Wetlands (2020) 40:1327–1337 https://doi.org/10.1007/s13157-020-01307-3 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. Knight1 & James T. Cronin2 & Madeline Gill1 & John A. Nyman3 & Blake E. Wilson1 & Rodrigo Diaz1 Received: 23 July 2019 /Accepted: 6 May 2020 / Published online: 17 July 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 pheno- types 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 Delta- type. 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 of NDVI (normalized difference vegetation index) from Landsat imagery has suggested that declines may have begun Die-back of Phragmites australis (Cav.) Tren. Ex Steud. was as early as 2015 (Ramsey III and Rangoonwala 2017). While detected in the Fall of 2016 affecting stands across the P. australis is often considered invasive across much of North Mississippi River Delta (MRD), Plaquemines Parish, America (Chambers et al. 1999), it can also provide several Louisiana, USA (Knight et al. 2018). Symptoms of die-back ecosystems services (reviewed in Kiviat 2013). Pragmites include retreat from deep water, reductions in stem density, a australis is the dominant emergent vegetation in the more clumped distribution of stems within a stand, and pre- Mississippi River Delta (Hauber et al. 2011), and declining mature senescence of leaf tissue (Armstrong et al. 1996a;van stands of P. australis raise concerns over the stability of the der Putten 1997). Widespread declines in plant health have regional wetlands. also been observed through remote sensing where an analysis Although die-back of P. australis is new to the MRD, the phenomenon has been reported from across its cosmopolitan distribution. Research by the European Research Project on * Ian A. Knight Reed Die-back and Progression concluded that a significant [email protected] factor in the decades of die-back in Europe was eutrophica- tion, exacerbated by stagnant water tables (reviewed in van 1 Department of Entomology, Louisiana State University, 402 der Putten 1997). This was by no means the only factor iden- Biological Sciences Bldg, Baton Rouge, LA 70803, USA tified in the die-back across this continent. Stress from soil 2 Department of Biological Sciences, Louisiana State University, phytotoxins, plant pathogens, and insect feeding damage were Baton Rouge, LA 70803, USA all implicated in producing symptoms associated with die- 3 School of Renewable Natural Resources, Louisiana State University, back (Armstrong et al. 1996a, b). In China, die-back of stands Baton Rouge, LA 70803, USA in the Dongtan wetlands of the Yangtze River estuary were 1328 Wetlands (2020) 40:1327–1337 attributed to the invasion of Spartina alterniflora (Loisel.). Mediterranean, and is the dominant lineage found in the The only North American account of P. australis die-back MRD (Hauber et al. 2011;Lambertinietal.2012). Die- occurred at Pointe Mouillee, Michigan, concurrent with cata- backs in the MRD appear to primarily, or at least more severe- strophic die-back of other emergent wetland species, and was ly impact the Delta-type (Knight et al. 2018). Also found attributed to rising and prolonged high water levels within the MRD is a phenotype of European lineage (EU- (McDonald 1955). type, haplotype M) (Saltonstall 2002; Lambertini et al. Increasing salinity can also cause die-back of P. australis 2012). The EU-type P. australis is a lineage that is often stands. Lissner and Schierup (1997) report die-back of considered invasive throughout much of North America and P. australis stands in Europe at sites where soil water salinity is frequently reported to be less susceptible to arthropod her- was greater than 15% [sic]. Concentrations of 20 ppt and bivores than native North American haplotypes (Park and greater have been shown to reduce shoot production (Eller Blossey 2008;Croninetal.2015). Beyond the MRD, Land- et al. 2014; Achenbach and Brix 2014), though no differences type (haplotype I) is found in more upland habitat, is widely were observed below 20 ppt. Soil water salinities of 28 ppt distributed along the Gulf Coast (Saltonstall 2002; Hauber promoted replacement of invasive P. australis by native salt et al. 2011), and has recently spread westward to California marsh species (Sun et al. 2007). Symptoms such as stunted (Meyerson