Controlling deer ticks
Considering the large amount of press given to mosquitoes and West Nile virus in recent years, it may surprise you to know that when it comes to public health, the most important arthropod in the nation is still the deer tick. This tiny tick is responsible for more people getting sick in this country than all other arthropods combined. During the past ten years, more than 100,000 cases of Lyme disease have been reported in the United States, in addition to cases of babesiosis and the more recently described ehrlichiosis. The pathogens that cause all three of these diseases are transmitted to humans by the bite of the deer tick.
The deer tick (Ixodes scapularis), also known as the black-legged tick, is found throughout the Northeast, Southeast, Midwest and some areas of the Southwest. Not coincidentally, Lyme disease is also reported in these areas, with the real “hot spot” being in the tri-state region of New York, New Jersey and Connecticut. In many of these areas, deer tick populations continue to increase. As their populations grow, so does the risk of contracting a tick-borne disease. As a consequence, there is increased pressure on landscape professionals to develop and implement programs designed to effectively control deer ticks and reduce the risk for Lyme disease, ehrlichiosis and babesiosis in both residential and recreational settings.
Life cycle of the deer tick
To understand how best to control ticks, you must understand the tick's life history. Deer ticks are parasites and must take blood to survive. They begin their life as tiny eggs, which hatch into sexually immature six-legged larvae that are about the size of a sand grain. These larval ticks feed on small animals, such as mice or birds, for several days, taking in blood until they become fully engorged and drop off the host, usually into the leaf litter.
The engorged larvae molt into sexually immature, eight-legged nymphs, which are about the size of a poppy seed. These nymphs then take a blood meal, usually on a small or medium-sized mammal, feeding for four or five days. The engorged nymph will drop off the host and eventually molt into a sexually mature, eight-legged adult.
The adult ticks will find their way onto a large mammal, usually a deer, where they will mate. The females will attach and feed for up to a week. They will then drop off and lay about 3,000 eggs which will hatch into larvae, and the cycle will begin again. The whole process takes two years, with each stage reaching its activity peak in a different season (see graph, page 52).
Although there are three active stages in the deer tick life cycle, it is the nymphal tick that has the most impact on public health. For example, we believe that more than 90 percent of all Lyme disease cases are due to bites from nymphal deer ticks. This is probably due to their tiny size, which makes them difficult to detect, and the fact that they're most active during the spring and summer months when people spend a lot of time outdoors. That's why most control efforts should be directed at this stage.
Landscape features associated with tick bite risk
Before implementing a tick control program, become familiar with the landscape features that favor deer ticks. Because deer ticks are primarily found in or near deciduous forests that support deer and a wide range of host animals, any residential or recreational areas near such woodlands are candidates for control measures. Our studies in Westchester County, N.Y., which is sometimes called the “Lyme disease capital of the world,” tell us that although nymphal deer ticks are most abundant in the wooded areas, they are also quite abundant in suburban edge habitats (the interface between woods and lawns) and on stone walls, a landscape feature common in many parts of the northeast (see photos, above right). Additionally, deer ticks may also be found in and around ornamental plantings if they are in close proximity to a woodlot.
Lawns are another feature that you must consider. Intuitively, you might not think that the open grass of a lawn would not be hospitable to ticks, given the fact that ticks need a humid environment to survive. And in fact, studies have shown that relatively fewer ticks are found on the open lawn compared to the woods, edge or ornamental plantings.
But that's only part of the story. When it comes to tick bites, lawns may present substantial risk. This is because risk for tick bites is a product of two things: the number of ticks in an area and the amount of time people spend in that area. For example, if there are ten times as many ticks in the woods than on a lawn, but you spend ten times as much of your time on the lawn, your risk of encountering a tick would be the same in both places, despite the fact that more ticks are found in the woods.
To treat or not to treat
Do you need to treat for ticks? Unfortunately, the answer to this question is not always obvious. Begin by conducting a thorough interview with the property owner to learn about their history of tick bites and tick-borne diseases. Ask the following:
Is the property located in a Lyme-disease-endemic neighborhood? You want to know if there are Lyme disease cases routinely being reported from the neighborhood. You can answer this question yourself by contacting the local health department about locally reported cases of Lyme disease.
Is there a history of tick bites associated with the property? If the owners routinely find deer ticks on themselves or their pets, it's a good bet ticks are found on their property.
