Integrating Biological Control Agents (Insects) with Herbicides for Suppression of Salt Cedar (_Tamarix spp._): Development of Cost-Effective Tools for Saving Water by Controlling an Invasive Phreatophyte

Project ID: 5000
Principal Investigator: Scott OMeara
Research Topic: Invasive Species
Funded Fiscal Years: 2008
Keywords: None

Research Question

Can biological control agents be integrated with slow-activity herbicides to enhance:

* The suppression of salt cedar growth and spread

* The cost effectiveness of reducing salt cedar growth and spread

* The establishment of biological control agents

* The efficacy and selectivity of chemical control alone

This question would involve aspects of timing of herbicide application and insect release, toxicity and symptomology of herbicides to both salt cedar and biological control agent, rates of chemical application, environmental variables, and costs.

Need and Benefit

Reclamation facilities are frequently plagued by salt cedar in the arid Western United States. Dense stands of salt cedar are heavy water consumers, with deep roots that are capable of reaching the water table at much greater depths than native phreatophytes such as cottonwoods and willows. This enables it to spread out over much wider portions of a flood plain and tap greater water supplies. Salt cedar also contributes to soil-surface salinity and increases the potential for wildfire frequency and intensity.

Current tools for control of salt cedar include chemical, mechanical, and most recently, biological methods. Chemical application when broadcast by fixed-wing aircraft or helicopter is relatively inexpensive and can be effective at controlling salt cedar. Slow-activity herbicides are defined for the purposes of this proposal (with specific reference to salt cedar and associated beneficial vegetation) as a rate, timing, active ingredient, and formulation dependent characteristic of a chemical application that produces relatively slow development of die-back and other symptoms. They essentially stress the plant and possibly induce resprouting with low rates of mortality. The advantage of slow-activity herbicides is that they can be used selectively to stress only the target species (salt cedar) and leave beneficial species unharmed. In contrast, fast-activity herbicides, defined as producing quick development of symptoms and high mortality and also dependent on application rate, timing, active ingredient, and formulation, have a broad spectrum of species efficacy and will damage or kill beneficial species in most cases.

Selective activity of herbicide applications is highly desirable in salt cedar infestations. The ability to control salt cedar while leaving beneficial species unharmed can expedite or eliminate restoration efforts necessary following salt cedar control. This would reduce long term management costs and time investments.

Mechanical controls can also be effective at controlling salt cedar if the root system is adequately pulled up and dried, but in practice this is a nonselective method as well. Above-ground mechanical control can be more selective, but is only effective for a short period of time, as salt cedar resprouts rapidly from root crowns and segments. Ground-based applications of herbicide alone or in combination with mechanical control are also effective and can be done selectively, but high costs often make this method prohibitive.

Biological control is emerging as a highly selective and sustainable method of salt cedar suppression that can be utilized in remote and diverse areas regardless of terrain or weather constraints that prohibit other management activities. Non-target feeding of the salt cedar leaf beetle (_Diorhabda elongate_) has not been observed in any open field situations. However, control efficacy and establishment of _Diorhabda_ beetles has seen variable success.

Based on experience and observations of _Diorhabda_ behavior from researchers, it is hypothesized that biological control agents may respond positively to resprouts and altered growth characteristics of salt cedar following application of slow-activity herbicides. The combination of the chemical and biological controls may have a synergistic effect on both control of salt cedar and establishment of beetles, providing a selective, cost-effective, and sustainable suppression tool.

Further studies into integration technologies that are both effective and selective in controlling salt cedar are needed to provide the most efficient tools to Reclamation managers with minimum associated costs and long-term viability. This scoping proposal is focused on examining the potential for integrating chemical and biological control technologies in light of herbicide/plant/beetle dynamics, costs, and involvement of collaborators and commitment of resources for further study.

Contributing Partners

Contact the Principal Investigator for information about partners.

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Last Updated: 6/22/20