Invasive Pest May Not Be Only Cause of Recent Louisiana Marsh Die-off

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A non-native insect infestation may not be the only factor involved in the ongoing die-back of a marsh grass in the Mississippi River’s “bird foot delta,” the ecologically and economically important part of coastal Louisiana where the river meets the Gulf of Mexico.

chart of vegetation change in the bird's foot delta
Changes in vegetation density in Louisiana's Mississippi River delta, May 2015-May 2016. Red and purple areas have 20% or more reduction in vegetation. USGS, public domain.

Non-native insects from Asia have been the main suspects in a recent, rapid die-off of an important marsh grass that helps stabilize Louisiana’s erosion-prone coastal wetlands. But a new USGS report, based on satellite imagery and a special technique for assessing marsh vegetation, shows much of the affected marsh began declining in 2015, more than a year before the latest die-off and insect infestation were discovered. The vegetation partially recovered in 2016; then a second decline began last fall.

The report does not specify the cause of the declines, but its results suggest the insect infestation may not be the only factor involved in the ongoing die-off of the grass Phragmites australis in the Mississippi River’s “bird foot delta,” the ecologically and economically important region where the river meets the Gulf of Mexico, and where fragile wetlands nourish large populations of ducks, wading birds, fish and shellfish.

The stakes are high, said phragmites expert Rebecca Howard, a research ecologist at the USGS’ Wetland and Aquatic Research Center in Lafayette, Louisiana. The state has lost about one-quarter of its wetlands—an area the size of Delaware—since the 1930s. Land managers and government agencies are working hard to protect and restore the remaining coastal wetlands, which provide flood protection to important shipping ports and inland communities, and act as nursery areas for a wide variety of wildlife.

“If phragmites stands fail to recover from the dieback, tens of thousands of acres of loosely-consolidated soils in the delta would become vulnerable to loss during coastal storms,” Howard said. “This could undermine the current Mississippi River shipping channels, pose a threat to navigation, and affect two wildlife refuges that support significant populations of wintering waterfowl.”

Commonly known in Louisiana as roseau cane, phragmites is considered the backbone of many of the delta’s coastal marshes. It has become the dominant plant in many places, and its dense, strong roots catch and hold sediment, retaining wetland soils and helping to build new land. So land managers reacted with alarm in spring 2017, when wide expanses of phragmites appeared dead or dying. About that same time, researchers found dense infestations of Nipponaclerda biwakoensis, a scale insect native to Asia, in affected phragmites stands. The insects, also known as mealy bugs, suck the grasses’ sap, weakening or killing them. Scientists from Louisiana State University, the U.S. Army Corps of Engineers and other agencies are working quickly to determine the extent of the phragmites die-off. According to preliminary estimates about 100,000 acres have been affected, including parts of Delta National Wildlife Refuge, managed by the U.S. Fish and Wildlife Service, and Louisiana’s Pass-a-Loutre Wildlife Management Area.

USGS scientists Elijah W. Ramsey III and Amina Rangoonwala, both experts in using remote sensing to track changes in marsh vegetation, went to work to pin down the timing of changes in hard-hit areas. Using satellite images of the Mississippi River delta and surrounding areas taken between 2014 and 2017, they applied a vegetation index that can identify changes in the amount of living marsh vegetation. Ramsey and Rangoonwala interpreted declines in the amount of living vegetation  as an indicator of declining health. To make sure any changes taking place in other plant species did not skew their results, they focused on a part of Delta National Wildlife Refuge that was mapped in 2011 and was dominated by phragmites at the time.

A stand of phragmites with many dead stems
Phragmites showing signs of stress at Pass-A-Loutre Wildlife Management Area, Louisiana, in April 2017. Rebecca Howard, USGS, public domain.

The phragmites appeared healthy from 2014 until summer 2015, when the images captured the start of a sharp, widespread decline that continued until spring 2016. During the 2016 growing season, the phragmites-dominated marsh either partially grew back, or remained stable, the researchers found. A new round of phragmites decline began in fall 2016 and continued through the end of the study period, in April 2017, but it was not as severe as the 2015-2016 dieback, Rangoonwala said.

“One big question is, what came first? The insect infestation or the dieback?” said Rangoonwala. “The insect infestation could have caused both the first die-off and the current one. Or the grasses may have been weakened from some unknown stress factor, and the insects may have come in later and taken advantage of that vulnerability." Howard noted that environmental factors such as increased salinity and high water can stress phragmites and affect its growth.

The USGS researchers plan to analyze images as far back as 2009, looking for trends in the phragmites health, and to step up the frequency of future mapping so they can provide an early warning of phragmites declines.

The report co-authored by Ramsey and Rangoonwala, “Mapping the change of Phragmites australis live biomass in the lower Mississippi River Delta marshes: U.S. Geological Survey Open-File Report 2017-1098,” was published July 28 and is available at https://pubs.er.usgs.gov/publication/ofr20171098 .