Investigating Plant Phenotype, Salinity, and Infestation by the Roseau Cane Scale as Factors in the Die-Back of Phragmit
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GENERAL WETLAND SCIENCE
Investigating Plant Phenotype, Salinity, and Infestation by the Roseau Cane Scale as Factors in the Die-Back of Phragmites australis in the Mississippi River Delta, USA Ian A. Knight 1
&
James T. Cronin 2 & Madeline Gill 1 & John A. Nyman 3 & Blake E. Wilson 1 & Rodrigo Diaz 1
Received: 23 July 2019 / Accepted: 6 May 2020 # Society of Wetland Scientists 2020
Abstract Die-back of Phragmites australis in the Mississippi River Delta (MRD), Louisiana, USA, is associated with large populations of nonnative Roseau Cane Scale (RCS), Nipponaclerda biwakoensis. Initial observations suggested different P. australis phenotypes displayed different susceptibilities to scale infestation and die-back, but the role of scale infestation on die-back was unknown. To understand the effect of RCS on P. australis, paired stands of Delta and European phenotypes in the MRD were monitored for stem heights, densities, and scale infestation over two years. A mesocosm experiment was conducted to assess whether RCS abundance and P. australis growth were dependent on water salinity and phenotype. Three Phragmites australis phenotypes were grown in small pools under fresh or mesohaline conditions, and RCS infestated or non-infested treatments. Scale densities were up to 7 times greater on the Delta compared to the European phenotype. Scale infestation resulted in 22%– 39% reductions in the proportion of stems with green leaf tissue for all phenotypes, and 12% reduction in stem heights for Deltatype. Salinity was detrimental to all phenotypes, reducing stem heights by 20% compared to freshwater. Our results provide evidence that the RCS can result in die-back symptoms similar to what is observed in the MRD. Keywords Die-back . Phragmites australis . Mississippi River Delta . Scale insect . Salinity . Phenotype
Introduction Die-back of Phragmites australis (Cav.) Tren. Ex Steud. was detected in the Fall of 2016 affecting stands across the Mississippi River Delta (MRD), Plaquemines Parish, Louisiana, USA (Knight et al. 2018). Symptoms of die-back include retreat from deep water, reductions in stem density, a more clumped distribution of stems within a stand, and premature senescence of leaf tissue (Armstrong et al. 1996a; van der Putten 1997). Widespread declines in plant health have also been observed through remote sensing where an analysis
* Ian A. Knight [email protected] 1
Department of Entomology, Louisiana State University, 402 Biological Sciences Bldg, Baton Rouge, LA 70803, USA
2
Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803, USA
3
School of Renewable Natural Resources, Louisiana State University, Baton Rouge, LA 70803, USA
of NDVI (normalized difference vegetation index) from Landsat imagery has suggested that declines may have begun as early as 2015 (Ramsey III and Rangoonwala 2017). While P. australis is often considered invasive across much of North America (Chambers et al. 1999), it can also provide several ecosystems services (reviewed in Kiviat 2013). P
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