Scientists at the LSU AgCenter are attempting to understand the primary factors influencing the die-offs and the implications of the roseau cane scale invasion.
State-wide census to determine the geographic extent of the scale in Louisiana
By inspecting roseau cane populations in parishes where the plant is known to occur over the 2017 growing season, researchers have developed a map indicating where the scale is known to occur. This will help agencies monitor the spread and help the public prevent unintentional introduction of the scale to un-infested parishes.
Update January 2019: Statewide sampling for 2018 indicate the scale has not spread to sites where it had already been recorded as absent, and has currently been detected in 12 parishes in southeastern Louisiana. Additionally, while exploring sites along the Texas, Mississippi, and Alabama Gulf Coast, the scale was only detected at a single location outside of Louisiana. The location of this site near a boat launch on J.D. Murphree WMA, suggests the scales were inadvertently transported here. How long the scale has been present at this site is unknown, but given it’s detection in early 2018, it likely predates the quarantine enacted to prevent further spread of the scale.
Monitoring of scale populations at sample sites across the lower MS River Delta
In addition to the statewide survey, periodical sampling of reference points across the lower delta are providing how scale populations change and how infestations affect Phragmites stands over the course of the growing season. With this data we can monitor changes in roseau stand density, growth, and changes in vegetation cover.
Update January 2019: One of the major questions following the harsh winter weather that hit Louisiana in early 2018 was whether the scale had been significantly impacted. As cold weather killed off much of the above ground biomass in the Delta, greatly impacting the appearance of Phragmites stands, and renewing alarm for the potential impacts of the scale.
Following bimonthly sampling events at monitoring sites across the Delta, 2018 has shaped up to be a very interesting. Scale populations quickly rebounded, and by May were already more numerous than peak populations observed in October 2017. Numbers continued to increase beyond what was observed in 2017, and populations observed in August were the highest observed in Louisiana to date. Interestingly, average numbers decreased by the final sample date in October 2018, unlike 2017 where October scale numbers were the highest for the year. The reason for this is unknown, but potential hypotheses include that scale numbers reached their carrying capacity or that the increased density of scales made them more susceptible to natural enemies or disease.
Despite higher scale populations, symptoms of die-off appeared much patchier than in 2017, with many sites showing signs of improvement, while others continued to decline. This inconsistent symptomology, along with results from controlled experiments conducted in Baton Rouge, indicate that while there is a clear relationship between scale infestations and die-off symptoms, there are likely other unidentified factors interacting with these scale infestations to produce the symptoms we observe in the field. While these data are still being analyzed and written up for publication, the scale does seem to play a role; however, the scale itself may not be the sole culprit.
Dr. Jim Cronin measuring roseau cane at a sample site in Plaquemines parish (Photo: Rodrigo Diaz)
Stand of resistant European invasive (near right) adjacent Delta variety (far center) (Photo: Ian Knight)
Different roseau cane varieties growing under salt and scale stress conditions at LSU Innovation Park
Testing roseau varietal resistance to scale infestations
Louisiana has several genetic lineages of roseau cane distributed across the state. Initial surveys have found that while the dominant ‘Delta’ variety of roseau in the lower delta appears to be susceptible to the scale, while an invasive ‘European’ variety appears less susceptible to the scale and does not show the die-off symptoms. Additionally, an experiment is being designed to address how abiotic stressors like prolonged inundation and increased salinity interact with infestations of the scale on susceptible and resistant varieties from the delta. The scale is also being assessed for the risk of infesting other Louisiana natives and agriculturally important grass species.
Update January 2019: Monitoring of paired ‘European’ and ‘Delta’ sites continued through September 2019. Trends from 2018 were even more clearly observed in 2019 with scale infestations on the susceptible ‘Delta’ cane greatly exceeding those observed on ‘European’. Populations were similarly low after the 2017-2018 winter, though scale numbers on ‘Delta’ variety stands quickly outpaced their European counterparts. Interestingly, the decline observed on the final sampling date in the marsh wide monitoring project was also observed in the ‘Delta’ variety stands, but not in the ‘European’ stands, where scale numbers increased relative to the previous sample date. On the final sample date of 2019, due to the decline in the Delta stands, there were no longer significant differences in scale densities between the two varieties. These differences were also replicated under controlled conditions in Baton Rouge, indicating that, as with many other herbivorous arthropods, the European variety is resistant to infestation by the roseau cane scale.
In addition to the paired plots study in the Mississippi River Delta, infestations of a common garden, consisting of different varieties of Phragmites from across the United States were conducted in summer 2018, and harvested in October. The project was conducted as part of the Master’s research of graduate student Leslie Aviles, and results will be updated once they have been analyzed.
Leslie Lopez (left) and Dr. Rodrigo Diaz (right) sample scales occurring in a roseau cane common garden at LSU (Photo: Madeline Gill)
Dr. Blake Wilson testing different insecticides for scale control on infested roseau cane (Photo: Madeline Gill)
Investigation of chemical management solutions
While options for chemical treatments in wetland habitats are likely to be limited, in case the scale is found to be an agricultural pest, various insecticides are being tested on infested populations of roseau cane.
