Evidence That Deer Population Reduction Prevents Lyme Disease and Environmental Destruction

Simply reducing deer numbers to natural levels, without any other actions of any kind taken, can break the cycle of Lyme disease.

The evidence that effective control of overabundant deer populations can prevent Lyme disease is summarized by the following observations:

· As deer numbers go up in a region or state, so do tick populations and human numbers of Lyme disease cases.  All regions with high Lyme rates have high deer populations.  Regions with low deer populations have low or zero Lyme disease rates1.
· According to national tick expert Dr Kirby Stafford lll PhD, Connecticut Agricultural Experiment Station, reducing deer densities to below 10-12 per sq mile has been shown to prevent ticks from perpetuating their species successfully and to substantially reduce tick numbers and human Lyme disease1.
· Evidence from a mainland study in Maine found few ticks where deer densities were below 15/mi2 (See Fig 1).

Figure 1. Relationship of tick density to deer
abundance (Rand et al. 2003).

Figure 1: In a mainland situation where deer roam freely, Rand's study 2 compared the abundance of adult deer ticks with the presence of deer along multiple 1000 ft transects within 8 study sites throughout southern Maine. Sampling included 74 transects, each examined from 1 to 3 years, for a total of 155 records (a total of 29 transect miles). Deer density, here in terms of ticks per square mile, was estimated from pellet group counts using a published conversion factor. As seen in the accompanying plot, deer density was highly positively correlated with tick abundance. In this study we found few ticks where deer densities dropped below 15/mi2.

· Within two years of implementing effective deer population reduction, Lyme disease cases were reduced by 90% in Mumford Cove and Groton Long Point, Connecticut3.  See pages 2,3 and 4 in the "Managing Urban Deer in Connecticut", 2007 edition, Connecticut Department of Environmental Protection 

CT DEP data

Figure 2: At Mumford Cove an attempt to control deer numbers using contraceptives for 3 years failed to prevent a rising deer population. In 2000 the deer numbers were reduced by hunters down to 10.4 deer per sq mile and have been held at that level since then. There are now only 2 to 3 Lyme cases a year in this community compared to 30 new cases a year prior to deer reduction. The deer are now maintained at a steady 10 or so per square mile very easily by two pairs of hunters once a year. There are now virtually no ticks to be found in this community.


(1) K. Stafford, Connecticut Agricultural Experiment Station Tick Management Handbook 2007
(2) Rand, P. W., C. Lubelczyk, G. R. Lavagne, S. Elias, M. S. Holman, E. H. Lacombe, and R. P. Smith, Jr.: Deer Density and the Abundance of Ixodes scapularis (Acari: Ixodidae). J. Med. Entomol. 40 :(2) 179-184, 2003.
(3) DEP study: Kilpatrick and LaBonte 2003 Deer Hunting in a residential community. Wildlife Soc. Bull. 31: 340-348
(4) Estimated physician confirmed case numbers based on CDC reported numbers for 2002, the last year for which lab reported cases were included in the case numbers. CDC estimates that annual cases are underreported by a factor of 6 to 12.

(click table to enlarge)

The environmental evidence that over 10 deer per sq mile is destroying woodlands:

Loss of native tree species: Evidence from the Connecticut Agriculture Experiment Station (Departments of Forestry and Horticulture): "Overabundant deer herds can eliminate native plant species and this can adversely affect other wildlife such as small mammals and birds that nest in shrubs and on the ground. Deer are impacting oak, sugar maple, pine and cedar forests in many parts of Connecticut, preventing regeneration of young saplings." “Managing the deer population is essential to maintaining the health of these preserves,” said Lise Hanners, Ph. D., state director of The Nature Conservancy in Connecticut. “At high numbers we know that deer restrict the growth of new trees, shrubs and flowers. Without some sort of management, the quality of the forests at these preserves will continue to suffer.”

Loss of wildflower species: Nature Conservancy and Ct. Ag. Experiment Station studies reveal loss of the following wildflowers in areas with over 10 to 12 deer per square mile: trout lilies, red trillium, lady slipper orchids and Canada lilies.

