VIA WEBINAR — At the recent Deer Associates annual gathering held virtually this year, there was a three-part update on some ongoing white-tailed deer and nilgai research designed to learn more about viable ways to manage further spread of the cattle fever tick.
Dr. Jeremy Baumgardt, assistant professor for research at the Caesar Kleberg Wildlife Research Institute, opened with some background noting in particular that the reason for concern is that both whitetails and the exotic nilgai have been proven to be hosts of bovine babesiosis which is deadly to cattle.
He pointed out that a large portion of the outbreaks of cattle fever ticks outside the permanent quarantine zone are concentrated primarily in Cameron and Willacy County as well as in Webb and Zapata County near Falcon Lake Reservoir.
“It seems that nilgai are at least partially responsible for the outbreaks in Cameron and Willacy County and whitetails are helping spread ticks in the Zapata County area,” Baumgardt told participants.
Whitetails can be treated for cattle fever ticks using ivermectin-treated corn in feeders. However, this tool may only be used during the non-hunting season. He also pointed out that this method is not entirely effective in all areas.
The bigger issue, though, is that because nilgai do not typically use feeders like deer, there is not an effective tick treatment method for nilgai. To that end, the team at CKWRI has been researching some other alternatives. For example, they ran some trials to see if wild nilgai would learn to eat pelleted feed from a feeder if introduced to other nilgai that do eat at feeders regularly. In short, they did not. Baumgardt also acknowledged that even if it had, doing this on a large operational scale was also not practical.
Another study evaluated the potential use of motion-activated sprayers that emit microscopic nematodes known to kill the ticks. The sprayers were strategically located at nilgai frequented fence crossings and game cameras were used to monitor the crossings. Nilgai movements were monitored with the use of GPS collars, and nilgai were captured multiple times to assess tick loads.
Another study involved whitetails that live in the area of the Falcon Reservoir. The whitetail population is high in this area because there is very little hunting pressure. Researchers theorize that is part of the reason why cattle fever tick infestation is so bad in this particular area.
Kathryn Sliwa, a master’s student with CKWRI, offered more details on the ongoing nilgai research. Specifically, she discussed the piece designed to better understand the movement patterns and behavior of nilgai and to determine an estimated home range size.
The study was conducted on three private ranches in Cameron County. The research project was focused in this particular area largely because there have been recent cattle fever tick outbreaks, Sliwa noted.
Thirty nilgai, 19 females, and 11 males were captured using the helicopter netgun technique. They were then equipped with a GPS collar and individual movements were tracked over the course of a year. GPS locations were received every hour.
Sliwa acknowledged that the study sites were not homogenous. The sites to the west had game fences and roads with high volume traffic, and another site was bordered by the Laguna Atascosa National Wildlife Refuge and the Gulf of Mexico. She told participants upfront that nilgai movement could very well be impacted by these differences.
From April 2019 to March 2020, Sliwa collected some 216,000 nilgai locations. What she found was home range size is different between sexes each month with males having larger home range sizes compared to females. Also, in the summer, both sexes had relatively comparable home range sizes, but during the fall and winter males had a larger home range size than females.
Sliwa also found that nilgai were most active during twilight – dawn and dusk. However, movement data also showed that nilgai were more active at night than during the day. As for distance traveled, the median total daily distance traveled was 1.7 miles for females and two miles for males.
In further analyzing the data, nilgai movement behaviors were separated into four classifications. Resident nilgai were defined as having overlapping seasonal ranges with periods of exploratory movements. There were also nilgai that appeared to have separate seasonal ranges for the summer and the winter. Still, another group exhibited nomadic behavior defined as “unpredictable” or “irregular” regardless of seasons, and then the final group encompassed individuals that left their area of residence to establish a new residency.
Sliwa told participants that 17 nilgai exhibited resident behavior of which 15 were adults. Seven of the collared nilgai had separate summer and winter ranges.
Four nilgai exhibited nomadic behavior and all four were adult males. However, the two nilgai that dispersed were young females.
“Female dispersal is more frequently seen in bird species and is rather uncommon for mammals,” Sliwa said. She speculated that this activity could be a result of female social structure or perhaps they dispersed to avoid inbreeding.
One female left the study area almost immediately after capture and traveled approximately 112 miles, “24 miles as the crow flies from the ranch she was collared on,” before settling in a new area of residence.
“This individual crossed county lines, which is a major concern for disease management,” said Sliwa.
The second female spent four months on the ranch where she was captured and collared. She then “wandered around for a bit” before settling into a new home range. That journey or “wandering period” encompassed about 40 miles.
Sliwa pointed out that this nomadic behavior increases the risk of new cattle fever tick outbreaks in South Texas. “Understanding the home range size and movement patterns of these nilgai is an important part of learning how to better manage the spread of these harmful ticks across South Texas,” she concluded.
Ashley Hodge, also a CKWRI graduate student, discussed her efforts to monitor whitetail movement relative to cattle fever tick management efforts along the U.S. and Mexico border. The objective was to reduce deer density on the study site, to evaluate space use and movement of the deer pre and post-removal to determine the efficacy of density reduction as a viable tool for cattle fever tick management.
The study area, which encompassed approximately 24,700 acres, was located on International Boundary and Water Commission land around Falcon Lake reservoir near Zapata. Researchers captured 102 whitetail does and 23 bucks using a net gun and helicopter in February of 2020. Each of the deer were aged, weighed, and body condition scored. All deer were scratched for ticks and those found were collected, counted and identified as to the species.
The deer were then fitted with a GPS collar. Locations were collected hourly for 52 weeks. The next part of the study entailed a large-scale removal of does. When the does were euthanized, they were also scratched for ticks.
As per the results, Hodge reported that the males had larger home ranges, averaging about 291 acres with several dispersals from some of the collared young males. Some of the males also took some long excursion events. She also recorded some males with relatively small home ranges in September which she told participants needs to be explored further.
The female home ranges, on average, stayed relatively small, about 242 acres, though a few had home range sizes the same as some of the dispersing young males.
She kept track of the water levels in Falcon Lake. The highest watermarks were between February and April. During that time, 18 different deer crossed from the United States to Mexico. From May to July, the lowest water levels for 2020, there were 76 crossing events from 18 unique deer. Between August and October when water levels were mixed, meaning some high levels and some low levels, there were 14 crossing events from seven unique deer.
“We didn’t see any crossings in August as that’s the time that the does are raising young fawns,” Hodge explained.
Between November and January, when water levels were mid to low, there was an increase again in the number of crossings with crossing from nine unique deer.
As for tick abundance, the range varied from one to 25 all the way up to over 100 ticks. One deer had 298 and another 306 ticks. In all 34.2 percent of the deer had no ticks, 47.5 percent had one to 25, and five percent over 100.
In summary, Hodge told participants that the smaller home range found for a lot of the does complicated the management effort using feeders with the ivermectin-treated corn.
“We’re going to have to place more of these feeders in the field if we want more of the population to be treated,” Hodge stressed.
Additionally, the dispersal events from young males across the border as well as some excursions across the border also contributes to more management issues and likely contributed to some of the local outbreaks. “We saw an increase in crossings moving into the fall,” she reiterated. “That supports USDA’s observation in terms of localized areas having higher infestations of ticks.”
Finally, she noted that much of the vegetation in this area is a really great habitat for these ticks to thrive.
“Some of our future work is going to be comparing these finer pre- and post-removal movements and space use, analyzing landscape metrics and tying that back into these isolated pockets of high tick numbers, and continuously monitoring tick abundance deer density, and potentially conducting future removals,” she concluded.