Roseau cane (Phragmites australis) is the dominant vegetation at the mouth of the Mississippi River. Vast stands of roseau cane are habitat for wildlife, protect oil infrastructure, reduce storm surge and maintain channel navigation. One main benefit of healthy roseau cane stands is the preservation of water flow in the primary and secondary channels. This is due to the density and thickness of stems that reduce water flow, keeping larger volumes of water in the channel, and thus reducing need for dredging. In the fall of 2016, outbreaks of the roseau cane scale (Nipponaclerda biwakoensis) were found associated with die-offs of the cane over thousands of acres at the mouth of the river. Because of the lack of information about the roseau cane die-off and the importance of cane stands for channel navigation, a team of LSU AgCenter researchers from different disciplines was assembled. A brief summary of research projects being conducted focusing on the roseau cane scale follows.
As of December 2021, the scale was found along every stand of roseau cane in coastal Louisiana. Compared to 2017 when the scale was mostly located in southeastern Louisiana, the distribution in 2021 revealed a rapid expansion. We can speculate that the long-distance dispersal of the scale can be attributed to storms that can break scale-infested stems and move them over several miles, and birds that transport immature scales or crawlers over migratory paths. More importantly, studies demonstrated that the scale only attacks roseau cane and not any other plant species. Field surveys in Plaquemines Parish revealed that the scale overwinters as gravid females, and by spring, crawlers colonize the regrowth of the cane. Outbreak conditions are found in late summer and fall when every roseau cane stem at the mouth of the river is infested with scales. These findings suggest that the scale is well established throughout Louisiana and has the potential to expand to Texas and Mississippi.
Several methods are available to manage scale insects in terrestrial ecosystems; however, these might not be suitable for wetlands. Field trials revealed that chemical control is not effective since the scale is protected by the leaf sheath, and nontarget damage could occur to nearby aquatic arthropods. Since there are several varieties or lineages of the roseau cane in Louisiana, researchers evaluated whether they might have resistance to the scale and thus the potential of being used in restoration trials. Results from field trials and surveys in the mouth of the river demonstrated that the European variety of cane had fewer scales compared to the Delta and Gulf varieties. Moreover, plant height and green coloration were reduced when the cane was grown under high salinity and exposed to scales. Thus, abiotic stressors can increase the susceptibility of the cane to the scale attack. Control options for the scale will need to consider their interactions with other biotic and abiotic stressors.
Due to the difficulty of access to remote wetlands at the mouth of the river, biological control of the scale is considered an ideal management option. Fortunately, surveys throughout Louisiana revealed the presence of three parasitoid species attacking the roseau cane scale. Parasitoid females laid one or multiple eggs inside the scale body. Then, the parasitoid larvae fed on the internal organs of the scale eventually killing it. Mortality due to these parasitoids ranged from 10% to 50% in Louisiana. In addition, since the roseau cane scale is native to China, scientists have been looking for new parasitoid species that could be used as biological control agents to help manage the scale in Louisiana. By increasing biotic pressure through parasitism, scale densities could be reduced below injury levels resulting in healthier cane stands.
Developing effective control options for insect pests is an ongoing process. While we’ve made substantial progress understanding the distribution, impact to the plant, interaction with other stressors and role of natural enemies of the scale, the cane die-offs are occurring in one of most complex river deltas of the world. As we develop management options for the scale, we must recognize that the mouth of the river is subject to massive abiotic pressures, including sea-level rise, higher frequency of storms, sediment diversion and land subsidence. Maintaining the health of the roseau cane stands and thus the navigability of the shipping channel will require a truly multidisciplinary approach.
Rodrigo Diaz is an associate professor in the Department of Entomology.
(This article appears in the winter 2022 issue of Louisiana Agriculture.)
Colony of scales attached to the stem of roseau cane. In late Fall, the densities of scales can reach up to more than 800 per stem. Photo by Rodrigo Diaz
Open water visible through thinning stand of roseau cane. Picture taken at the Bird’s Foot Delta of the Mississippi River, Plaquemines Parish, Louisiana. Photo by Rodrigo Diaz