What is the proximity to woods? Properties that border woodlots typically present the most risk, with tick numbers generally declining as you move farther from the woods.
Are deer observed on the property? Deer are important hosts for I. scapularis and are good indicators of their presence. Woodlots in the Northeast and Midwest that support deer are likely sources of deer ticks.
Is the property shaded? Ticks require high humidity to survive and do not do well on lawns or fields that are routinely exposed to direct sunlight.
If the answer is “yes” to all of the above questions, you should consider a tick-management plan. If the answer is “no” to all of the questions, an aggressive tick management program is not warranted. However, if the answer is “yes” to some of the questions, and “no” to others, the landscaper and the homeowner need to agree on a plan they are comfortable with given the estimated level of risk.
Research shows that the public often misidentifies several common insects as ticks, so keep in mind that a homeowner's description may not be reliable. Therefore, it is good practice to sample the property yourself before making treatment decisions. Although there are a variety of tick sampling methods, the best way to collect host-seeking ticks is by drag sampling. This is conducted by “dragging” a 1-square-meter flannel cloth over the ground (see photo, top left). Ticks looking for hosts will attach to the drag cloth. When you turn the cloth over (about every 20 meters), you'll find the ticks crawling on the cloth. You can collect them for later identification.
This type of sampling has limitations. Keep in mind the seasonality of the different stages (see graph, above) and the fact that ticks are less active on windy or cold days. If you sample when ticks are not very active, a misleading negative finding may result. Also, less than 10 percent of the tick population is collected at any one time with drag sampling, so a lot of sampling may be required to find any ticks if populations aren't very heavy. One suggestion is to drag-sample the same 1,000 square meters on three or four different days. If you find deer ticks, your decision on whether to begin tick management will be easier.
Developing a tick management plan
An effective strategy for controlling deer ticks and reducing the risk of tick bites should employ an integrated approach using a variety of methods. Following are the most practical methods currently available:
- Personal protection
Educate homeowners about ways of avoiding tick bites. Wearing light colored, tightly woven clothing, tucking pants into socks and shirt into pants, using insect repellents and frequent checking to remove crawling or attached ticks reduce the risk of tick-borne diseases if conscientiously practiced. However, most people will not do these things on a daily basis, so environmental management may be necessary in high risk areas. At the least, you should incorporate good personal protection practices whenever you are working in areas where ticks may be present.
- Habitat modification
The key here is to make the environment as inhospitable to ticks as possible while maintaining the type of landscape you want. This includes keeping grass cut short, removing brush piles, keeping the property free of leaf litter and trimming vegetation around lawns, edge habitat and stone walls to increase exposure to the sun.
- Deer exclosure
Research has shown that enclosing property with a deer fence can effectively reduce deer tick numbers inside the exclosure. This method, however, is expensive and is not practical in many cases.
- Chemical control
The use of chemicals is currently the most effective method available to reduce ticks in residential and recreational areas. With just one application, chemicals can significantly reduce numbers of nymphal deer ticks and lower risk for contracting Lyme disease. The critical point is the timing of the application — from late May to early June. This knocks down nymphal ticks when they reach their activity peak and should keep numbers down throughout the spring/summer high risk period.
Because deer ticks do not move far on their own, the only ticks subsequently appearing in the treated area should be the engorged ticks transported by animals. These ticks won't present a risk until the next season. Areas treated should include lawns, edges and stone walls. Both liquid and granular formulations can be used, but be sure to get good penetration of vegetation. A fall application may be employed to kill adult ticks. The primary chemicals approved for tick control include carbaryl and pyrethroids (see the “Insecticide Update,” this issue). Check with your local extension agency or pesticide bureau for a listing of the chemicals approved in your state. Be sure to follow label instructions.
An important consideration when developing a tick management plan is that risk reduction will not be 100 percent. Inform property owners that it's likely at least some ticks will survive in even the most successful treatment program.
New tick control strategies are currently being researched, including the use of biological agents such as fungi and nematodes. These should eventually provide additional control tools for landscape managers. In the meantime, armed with knowledge of tick life cycles and the appropriate chemical tools, landscape professionals should be able to successfully keep these disease-spreading pests under control.
Dr. Richard C. Falco is a medical entomologist at the Louis Calder Center, Fordham University (New York, N.Y.) and New York Medical College (Valhalla, N.Y.).
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