Unmanned aircraft systems and tracer dyes : potential for monitoring herbicide spray distribution

Abstract

Chemical control of nuisance aquatic vegetation has long been the most widely utilized management tool due to its high level of efficacy, limited environmental impacts, and relatively low cost. However, unprecise application of herbicides can lead to uncontrolled invasive plants and unintended management costs. Therefore, precision herbicide delivery techniques are being developed to improve invasive plant control and minimize impacts to non-target plants. These technological advancements have the potential to enhance aquatic ecosystem protection from invasive species while reducing associated management costs. Despite the benefits of using registered herbicides for aquatic plant control in efforts to restore aquatic habitats, their use is often misunderstood and opposed by public stakeholders. This can lead to significant challenges related to chemical control of nuisance aquatic vegetation. Thus, US Army Corps of Engineers (USACE) Districts seek improved methods to monitor and quantify the distribution (i.e., amount of herbicide retained on plant foliage compared to those deposited into the water column) of herbicides applied in aquatic systems. Monitoring herbicide movement in aquatic systems can be tedious and costly using standard analytical methods. However, since the inert fluorescent tracer dye Rhodamine WT (RWT) closely mimics product movement in the aquatic environment it has been used as a cost-effective surrogate for herbicides tracing. The use of RWT (or other inert tracer dyes) can be an efficient way to quantify herbicide retention and deposition following foliar treatments. However, the collection of operational spray deposition data in large populations of invasive floating and emergent plant stands is labor intensive and costly. One proposed solution is the use of remote sensing methods as an alternative to traditional in situ samples. Specifically, using unmanned aircraft systems (UAS) in conjunction with RWT could provide more efficient monitoring and quantification of herbicide spray distribution and in-water concentrations when using RWT in combination with herbicides. A better understanding of UAS capabilities and limitations is key as this technology is being explored for improved and integrated management of aquatic plants in the U.S. This technical note (TN) provides a review of literature to assess the state of knowledge and technologies that can assist USACE Districts and partners with tracking herbicide movement (using RWT as a surrogate or additive), which could improve operational monitoring, thus reducing the level of uncertainty related to chemical applications and non-target impacts, and thus improve management in aquatic systems.