious to see but I think what will happen is we'll have sort of this hide-and-seek kind of process where tamarisk will be found sparsely in the riparian stands. And then the beetle will come in, defoliate, and knock that back. But in the meantime, tamarisk is getting established someplace else. So that's my guess of how things are going to go.
Science Moab is a nonprofit dedicated to engaging community members and visitors with the science happening in Southeast Utah and the Colorado Plateau. To learn more and listen to the rest of Tim Graham’s interview, visit www.sciencemoab.org/radio. This interview has been edited for clarity.