et al. 2010). Considered to be a distinct subspecies plant growth are exacerbated by greater salinities and (P. australis berlandieri), Land-type occurs in less saline con- prolonged exposure (Hellings and Gallagher 1992; Howard ditions, potentially explaining its rarity in the MRD and Rafferty 2006). Saltwater intrusion resulting from eustatic (Achenbach and Brix 2014). sea level rise, subsidence, and changes in weather patterns as a The goal of this study is to investigate the role of RCS in result of global climate change are expected to exacerbate the die-back of P. australis by determining the susceptibility salinity stress (reviewed in Day Jr et al. 2005). of different P. australis phenotypes to the scale, and testing The discovery of extensive die-back in the MRD precipi- how scale herbivory and salt stress interact to affect plant tated the discovery of heavy infestations of the non-native health. The first objective was to evaluate stand conditions Roseau Cane Scale (hereafter referred to as RCS), and determine whether the two common P. australis pheno- Nipponaclerda biwakoensis (Kuwana) types found in the MRD (EU-type, Delta-type) differ in RCS (Hemiptera:Aclerdidae) (Knight et al. 2018). Subsequent sam- abundance (number of scales per stem) and frequency of par- pling by our group revealed that the RCS was present in 11 asitism. Five contiguous pairs of EU- and Delta-type stands parishes in southeastern Louisiana (Knight et al. 2018). The were censused over two years. For the second objective, a scale appears to have arrived with three Encyrtid parasitoid controlled mesocosm experiment was conducted to test wasps known from their native range, Astymachus japonicus whether salinity level and P. australis phenotype influenced Howard, Neastymachus japonicus Tachikawa, and susceptibility to the RCS and whether the RCS alone or in Boucekiella depressa Hoffer (Tachikawa 1970; Kaneko combination with salinity could cause the die-back symptoms 2004; Knight et al. 2018). Native to Japan, China, and observed in the MRD. Korea, RCS is of little to no concern to local P. australis populations except as a pest where P. australis is grown com- mercially (Kuwana 1907; Qin et al. 2003;Brixetal.2014). Materials and Methods The effect of the RCS on P. australis health has not been studied; however, feeding by sap sucking insects can result in Paired Stands of EU- and Delta-Type Phenotypes physiological changes, which may reduce airflow to sub- merged roots (Armstrong et al. 1996b). Other damage by Pass-a-Loutre Wildlife Management Area (Pass-a-Loutre) is a sap-sucking insects includes injury by direct removal of plant 46,540 ha reserve located in the Mississippi River Delta and nutrients and sugars, as well as reduced photosynthesis from managed by the Louisiana Department of Wildlife and the accumulation of sooty mold growing on excreted honey- Fisheries. Pass-a-Loutre is a coastal freshwater marsh with dew (Bowling et al. 2016). Severe infestations of invasive sap- average salinity levels less than 1 ppt that is fringed by beach sucking, scale-like, insects in bamboo can result in death of and barrier island habitats with P. australis being the domi- the stand (Hodges and Hodges 2004). Currently, there are no nant vegetation of the region (CPRA 2019). Mean water depth data on the role of RCS infestations in P. australis die-back, across sites at the time of sampling ranged from 7 to 44 cm. but initial observations of relatively low scale densities and Mean annual air temperatures over the duration of the study better apparent health of certain P. australis phenotypes in the ranged from 17.2 to 25 °C and sites consisted of Balize and MRD implicate RCS as a potential factor in the die-backs. Larose soils (Natural Resources Conservation Service 2019). Within Louisiana there are several genetic lineages with Across its invasive range, exotic Haplotype M exhibits distinctive phenotypes. Delta-type (haplotype M1) is geneti- growth habits that distinguish it from native haplotypes. cally similar to populations found in North Africa and the Relative stem heights are comparable among haplotypes for Wetlands (2020) 40:1327–1337 1329 young shoots (Vasquez et al. 2005), but non-native index of degree of plant senescence. Percent green was esti- P.