Biocontrol Efforts on Tejon Ranch
By Conservancy Stewardship Manager Laura Pavliscak
Photo: The invasive tamarisk tree dominates the drainage of Lower Tejon Creek.
This summer marked an important first for the Conservancy—using biological control to manage an invasive species of concern on the Tejon Ranch. Biocontrol is the purposeful introduction of a controlling agent like an herbivorous insect on a target pest species. In our case, we introduced a beetle that specializes in defoliating a highly invasive tree species in our creeks. Tamarisk or salt cedar (Tamarix ramosissima), is a water-loving shrub or small tree from Asia. In the early 1800’s, several species of tamarisk were planted in the American west as rapidly-growing, heat-tolerant cover for both horticultural purposes and erosion control along river banks. As a plant, it was hugely successful, dramatically expanding into new areas, and outcompeting native species. Today, tamarisk has been shown to dramatically lower the water table, increase fire frequency and soil salinity, and significantly decrease biodiversity of native flora and fauna. It is considered highly invasive in the California Invasive Plant Council’s statewide invasiveness ranking.
Photo: Mature tamarisk trees at Comanche Spring.
Typical management methods for this species involve cutting woody stems and immediately painting the stumps with herbicide. An individual plant can have many dozens of stems, so this method is not only extremely laborious, but also expensive and slow going. Sometimes stumps re-sprout and need to be treated again. For land managers, this means that effectively managing tamarisk is an expensive and time-consuming practice that may limit implementation in landscape-scale infestations.
Photo: Tamarisk saplings (foreground) with mature trees at Comanche Springs.
On Tejon, tamarisk is generally associated with drainages and is found in low densities throughout the Ranch. However, there are two drainages in the San Joaquin Valley where tamarisk dominates the creek-dwelling community, decreasing the quality and availability of habitat for native species. This summer we have embarked on a collaboration with researchers from UC Santa Barbara Riparian Invasion Research Laboratory to release a specialized invertebrate in these infested creeks that specifically targets tamarisk. The tamarisk leaf beetle (Diorhabda carinulata) is a highly vetted herbivorous biocontrol species that has been used to manage tamarisk in watersheds in the American southwest since the early 2000’s. It works by consuming and damaging large amounts of tamarisk leaves, defoliating trees each growing season, and effectively starving the tree to death over several years. Release sites are monitored monthly to determine the effect beetles are having on tamarisk over time, and also how the surrounding vegetation and invertebrate communities change. Although abatement takes place over several years, using this method requires very limited labor and financial investment to make a real difference, which is extremely useful for a small land trust like ours.
Photo: The tamarisk leaf beetle (Diorhabda carinulata).
Beetles were released in mid-June and we just completed our initial round of monitoring. The first step is to get the beetles established so they can reproduce and get started on their prolonged feasting. There have been no dramatic changes in the tamarisk populations so far, but we are excited to see how things might change as time goes on. If all goes well and the beetles establish, we should start to see stress in our tamarisk populations next spring—fingers crossed!
Photo: A bag is used to contain the initial release of beetles in Tejon Creek.