Where does the roseau cane scale come from?
The roseau cane scale (RCS), Nipponaclerda biwakoensis, is found on roseau cane, Phragmites australis, in wetlands in China and Japan. It is unknown when or how the scale arrived in Louisiana. Given the degree of infestation in the Mississippi River Delta, it is likely that the scale arrived on shipping containers that pass through the Delta.
Are the RCS responsible for the die-offs in the Mississippi River Delta?
The short answer, is we don’t know. Scale insects feed on plant fluids. Since the scale is not thought to transmit disease, a single scale is insignificant, but heavy infestations may result in severe plant stress. Given the apparent sudden arrival and expansion of the scale, and it coincidence with the die-off syndrome, the scale is a strong suspect. However, die-off of roseau cane, has also been observed in Europe where the scale is not present. It may be possible that the scale is only a single factor in the die-offs, or they may be opportunistic pests exploiting cane that are already stressed due to other environmental factors. Experiments are underway at the LSU AgCenter to determine the effect of the scales on roseau cane under controlled settings.
How does the RCS spread?
Most roseau cane scales are only mobile during their first instar (life stage). This first instar is called a crawler and is the only life stage that has legs. Male scales keep their legs their entire lives, but are very rare. When these crawlers hatch they will disperse to find a new spot to feed. These crawlers are almost microscopic, and can be dispersed by a variety of factors including the wind, on animals like birds, and possible even by humans who come in contact with the cane. Once a crawler finds a suitable place to settle, it begin feeding, lose its legs, and will remain in that location for the rest of its life. Female scales store their eggs inside their bodies until they hatch, so the scale can spread if infested plant material is moved to areas where the scale does not occur.
Do the RCS infest any other plants in Louisiana?
While RCS has been reported on plant species other than roseau cane in China, it has only been observed on Phragmites in Louisiana. Research is underway at the LSU AgCenter to determine the risk the scale poses to other environmentally and agriculturally important grass species. While this work is ongoing, preliminary data suggests the host range of the scale in Louisiana is restricted to roseau cane. This will be updated as the project progresses.
What is being done to control the RCS?
Because the scale occurs in wetlands, management options are limited. There are no insecticides approved for use on wetlands, and it is likely that any chemical control methods would have severe unintended impacts on non-target species. Fortunately, there are native and exotic natural enemies of the scale. Several species of wasp have been observed parasitizing scales in Louisiana. Some of these wasps are native, while others appear to have arrived with the scale. Given the severity of the scale infestations in the Delta, the degree to which these wasps will help keep the scale in check is unknown. Naturally occurring populations of natural enemies like the wasps often lag behind their hosts, and our wasps may not be able to keep up with the scales’ explosive growth. There may be potential for augmentative releases of lab reared parasitoid wasps in the spring to bolster natural populations. Additionally, there appears to be differences in susceptibility to the scale and die-off symptoms among different genetic lineages of roseau cane, which may provide resistant source material for revegetation efforts in areas where the cane has died off.
What can the public do to help?
The primary concern at the moment is limiting the spread of the scale. Please do not intentionally move the scale or infested cane. Care should also be taken to rinse clothes and vehicles that have come in contact with roseau cane, especially when moving from infested to uninfested regions.
You can also help us keep our distribution map up to date! We can’t be everywhere at once, and the time we can spend updating our distribution map is limited. You can help by submitting your observations to our Qualtirx survey form (https://lsu.qualtrics.com/jfe/form/SV_0j0TdVkA4eNmak5).
How did the freezing weather during the winter of 2017-2018 affect roseau cane and the scale.
Roseau cane grows and thrives in climates with colder winters than Louisiana. That being said, the freezing weather appears to have damaged much of the above ground biomass of roseau cane at sites monitored by the AgCenter. What remains to be seen is how vigorously these stands recover over the 2018 growing season. If the scales are, at least in-part, responsible for the die-back symptoms observed in the Delta, it is likely due to their disruption of roseau cane’s ability to sequester energy in its roots which the cane uses to regrow after disturbance events like freezing weather or hurricanes.
Much like roseau cane, the scales are found at much higher latitudes than Louisiana (Louisiana is likely the furthest southern reported population of the scale in published literature). Exactly how the scale will respond this exceptional winter is currently being investigated, but it is unlikely the freezes will produce any lasting effect on scale populations. Data collected February of 2018 shows live populations of scales are still present at sample sites. While numbers are reduced, compared to October 2017, it is unknown whether this is simply part of a natural cycle. Sampling conducted in 2018 will allow researchers to compare current populations to 2017 populations and may provide some insight into how the scale responds to harsh weather.