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June 11, 2013
Decline in tick numbers only part of the 4-poster story
Four-posters appear to reduce tick numbers while not changing deer behavior or endangering humans, according to the October 19 presentation in the school auditorium by Cornell Cooperative scientists Dan Gilrein and Susan Walker, who explained the 4-poster study results.
The 4-poster device is a bait station that is designed to apply a permethrin-based tickicide to deer in order to kill ticks. The devices contain corn, and when a deer feeds from the station, rollers coated with an oily liquid containing permethrin are transferred to the deer’s head, neck and ears. The permethrin solution used on the devices is a registered pesticide in other states, Dr. Gilrein noted. Currently in New York, that solution can only be used under a special research permit.
The objectives of the study, Dr. Gilrein said, were to assess human and wildlife-associated risks due to change in deer movement and behavior; examine whether there is an increased human exposure to permethrin through handling and consuming deer who use the 4-poster devices; and the efficacy of the 4-poster system in controlling tick densities.
Both Dr. Gilrein and Ms. Walker emphasized that the final analysis of data and complete study will be released in the spring and that the information was just the preliminary results of the three-year study with the town.
The study evaluated the drop in tick numbers from 2008 through 2010, as determined by yearly tick drags performed in June. As explained by Dr. Gilrein, there have been significant drops in the number of lone star tick males, females and nymphs as well as black-legged nymphs since the start of the study (see table). There were very few adult black-legged adult ticks detected in June. Since they’re not typically active then, changes in that population did not provide a reliable statistic.
“The declines from 2008 have been very, very high overall,” he said. In North Haven, the control site where there are no 4-posters, “we have a fairly high drop but it doesn’t nearly match the Shelter Island level.”
Dr. Gilrein suggested that the overall trend may have something to do with the environment, specifically the cooler, wet weather on the East End in 2009, a condition that impedes tick movement.
But Cornell will have to apply statistical analysis to the data to account for different variables, which “will help us to understand how convincing this is and what level of significance to attach to it,” Dr. Gilrein said.
He added, “The whole story is not told yet, either, because the [tick drags] next June will reflect the entire three years of having these devices.”
Audience member Pat King asked what sort of variables the scientists take into account when performing sampling. Dr. Gilrein responded that they don’t perform sampling during wet weather or when there’s dew on the grass. They also track the time and sequence of sampling locations.
“We try to think of all the possible variables that come into it, and as best we can, manage those to make it as consistent as possible.”
DEER MOVEMENT AND BEHAVIOR
Ms. Walker explained that through observation of deer movements with GPS and radio-frequency tracking collars, Cornell scientists haven’t detected any significant change in deer movements as a result of the 4-posters.
Deer spend most of their time in relatively small areas year after year, Ms. Walker explained, known as their core areas and home ranges. If the devices entice deer to leave or expand their home ranges, it could result in increased collisions between deer and cars or a change the impact that deer have on vegetation.
A comparison of tracking data of core areas and home ranges on Shelter Island versus North Haven since the start of the study showed no significant difference in the changes over time in the two areas. “The devices are minimally influencing how these deer use the landscape,” Ms. Walker explained.
Ms. Walker also noted that the extra food source hasn’t resulted in significant changes in the number of deer offspring. Though there has been a population increase since the start of the study, it can be attributed to the drop in the hunter harvest since restrictions were imposed in areas with 4-posters, Ms. Walker explained.
From the audience, Councilman Glenn Waddington said that at the start of the study, the town cut back on its nuisance program in areas where there are 4-posters so as not to skew the results. He asked whether the nuisance program could be expanded again if there is a future 4-poster program. Ms. Walker responded, “I wouldn’t recommend reducing the scale of your management program, because you’ve seen positive trends in the decline in your deer-vehicle collisions.
She explained that the town would have to strike a balance between its 4-poster program and its management program: “I think that the two can blend together quite nicely.”
One of the biggest costs with the devices, she noted, is supplying corn to feed the deer, and the more deer there are, the more the town has to supply.
Increased exposure to humans
Dr. Gilrein first noted that permethrin is not new to the East End — the chemical is already widely used in broadcast spraying. The advantage of a “host-targeted” use of permethrin through the 4-posters rather than broadcast spraying is that it reduces the risk of runoff, Dr. Gilrein said.
Scientists took samples of permethrin residue on deer’s coats, muscles and livers in 2008 and 2009. Permethrin residue was found on deer hides from both Shelter Island and North Haven (where there are no 4-posters), as expected. It was likely found on North Haven due to broadcast spraying, though it was present in larger amounts on Island deer hides.
In December of 2008, results showed that 3 of 13 deer sampled on the Island the previous fall tested positive for permethrin in their neck muscle tissue. Ms. Walker said that this was a result of cross-contamination: hair from the hide got onto the meat during the sampling of three deer. The other 10 Island samples and the four North Haven samples showed no permethrin in the meat.
After the scientists revised their method of cutting the sample hide in 2009 to avoid transferring hair to the meat, no permethrin was detected in the muscle samples from the Island or North Haven. No permethrin was detected in the livers of any samples in 2008 or 2009.
Ms. Walker said according to a 2009 New York State Department of Health report reviewing the findings, “Results of preliminary sampling for permethrin indicate that the health risks of handling and of consuming venison or liver from deer that have visited a 4-poster device on Shelter Island are very low.”
For this winter’s sampling, Ms. Walker explained, “we’re actually going one step further … We now want to know when a hunter handles the samples themselves, what the residues are going to be like.” The sampling will be performed according to Cornell’s White-tailed Deer Harvest and Safe Handling Procedures for hunters, so that “we can tell hunters what the residues will be like if you follow this safe handling document.”
The final study is expected to be released by the DEC by May, at the latest.