First, I would like to thank every- one who has taken the time to write and express their opinions about ticks and tick-borne illnesses on Martha’s Vineyard. This is a serious problem, and as we search for solutions as a community, it is important to have as many opinions and as much factual information as possible to guide the decision-making process.

In that spirit, I want to lay out, in some detail, why I have come to the conclusion that reducing the number of deer on the Island is our best hope for reducing the number of ticks and the epidemic of tick-borne illness.

Many comments focused on the fact that both deer tick larvae and nymphs feed primarily on white-footed mice and that the mice act as the reservoirs where the deer ticks acquire the diseases they later transmit to humans; therefore we should focus on reducing the number of mice rather than the number of deer.

There are good ecological reasons to focus on deer instead of mice. However, for me, the choice to focus on deer is one of feasibility based on numbers of mice versus deer, since I estimate that there are probably 50 to 200 times more mice than deer on Martha’s Vineyard.

The latest estimates for deer numbers that I am aware of are based on 40 to 50 deer per square mile of wooded habitat. Assuming that about two thirds of the Island (70 square miles) is suitable deer habitat yields estimates of between 2,800 and 3,500 deer on the Vineyard.

The density of white-footed mice is generally estimated at four to 15 per acre or 2,560 to 9,600 mice per square mile. Since deer and white-footed mice share similar habitat, we can also assume 70 square miles of mice habitat, which works out to between 179,200 and 672,000 white-footed mice on the Island, with a middle figure based on 10 mice per acre (6,400 per square mile) of 448,000.

Acorns are an important food for white-footed mice; given the number of oak trees on the Island, I assume the white footed mouse population is closer to the middle or high end of the normal population density. Thus I believe it is likely that there could be 500,000 or more white-footed mice on the Island during years with lots of acorns.

There are no data on the density of mice needed to maintain the reservoir of Lyme and other tick borne illnesses that the ticks acquire from mice, so we can only guess at how much we would need to reduce the density of white-footed mice to break the disease cycle or reduce the number of deer ticks. However, the number would almost certainly be in the hundreds of thousands.

In addition, while most female deer bear one or two fawns per year, white-footed mice produce two to four litters averaging five young, i.e. 10 to 20 pups per season. Even if it were possible to dramatically reduce the number of mice, their high reproductive rate means the population could rebound very quickly, requiring killing tens of thousands of additional mice every year to maintain the reduced population.

Dr. Sam Telford of Tufts University described another way to calculate the relative effectiveness of focusing on deer versus mice. He estimates that a single mouse could feed 480 larvae each year (40 larvae per mouse per week x 12 weeks of larval season) while on average one deer feeds 96 female ticks. Each female deer tick can lay 2,000 or more eggs. Thus removing one female deer from the population has the potential to prevent more than 192,000 larvae from hatching. He concludes that “one could thus argue that a single deer produces enough larvae to provide 400 mice with ticks . . . or that the reproductive potential of one deer compensates for removing the contribution of 400 mice.” 

An alternative to reducing the number of mice is the use of tick tubes to kill the ticks feeding on the mice. Tick tubes consist of an open ended tube stuffed with cotton treated with permethrin. The mice take the treated cotton back to their nest, where the permethrin gets on their fur and kills ticks feeding on the mice, apparently without harming the mice.

Commercial tick tubes are manufactured by Damminix; the numbers used in the following calculations are taken from their website. Tick tubes, when used in the recommended numbers (48 per acre of mice habitat) are effective in reducing deer ticks in small areas such as a yard. Presumably if we put out enough tick tubes, they would also be effective over much larger areas. However, once again it is a matter of numbers and cost. If we assume that 25 per cent (16,000 acres) of the Vineyard is deer tick habitat, we would need 768,000 tick tubes at the recommended density. On the website, Damminix charges $130 for 48 tubes, enough to treat one acre. At $130 per acre and 16,000 acres, the annual cost for one application would be over $2 million per year. Damminix generally recommends two applications per year, one in the spring when the deer tick nymphs are active and another in July when the larvae hatch, so the actual cost to treat 25 per cent of the Island would be over $4 million annually.

Even with Martha’s Vineyard’s long history of extremely generous philanthropy, reducing the number of mice or using tick tubes to kill the deer ticks feeding on mice appear to be too difficult and too expensive to be feasible. Thus I believe that if we really want to reduce the number of deer ticks and the incidence of the diseases they carry, the only feasible and affordable way is to break the reproductive cycle at the deer, where the female ticks go to mate and get the final blood meal they need to produce 2,000 or more eggs.

The final, but very important point with regard to trying to reduce ticks and tick-borne illnesses by significantly reducing numbers of mice or widespread dissemination of tick tubes is that even if we could afford them, these methods would only be effective against deer ticks. Lone star ticks appear to be spreading across the Island and may soon be a serious problem over much of the Vineyard. Lone star larvae and nymphs feed on medium-sized mammals such as raccoons, skunks and rabbits, but rodents such as white-footed mice do not appear to be an important food source for lone star ticks. Therefore, reducing the number of mice or using tick tubes will not reduce or slow the spread of lone star ticks.

Lone star larvae, nymphs and adults do feed on white-tailed deer. Therefore, I believe reducing the number of deer is our best chance to slow or stop the increase in lone star ticks as well as reduce the number of deer ticks.

In a future piece, I plan to discuss the options for reducing the number of deer ticks and lone star ticks, including four-poster feeding stations, birth control/sterilization and managing the deer herd.

Richard Johnson is a longtime Island biologist who leads the tick-borne illness prevention program for the Martha’s Vineyard Boards of Health.