Producing a More Effective Management Strategy for WNS-Susceptible Bats in Florida

  • A biologist rappelling into a cave to survey for hibernating bats. (Photo credit, Jonathan Mays)

  • A colony of southeastern myotis bats.

  • A tri-color bat hibernating in a cave.

  • A biologist prepares to enter a cave for a bat survey.

  • Tri-colored bats roosting in the weep hole of a culvert.

  • Southeastern myotis bats spilling out of a weep hole.

  • Map of probable summer origin of bat hibernating in Florida cave according to stable isotope analysis.

In some caves near Albany, New York in 2007, biologists noticed bats were sick and dying with a powdery white substance on their noses. The culprit came to be known as white-nose syndrome (WNS), which has since killed millions of bats throughout North America. White-nose syndrome is caused by a fungus, Pseudogymnoascus destructans, or Pd for short, which grows in cold, dark, and damp places, like caves. Pd attacks the bare skin of bats and causes them to be more active than usual, fly outside in the daytime or during winter, ultimately causing them to burn away fat needed to survive the winter. The syndrome can also destroy bats’ wing tissue and may disrupt their water and electrolyte balance. It is not known definitively where Pd came from, although it’s widely accepted that the disease was brought over from Europe, but it spread quickly through many bat populations in North America. WNS is currently found in 35 states and 7 Canadian provinces; Florida is currently the only state east of the Mississippi River that is still free from WNS.  

To be fully prepared to respond to impacts from WNS on Florida’s susceptible bat species, researchers from FWRI are monitoring populations of cave-dependent bat species and conducting WNS surveillance activities. Continued monitoring of Florida’s cave bat populations prior to the arrival of WNS is necessary to develop a baseline assessment of cave bat populations and to determine population trends, allowing biologists to better assess impacts from the disease once it reaches Florida. Surveillance will aid in early detection of the disease, allowing biologists and managers to rapidly implement appropriate response actions, accurately track the spread of WNS, and contribute most effectively to the national WNS response effort.  

Biologists and researchers have five main goals for this study:  

  • Determine the distribution of cave bats in Florida caves and identify important cave roosts for protection 
  • Determine if Florida caves are susceptible to Pd and guide future disease monitoring and surveillance. 
  • Evaluate factors affecting cave occupancy and abundance of bats. 
  • Use stable hydrogen isotope analysis to determine probable summer origins of tri-colored bats hibernating in Florida caves to determine the potential of natural migratory movements to spread Pd to Florida. 
  • Determine the importance of culverts as winter roosts, determine their ability to sustain Pd, and identify structures suitable for future treatment research and implementation. Identify factors that make a culvert a suitable winter roost for bats, and provide guidelines for the Florida Department of Transportation (FDOT) to reduce bat conflict during roadwork. 

Research for this project was carried out in northern Florida, specifically the karst areas of northwest Florida (Jackson and Washington counties) and north-central Florida (Alachua, Marion, Gilchrist, Citrus, Levy, and Sumter counties). Culvert surveys have occurred throughout the panhandle and the northern half of the peninsula. 

During each survey, at least two biologists worked together to thoroughly search the cave or culvert and count the number of bats of each species present. For large clusters, biologists estimated the number of bats present based on 2,000 bats per square meter (Gore and Hovis 1998) 

Disturbance to bats was reduced during each survey by using red lights, minimizing noise, and limiting the time spent inside each cave. Furthermore, researchers visited each cave only once per winter. Florida is outside the current known range of Pd and WNS, and biologists only used equipment that had not been used in states where Pd or WNS was confirmed or suspected. To minimize the risk of human transfer of Pd to bats or caves, biologists followed recommended decontamination protocols, which included wearing disposable coverall suits in each cave and cleaning gear moved between caves with Clorox wipes or a Lysol IC solution.  

This is a multi-year project that continuously progresses along with the national goals to combat WNS. Initial research on natural cave roosts has been largely completed and is in the process of publication. In addition to assessing the cave habitat, researchers have collected baseline information on distribution, abundance, and population trends of cave bats prior to the onset of WNS. Analysis on the migration patterns of tri-colored bats using stable isotopes is nearing completion and the manuscript currently in preparation. Currently biologists are wrapping up field work and analysis on the use of culverts as a roost by bats, evaluating both habitat selection and seasonal variation in use. Additionally, these biologists are collaborating with HSC’s Long-term Bat Monitoring Program to conduct acoustic surveys to determine the statewide distribution of cave bats in all seasons, determine the importance of karst areas and other habitat types to bats, and to determine baseline productivity prior to the likely onset of WNS. 

Although this project is ongoing, biologists have already learned valuable information about Florida’s bat populations, in regard to WNS and Pd. Researchers detected three species of bats roosting in Florida’s caves, all of which are susceptible to Pd: tri-colored bats (Perimyotis subflavus) in 126 (77.8%) caves, southeastern myotis (Myotis austroriparius) in 51 (31.5%) caves, and a single Rafinesque's big-eared bat (Corynorhinus rafinesquii). Tri-colored bat occupancy was significantly positively influenced by cooler temperatures, larger cave entrance size, and distance to waterways, as well as fewer number of entrances, presence of solution holes, and a lack of flooding. Tri-colored bat abundance increased in longer, cooler caves further from waterways with presence of solution holes, a single entrance and no signs of flooding. Southeastern myotis occupancy was positively associated with longer, domed caves. More than 90% of caves surveyed each year had a wall temperature below the upper critical growth limit for Pd (19.8 °C), indicating that most Florida caves are susceptible to Pd infection. 

Biologists also found that culverts were used by southeastern myotis and tri-colored bats in both winter and summer, and by big brown bats in the summer only. Southeastern myotis were the most common species in both seasons, occupying 55.3% and 29.6% of culverts in the winter and summer, respectively. Researchers documented the highest proportion of occupied culverts and the largest southeastern myotis colony in a culvert reported in the literature. Tri-colored bats occupied an average of 13.8% of culverts in winter and always had <10 bats, but in summer occupancy dropped to two bats in a single culvert. 

This project is always evolving with the national priorities for addressing WNS related concerns. This study helps support the North American Bat Monitoring Program and the National White-nose Syndrome Response Team. Additionally, these results can inform models of disease spread and provide guidance to WNS monitoring and surveillance and cave conservation in the southeastern US. Lastly, research into bat selection of culverts and seasonal use will be useful for FDOT and other transportation agencies in the southeast to safely conduct road maintenance.