Update January 2019: Due to the concealed nature of the scale, a management plan which targeted new hatched scale insects which are still mobile and more susceptible to insecticide applications seemed to be a logical solution to the ineffectiveness of previous insecticide trials. In agriculture and horticulture, scale insects can be difficult to treat with foliar insecticides, due to a waxy protective coat. To get around this issue, managers will monitor for the emergence of newly hatched scales which still have legs, are mobile, and do not yet have their waxy shell. A similar protocol was attempted in spring 2018, when the first generation of crawlers was detected in stands of roseau cane.
Few differences were observed, but after several weeks of monitoring, there appeared to be a trend of reduced scale populations in some of the treated stands, though the results were not statistically significant. In an attempt to further express these differences an additional application of insecticides was made and the plots were monitored for several more weeks. Unfortunately, not only did the anticipated differences not occur, but some of the treated plots eventually became much worse than the untreated plots. These mixed results indicate that while there may be some efficacy of foliar applications, if the timing and chemistries were refined, the treatments did not eliminate the scales, and potentially disrupted the natural enemies resulting in worse infestations in some treatments.
Several attempts were made to assay the scales under controlled laboratory conditions, during the spring and summer of 2018. Unfortunately, while the scale flourishes in the natural environment, it is surprisingly difficult to work with in the laboratory. Ultimately, a technique was developed to determine the effect of different insecticide treatments on crawlers; however, of the chemistries tested, the only products which produced significant mortality are far too toxic for use in wetland environments.
A new technique to better assess the efficacy of chemical treatments on settled scales is currently being developed and will hopefully be implemented in the early months of 2019.
Roseau cane scale host-range studies
As part of her Master’s thesis research, graduate student Leslie Avilés, has been assessing the potential risk of the roseau cane scale to other grass species of ecological and agronomic significance in Louisiana.
In controlled greenhouse studies, none of the agronomic grasses, corn, sorghum, sugarcane, or rice showed any signs of infestation. Dead crawlers were observed on the surface of these plants indicating the inoculation was successful, but that the scales failed to establish themselves. Of the marsh grasses tested, settling was observed on California bulrush and smooth cordgrass; however, these individuals did not develop into adult scales as they did on the roseau cane controls.
A field survey of alternative hosts was also conducted in the field, to observe for the potential of spillover from heavily infested stands of roseau cane in the Delta. Despite record scale densities on the roseau cane, scales were only observed in wild rice, and at densities far below those of roseau cane.
Results from these projects will be formally presented as part of Leslie’s Master’s thesis; however, these preliminary results suggest the scale should not be of concern to Louisiana agriculture or to any of the other marsh grasses found in coastal Louisiana.
Assessing soil and site specific effects
During the 2018 season, soil samples have been collected from monitoring sites throughout the year. Additionally, soils from healthy and die-back sites are now being analyzed for sulfides, a known phytotoxin associated with die-offs in Europe. Soils from these healthy and die-off sites are also currently being assessed in the greenhouse to determine the site specific effects of soil on roseau cane health.
Research Associate Seth Spinner collecting soil samples from a die-off site at Pass-a-Loutre WMA
Screening Phragmites for plant pathogens
In order to determine the potential role of plant diseases in the roseau cane die-offs, the lab of Dr. Rodrigo Valverde is working to isolate and culture microorganisms from stem and leaf tissue of die-off affected Phragmites. These cultures will be identified using morphological and molecular methods. Healthy roseau cane leaf tissue, and ultimately healthy whole plants, will be inoculated in the lab and the greenhouse to confirm their susceptibility to these potential pathogens and to determine their role in the die-offs.
Currently several microorganisms have been isolated and tentatively identified from samples taken in the Mississippi River Delta. Some of them include: Stagonospora sp. (endophyte), Fusarium concolor, Fusarium equiseti, Fusarium incarnatum, Cladosporium tenuissimum, Drechslera euphorbiae, Aspergillus niger, and Bipolaris sp.
Cultures of microbes collected from roseau cane in the MS Delta
Restoration plot monitoring
In October 2018, the Coastal Protection and Restoration Authority (CPRA) established revegetation plots at several locations in the Pass-a-Loutre Wildlife Management Area. These sites were chosen in an attempt to restore sites in the Delta where Phragmites die-offs had ceded to open water. Plantings included locally abundant as well as resistant varieties of Phragmites as well as other ecologically important marsh grasses.
These sites will be monitored by LSU to determine the success and survivorship of the plantings. Researchers will also be monitoring for infestations by the scale, soil chemistry, and water regime, to better understand the factors behind the die-offs and improve upon future planting efforts.
Phragmites plantings at rattlesnake flats restoration sites at Pass-a-Loutre WMA
Longitudinal population dynamics
In addition to the projects listed above, a new experiment is underway sampling sites along a North-to-South gradient. By visiting easily accessible sites on a biweekly basis over the season, we will get a fine temporal scale resolution of how scale populations emerge and change throughout the year.
Update January 2019: Season long monitoring of these accessible sites has yielded considerable information on the timing of scale emergence and how scale populations change through time. The project is ongoing, and will be wrapped up once the first major emergence of crawlers is observed which is anticipated late March or April.
In addition to monitoring scale populations, undergraduate student Keyla Pruett has expanded upon the sampling efforts of this study to include the multiple parasitoid wasp species, which attack the scale in Louisiana, to better understand their biology and potential role in managing the roseau cane scale.
Undergraduate Keyla Pruett examines roseau cane scales for parasitoid wasps