Pilot Testing and Protocol Development of Apple Snail Suppression at Mandalay National Wildlife Refuge

Science Center Objects

The maculata apple snail (Pomacea maculata) has invaded many coastal and some inland areas in south Louisiana. Reports have indicated that the freshwater snails have significantly reduced the amount of submerged aquatic vegetation (SAV) and floating leaved vegetation (FLV) available for waterfowl and other wildlife.

Figure 1. Apple snail remains after being preyed upon by a raccoon at Mandalay National Wildlife Refuge. Circled on the left is the operculum with some flesh still attached. Circled on the right is the ovary, presumably left because it contains the same poisons used to protect the egg masses. The snail’s shell is at the top of the picture. (J. Carter, USGS 2017)

The Science Issue and Relevance: The maculata apple snail (Pomacea maculata) has invaded many coastal and some inland areas in south Louisiana. Reports have indicated that the freshwater snails have significantly reduced the amount of submerged aquatic vegetation (SAV) and floating leaved vegetation (FLV) available for waterfowl and other wildlife. There is also evidence that native predators such as alligators and raccoons are preying on the snails (Figure 1). Apple snail egg masses are easily visible because of their bright pink color and because they are deposited on objects sticking out of the water. Each female can lay an egg mass any time during the year provided the weather conditions are suitable. Their egg masses contain between 800 and 2,000 eggs, and during the most active months (summer) they may lay an egg mass every 4 - 7 days. Thus, a single female could produce more than 52,000 eggs from the beginning of April to the end of September; one hundred females could produce over 5 million eggs. Oftentimes, egg masses do not hatch out completely or are lost to flooding, but the high number of eggs in each clutch ensures that the ecosystem is flooded with apple snail hatchlings. Many animals are known to feed on the snails at every stage of their life history; however, the apple snail’s reproductive effort overwhelms the predators’ abilities to impact the population. A recent USGS-led study in Mobile, Alabama, demonstrated that spraying apple snail egg masses with coconut oil or PAM cooking spray reduced hatch success by as much as 80%. In this study, we will test the idea that a concerted egg mass reduction effort will allow the predator community to “catch up” and reduce the apple snail population and their impact on SAV and FLV. Our study objective is to develop and test protocols for apple snail egg mass suppression using a combination of physical destruction and sprayed on food oil. The study will occur in Mandalay National Wildlife Refuge near Houma, Louisiana, which has a confirmed population of apple snails and impacts to aquatic vegetation.

Figure 2. (A) Mandalay NWR is in Terrebonne Parish, Louisiana. The refuge boundaries are outlined in yellow. (B) Keyhole cuts outlined in red will receive egg mass suppression. Keyhole cuts outlined in green are control areas.

Methodology for Addressing the Issue: Due to accessibility issues we will be working in keyhole cuts of dredged canals intersecting Refuge property (Figure 2). Three sites at keyhole cuts will receive eradication treatments that will be compared with two similar keyhole sites without treatment. Weekly egg mass suppression will occur at treated sites for two to three months. At the start and end of the experiment, we will measure: 1) the SAV and FLV community’s species richness and dominance to determine the egg mass suppression’s impact on aquatic vegetation; 2) egg mass counts to monitor snail reproductive output; and 3) apple snail population density as a measure of adult recruitment Figure 3.

Future Steps: The field work component for this project should conclude by October 2017. Analysis and write up will conclude by March 2018. Depending on the results of this study, a follow-up study will be conducted to refine the techniques and protocols developed to provide guidance for managers on apple snail mitigation.

Keywords: Maculata apple snails, invasive species, control, Mandalay National Wildlife Refuge

Related Projects:

  • An experiment to determine if maculata apple snails will eat native frog eggs.
  • An experiment to determine if a diet of female maculata apple snails will make alligators ill.
  • A camera trap study to identify predators of maculata apple snails at Mandalay NWR.
  • A shell removal study to determine the impact of raccoon predation on maculata apple snails at Mandalay NWR.
  • An experiment to determine the rate of maculata apple snail predation in a Louisiana swamp forest.
Jacoby Carter (on left) and Brian Pember (on right), US Fish and Wildlife Service, Mandalay National Wildlife Refuge manager, are checking a snail trap. The trap had caught 10 snails in one day. Location: Speckelbelly Canal, Mandalay National Wildlife Refuge. Date of Photo: August 2, 2017. Photo credit: Sue Wilder, FWS.