Loss of bird species: Connecticut Audubon's 2007 State of the Birds report lists the following birds as reduced or endangered by excessively deer-browsed forests with over 10 deer per square mile: Wood thrush, hooded and other warblers, house wrens, song sparrows, eastern towhee, indigo buntings, yellow breasted chats and golden winged


What is the role of hunting?

Under the current archaic and obsolete restrictions, largely in place since a happier time before the explosion of the deer population and consequent epidemic of Lyme disease, sport hunting does not by itself usually reduce deer numbers. Hunting is not traditionally intended to reduce deer numbers. But it can. Pro-actively working with volunteer sport hunters to reduce deer populations in a safe, controlled, and managed deer population reduction program can be highly effective.

How safe is hunting as a deer reduction method? What is the safety record for Connecticut?

The following answer is provided by the State's Hunter Safety Program Administrator and Wildlife Biologist, Mark Clavette

Hunting is among the safest of all outdoor activites and has one of the lowest accident rates for all forms of outdoor recreation. Hunting accidents are rare and most injuries while hunting are self-inflicted or involve members of the same hunting party. The safety record of hunters has improved substantially over the years due in large part to mandatory hunter education which has produced an extremely safety-conscious generation of hunters. The latest data from the International Hunter Education Association indicate a national rate of 6 hunting related firearms accidents per 100,000 hunters.

Connecticut enjoys one of the best safety records among the states. In a 24 year period, Connecticut has had an average of 4 accidents (all reported injuries while hunting) per year. The last several years there were one or two accidents reported. This is among some 60,000 firearms hunters and 13,000 archery hunters, who spend a conservative estimate of 789,000+ days afield each year during the various seasons.

What is a controlled hunt?

Controlled or managed hunts are those that are planned for deer reduction and use selected experienced hunters with impeccable personal references in controlled conditions which would include some or all of the following: A controlled hunt describes a process by which a limited number of hunters are specifically authorized to enter designated open space parcels under prescribed conditions. The days and hours of hunting, elevated tree stand locations, weapons permitted, and other details are dictated by the host organization. In most cases the properties are closed to the public for the duration of the hunt. In all cases there are communications with the public, particularly adjacent property owners, through newspaper coverage, direct mailings, and/or sign posting. The use of elevated tree stands for each hunter, so that the hunters are all aiming down at the ground beneath them where the deer are attracted by bait, ensures safety even in residential areas and most if not all the venison is donated to food pantries. Controlled hunts have a very good safety record, the only injuries that have occurred in Connecticut since this type of program began being injuries to hunters themselves. For more articles on Controlled hunts in Connecticut see the Alliance website.

Some communities have chosen to pay professional hunters and obtain special permits from DEP specifically for deer population control. This can be quicker but is quite costly initially. Whatever the method or mix of methods chosen, the cost benefits of a healthy human population without tick borne illnesses or deer vehicle accidents soon outweigh the initial investment in deer control programs. Audubon Societies in New Jersey and Connecticut and the Nature Conservancy in Connecticut have already begun using sport hunters to reduce deer numbers on their properties in order to save habitat for birds and to stop the ongoing destruction of native wildflowers.

Integrated approach to deer and tick reduction:

Until deer numbers have reached the ideal level of 10 or so per square mile there will still be some ticks around so precautions must be continued until tick counts have fallen dramatically which can take up to 2 years. A combination of deer population reduction with tickicides for the remaining deer which can continue to be applied out of hunting season is perhaps ideal. On Shelter Island in New York this is the approach they are taking, combining rapid reduction of deer numbers with the use (for just part of the year) of the "4-poster device" to kill ticks on the deer while they feed in order to help "mop up" residual ticks that are still looking for a deer host. If and when contraceptives for deer become effective enough to use on free ranging deer, their use might help maintain lowered populations of deer in more developed areas where access for hunters is already limited. There is currently no available contraceptive for deer, nor even an experimental one that is effective at preventing population growth. As deer have a natural life span of 12 to 18 years, initial herd reduction is essential in order to get numbers back into balance with nature.

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