Claudia Husseneder, Foil, Lane D.
Claudia Husseneder, Lane Foil, Ben Aker, Darrius Davis and Patrick Rayle
In April 2010, the Deepwater Horizon oil spill in the Gulf of Mexico caused the largest ever human-made accidental marine oil spill. Almost 5 million barrels of oil were discharged in four months, which severely affected marine and coastal habitats. Coastal Louisiana received the most contamination of all Gulf states from the oil and dispersant mixture.
As pictures of dead seabirds, turtles and dolphins filled the news, people working in the marsh also noticed fewer insects, indicating a serious disturbance of the natural ecosystem. LSU AgCenter scientists conducted research combining population genetics and census studies using insects as bioindicators of marsh health.
The greenhead horse fly, Tabanus nigrovittatus (Figure 1), was identified as an indicator species for toxic effects of oil intrusion and recovery of marsh. This fly is one of few insects native to spartina marshes from Texas to Nova Scotia. The carnivorous larvae develop for three to nine months as top invertebrate predators in the marsh soil. Their abundance reflects the health of the food web in the mud of tidal marshes. The AgCenter team followed the fate of horse fly populations for six years after the oil spill.
Horse flies were collected from June to October 2010-2016 from oiled locations (Grand Isle, Elmer’s Isle, Grand Bayou) and locations unaffected by oil (Cameron Parish, Rockefeller Wildlife Refuge). They were captured with canopy traps and counted. The team extracted DNA and established the genetic profile of each individual via genotyping.
Immediately after the oil spill in 2010 and 2011, horse fly trap catches were significantly lower in oiled than in control sites; yet numbers increased by 2016. In 2010 and 2011, almost all oiled populations showed genetic bottlenecks, i.e. reduction in genetic diversity due to population crashes. All bottlenecks in formerly oiled locations had disappeared by 2015 and 2016, indicating recovery. No population from unoiled areas showed any genetic bottlenecks.
Population genetic analyses documented changes in genetic signatures of oiled and unaffected populations from 2010 to 2016. The 2010 populations of oiled and control areas fell into separate genetic clusters, indicating a lack of migration. In 2011, genetic signatures from control populations began showing up in oiled regions, indicating migration from unaffected to oiled regions that continued in 2015. By 2016, essentially the same genetic signatures were found in individuals and populations from oiled and unaffected regions, suggesting increasing migration of individuals from control populations into formerly oiled areas.
The research team initially expected the horse fly populations from summer 2010 would represent a pre-event baseline, since this generation emerged before the oil spill made landfall. However, polarized light reflected from oil sheens is thought to resemble that reflected from fresh water, so flying insects landing on these surfaces in search of water become trapped. This phenomenon is the likely cause of the acute adult mortality in oiled areas.
Population expansion in formerly oiled areas was likely fueled by migration, successful reproduction in oiled areas and increased survival of larvae developing in formerly oiled areas due to decreased oil residue toxicity and increased availability of their food web. Horse flies are strong fliers, and shrinking genetic distances over the years between populations from nonoiled and oiled areas indicate increasing migration rates. Tabanus nigrovittatus [HC3] females can lay more than 200 eggs per batch and can produce the first batch without a blood meal. Pursuit of a blood meal and subsequent egg-laying ensure dispersal of flies and recovery of depleted areas. Most microalgae and 90% of tiny soil organisms, which form the base of the food web in marsh sediments, recovered within three years after the oil spill. The terrestrial invertebrate community, mostly plant-feeding insects, had largely recovered one year after the spill. The guts of horse fly larvae contain mostly DNA of plant-feeding stink bugs.
The dispersal ability, high reproduction rate and restoration of the larval food web in oiled marshes was, therefore, vital for horse fly recovery. Horse fly populations’ sensitivity to oil contamination evidenced by the immediate population crash after the oil made landfall and their recovery following the recovery of spartina marsh grass and the invertebrate food web makes this species a valuable bioindicator to assess the process of marsh reconstitution.
Long-term costs from oiling in Louisiana marshes include erosion of marsh islands due to reduction of plant roots. Coastal restoration projects have begun to combat land loss using freshwater diversions, which are expected to decrease salinity in certain zones. To establish baselines of invertebrate communities in different salinity zones in anticipation of these changes and future challenges to the marsh, the AgCenter research team (Figure 2) surveyed invertebrate communities in 2017 and 2018 in different marsh salinity zones in Louisiana estuaries.
Canopy trapping detected three species of horse flies in low-, mid- and high-salinity zones. Most abundant at all locations was the Tabanus nigrovittatus horse fly bioindicator species, with spring and fall activity peaks. Using dissection techniques, researchers found that T. nigrovittatus females were the only horse flies capable of producing eggs without a bloodmeal. This ability explains the higher abundance relative to the other two species because it is advantageous for survival in harsh environments like the saltmarsh.
The AgCenter researchers also identified insect species by sequencing DNA extracted from marsh sediment. While horse fly DNA was present at all sites, researchers detected only eight species of springtails, flies and plantfeeding bugs at high salinity versus 19 species of predominantly springtails, flies, moths, beetles and plant-feeding bugs at low-salinity sites.
The lower insect biodiversity detected in soil at high salinity sites was confirmed by monthly sweep net insect inventories, which showed that 39 insect families resided at high salinity compared to 61 families at low-salinity sites. From these families, 26 species were flagged as bioindicators for salinity levels. At low salinities, a damselfly, Rambur’s Forktail (Ischnura ramburii) and a sharpshooter, Draeculacephala portola, were common, easily collected indicators. Rambur’s Forktail is predaceous as both an immature and adult. The immatures, which develop in the water, are tolerant of the brackish water at the low-salinity sites but cannot develop in highly saline water. The sharpshooter is a specialist on spartina cordgrasses and prevalent in the southeastern Atlantic and Gulf coastline.
At high-salinity sites, a plant-feeding bug, Ischnodemus conicus, and the predaceous black-headed melyrid Collops nigriceps are useful indicators (Figure 3). Both species are associated with the highly salt-tolerant smooth cordgrass that dominates Gulf and Atlantic coast salt marshes. The presence of these insect bioindicators can be used to predict that a certain habitat belongs to the salinity level that the insects are associated with and provides specific plant hosts for the herbivoruous insects or a stable food web for the predaceous insects.
The AgCenter database with seasonal distributions of invertebrate predators, parasites, herbivores and detritus feeders provides important baseline data in different salinity zones for assessing changes in marsh health due to future pollution and changes in salinity caused, for example, by erosion, subsidence, sea level rise or freshwater diversions. When combined with plant inventories and land mass surveys, the invertebrate surveys will likely be indicators of future population trends of commercially important invertebrates like shrimp as well as vertebrates dependent upon tidal marsh estuaries.
Claudia Husseneder is Paul K. Adams Professor of Urban Entomology, Lane Foil is Pennington Regents Chair for Wildlife Research, and Ben Aker, Darrius Davis and Patrick Rayle are graduate students in the Department of Entomology.
(This article appears in the spring 2020 issue of Louisiana Agriculture, which focuses on entomology.)
Figure 1. The greenhead horse fly (Tabanus nigrovittatus) was used as a bioindicator of decline and recovery of marsh health after the Deepwater Horizon oil spill. Photo by Claudia Husseneder
Figure 2. The research team included Department of Entomology students, from left to right, Darrius Davis, Ben Aker, Erin Stevens, Mike Becker, Patrick Rayle and Julian Lucero. Photo by Claudia Husseneder