Claudia Husseneder and Lane D. Foil

LSU AgCenter researchers investigated populations of the saltmarsh greenhead horse fly (Tabanus nigrovittatus) as bioindicators of marsh health in the aftermath of the April 2010 Macondo oil spill in the Gulf of Mexico.

Acute oiling in the Gulf of Mexico subsequent to the Deepwater Horizon platform collapse created visible impact on highly prominent vertebrates, particularly birds, marine mammals and turtles along the coast. In addition, coastal residents near oil-impacted areas recognized a suspicious absence of insects that usually “bug” fishermen in the marsh, such as the greenhead horse fly. This observation led the researchers to hypothesize that the oil residue affected the invertebrate ecosystems in tidal marshes and that horse fly residents of the marsh community could serve as bioindicators for wetland health because they require both aquatic and terrestrial habitat for survival. To test this hypothesis, the researchers first compared population abundance based on a census of trapped horse flies between areas of the marsh that were not affected by oil intrusion and those that have been affected by the oil spill. Second, they used genetic markers to compare population genetic and mating structure between populations from nonoiled and contaminated areas.

The researchers chose T. nigrovittatus as an entomological model for coastal ecology studies and as a bioindicator of the impact of the intrusion of oil into coastal marshes because this species is one of the few insect species native and bound to coastal marshes ranging from Texas to Nova Scotia. Its larvae are the top predators in the marsh and thus vulnerable to changes in the environment. Active adults would have developed as larvae in the marshes over the nine to 12 months prior to emergence, and thus adult populations can be direct indicators of marsh quality after prior insults.

The entomologists studied the impact of the 2010 oil spill on horse fly population abundance and genetics in Louisiana marshes. Horse fly populations were sampled biweekly from nonoiled and oiled locations immediately after the oil spill in June 2010 until October 2011, with canopy traps baited with dry ice, which is known to attract female horse flies. Nonoiled trap sites were located in southwestern Louisiana (Ship Channel in Cameron Parish, Rockefeller Wildlife Refuge at the border of Cameron and Vermilion parishes and Cypremort Point in St. Mary Parish). Oiled trap sites were located on Grand Bayou, Grand Isle and Elmer’s Island in southeastern Louisiana.

Adult horse fly capture rates showed severe crashes of horse fly abundance in oiled areas. Trap data indicated that the adult horse flies had been affected immediately after the oil reached Grand Isle, Elmer’s Island and Grand Bayou (an average of 1.27 to 4.8 flies captured per hour in 41 trap days); both of these areas are notorious for greenhead attacks each summer. In comparison, fly activity remained significantly higher at the nonoiled locations at Cypremort Point and Cameron (an average of 36.6 to 92.2 flies per hour in 60 trap days).

In 2011, the researchers found no significant change in the adult horse fly populations compared to 2010. However, there were site specific results that indicated potential recovery of horse fly populations. For example, there were two trap sites at the Grand Bayou location where 21 to 24 flies per hour were captured in August 2011. The significance of these data is that there are potentially islands of recovering marsh within affected areas.
 
  Genetic markers revealed that the majority of oiled populations went through a “genetic bottleneck,” a loss of genetic diversity, because of a severe reduction in population size, fewer breeding parents and fewer migrants among populations as compared to populations from areas not affected by the oil spill.

Similar to the immediate impact on the adult population, horse fly larvae were also affected. Larvae were isolated from 80 percent of the substrate samples obtained in 2011 from Cameron, 75 percent of samples from Cypremort Point (nonoiled areas), 38 percent of the sediment samples from Grand Bayou, but were absent in all samples from Grand Isle (oiled areas).

The next steps in this research are to describe the invertebrate food web of horse fly larvae and assess marsh health by conducting chemical and toxicity analyses of sediment samples and genetic analyses of larval guts from archived (2010-2011) and new collections several years after the oil spill. The goal of future studies is to establish links of toxic components found within marsh sediment samples with the absence of developing horse flies as well as elements of their food web and compare these data to healthy marsh ecosystems.

The techniques developed to measure the elements of the food web could be used for time- and cost-efficient screening to survey marsh health and potential need for remediation. Thus, the fishermen’s nemesis could serve as a model for environmental impact assessment. Perhaps, it is of some comfort to keep in mind that blood sacrifice to a plethora of greenheads is one sign of a healthy marsh ecosystem.

Claudia Husseneder and her co-author, Lane D. Foil, are professors in the Department of Entomology. Foil is the Pennington Chair for Wildlife Research.

This article was published in the summer 2014 issue of Louisiana Agriculture Magazine.