Roseau cane (Phragmites australis) Dieback in the Mississippi River Delta
Widespread die-offs of Roseau cane (Phragmites australis) have been reported in Plaquemines Parish since the fall of 2016 (Knight et al. 2018; Fig. 1). Roseau cane is widely used for erosion control at the mouth of the Mississippi River because of its ability to grow at depths unsuitable for other marsh plants. Due its aggressive growth and strong rhizomes, Roseau cane increases soil accumulation, provides habitat for fisheries and migratory birds, protects oil and gas pipelines, and prevents shoreline erosion from direct exposure to storm surge.
Upon close examination, state biologists found that cane die-offs were associated with the presence of a scale insect. This scale insect was recently identified by Dr. Scott Schneider (USDA-ARS) as Nipponaclerda biwakoensis (Kuwana) and is native to China and Japan (McConnell, 1954). It is now known as the roseau cane scale (Fig. 2). The roseau cane scale feeds on the cane sap and can be found along the stems. Interestingly, the Mississippi River Delta (MRD) is comprised of a mixture of multiple varieties of roseau cane including our “native” type (Gulf variety) along with invasive nonnative varieties originating from Europe and North Africa (Lambertini et al. 2012). These nonnative varieties include Delta which dominates the MRD, the well-known European invasive variety (Haplotype M), and several other uncommon varieties. These different varieties may differ in their resistance to the scale, but this has never been examined.
Infestations of the scale have been reported throughout the MRD including Venice, Grand Bay, Jaquines Island, Double Bayou, Pass A Loutre Wildlife Management Area, and Delta National Wildlife Refuge. Based on our surveys, scale infestations in these areas have reached more than 700 per stem causing extensive damage to the plant (Knight et al. unpublished). Three parasitoids have been found attacking the scale. The parasitoids were identified by Dr. John Noyes (British Natural History Museum) as Neastymachus japonicus, Boucekiella depressa, Astymachus sp., and they are native to Asia (Japoshvili et al. 2016). While scale populations are still occurring at damaging levels, without the mortality from these parasitoids, the impact on roseau stands may be worse.
Management options for roseau cane scales have not been studied. However, in China, roseau pests are controlled with winter burns, removal of crop residues and spring submersion of plants (Brix et al. 2014). The use of insecticides is generally not recommended in aquatic habitats; however, they could be an option if reduced-risk application methods can be developed.
Because of accelerating loss of Louisiana’s coast, it is imperative that the state develops short and long-term management plans for monitoring the health of Roseau stands and mitigating impacts of this invasive scale. In response to this critical problem, a group of us at LSU have gotten together to investigate the causes of roseau cane dieback, develop a plan to manage the health of the marsh ecosystem, and restore the vegetation in dieback sites. Partners in this project include Rodrigo Diaz (Fig. 3), Mike Stout and Blake Wilson (Entomology), Rodrigo Valverde (Plant Pathology) and Andy Nyman (School of Renewable and Natural Resources). Our work is supported by grants and awards from the Louisiana Department of Wildlife and Fisheries, Coastal Restoration and Protection Authority, LA Department of Forestry and Agriculture, National Resource Conservation Service, and the United States Department of Agriculture. Our work on this problem has been featured in the New York Times, The Washington Post, and various local news media sources. Most recently, Country Roads Magazine wrote an article on our work (https://countryroadsmagazine.com/outdoors/knowing-nature/roseau-cane-scale/).
Currently, our team is pursuing 7 objectives:
(Objective 1) Educate the public about the importance of roseau cane in mitigating coastal erosion along the Mississippi River Delta, and management of the roseau cane scale
(Objective 2) Monitor distribution and range expansion of roseau cane scale along the Louisiana Gulf Coast
(Objective 3) Assess host-plant specificity of the roseau cane scale
(Objective 4) Conduct experimental tests of the effects of scales and other stressors on roseau cane growth and survival (Fig. 4)
(Objective 5) Quantify secondary metabolic diversity among the different varieties of roseau cane, evaluate whether plant resistance or tolerance to scale insects and other herbivores is related to a particular secondary metabolite or the total diversity of metabolites, and whether plant stressors (e.g., scales, flooding, eutrophication) affect secondary metabolite profiles
(Objective 6) Evaluate, under laboratory and greenhouse conditions, the response of roseau cane to simultaneous infections/infestation of selected plant pathogens and insects
(Objective 7) Conduct experiments to evaluate which plant species or roseau varieties are most efficacious for restoring dieback areas in the MRD and develop restoration protocols
