Habitat-Targeted Chemical Control
Habitat-targeted chemical control refers to the application of an acaricide (a pesticide that kills ticks and related organisms) to the environment in an effort to suppress or manage tick populations. It is the most effective and least expensive way to control ticks in small- or moderately-sized areas. However, the use of habitat-targeted or area-wide chemical control is often perceived by the public as having undesirable environmental effects. This view undoubtedly stems from the widespread use of persistent chlorinated hydrocarbon insecticides (e.g. DDT, chlordane, heptachlor) that were in common use during the late 1940s through the 1970s. This class of pesticides not only lasted a long time in the environment, but also tended to bioaccumulate within the food web. During the 1970s, many of these pesticides were banned or had their use significantly regulated. They were replaced by classes of pesticides known as the organophosphates (e.g. diazinon, chlorpyrifos, propoxur) and organocarbamates (e.g. carbaryl) that were more biodegradable (less persistent) than the chlorinated hydrocarbons, but generally more toxic to mammals. By the late 1990s, many of the organophosphate acaricides were no longer available for tick control and have largely been replaced by synthetic pyrethroids (e.g. bifenthrin, cyfluthrin, deltamethrin, cyhalothrin, permethrin) and natural pyrethrins. The advantages of the pyrethroid compounds are that they are generally less persistent in the environment and exhibit low mammalian toxicity, compared to organophospates. However, the acaricides currently available are still broad-spectrum and, as such, can have significant impacts on a variety of non-target organisms.
The ultimate goal of any habitat-targeted control program is to kill the greatest number of ticks, while minimizing adverse environmental effects, by using the least amount of acaricide. In part, this can be achieved by limiting treatment to just those areas with high probability of human-tick encounters. Rather than treating large expanses of woodland, barrier applications to vegetation in areas with significant human activity will dramatically reduce exposure to ticks while minimizing the potential for unwanted environmental impacts. Studies in New Jersey involving barrier acaricide applications have shown that impacts to non-target organisms may be significant, but short-lived.
A thorough knowledge of tick biology, behavior, and ecology can reduce acaricide use by eliminating unnecessary multiple applications and by targeting only those habitats capable of supporting ticks. The three most important factors in the development of a successful and environmentally sound habitat-targeted tick reduction strategy are the seasonal activity, habitat preference, and questing behavior of the tick species to be controlled. For example, applications to control blacklegged tick nymphs should be conducted during their peak activity period in late May and concentrated in the litter layer where they quest. A single application made at the right time and location can control 90% to 100% of nymphs in the target area, thus eliminating the need for repeated applications. Similarly, applications intended to suppress adult blacklegged ticks should be made around late October and directed toward shrub layer vegetation where they quest. A single application of carbaryl directed at fall populations of adult blacklegged ticks adults resulted in 95% to 100% control, and because few ticks are available to overwinter, control is maintained during the subsequent spring. However, such applications do not seem to have an impact on other stages of blacklegged ticks or other tick species, which are inactive in the fall. The larval stage is the most difficult to control because hatching of eggmasses occurs over a protracted period and the distribution of larvae is extremely patchy. Although blacklegged tick larvae do not pose a public health threat, control may be achieved with a late July-early August application directed at the litter layer.
Lone Star Ticks
Although sharing the same type of habitat as blacklegged ticks, efforts to control the lone star tick require modifications in both timing and location of the application. Since there is no fall activity peak, any control of adult lone star ticks must be confined to spring. However, seasonal activity patterns of lone star tick nymphs and larvae are similar to those of the blacklegged tick. In general, the duration of activity of the various stages of the lone star tick is longer than that of the blacklegged tick, and since all active stages of the lone star tick quest both at ground level and in the shrub layer, both areas need to be treated. Differences in activity periods and habitat must also be taken into consideration when attempting to control American dog ticks. Because of its host associations, control of this species will be most effective when directed at tall grasses and shrubs in fields, along trails, rights-of-way, and woodland edges. Since the brown dog tick is primarily an indoor pest, timing of any application is less critical.
Selecting an Acaricide
Seasonality, habitat affinity, and questing behavior also affect the selection of acaricide formulations. For example, adult blacklegged ticks quest in shrub layer vegetation in both fall and spring when deciduous foliage is absent. In such situations, the use of liquid sprays applied to shrubs would be the obvious choice. In contrast, since the immature stages of this tick quest on or within the leaf litter during the growing season, the application must penetrate the foliage and reach the litter layer to be effective. Granular formulations are ideally suited to habitat-targeted application of granular and liquid acaricide formulations for this purpose, but liquid formulations can be just as effective providing that sufficient pressure is used to penetrate the foliage and physically disturb the leaf litter. Chest-mounted cyclone spreaders or modified mist blowers have been used successfully to apply granular acaricides, while high-pressure hydraulic sprayers are best suited to apply liquid formulations. Other considerations affecting the selection of formulation include availability of equipment, size of the area to be treated, density of vegetation, and cost. Aerial applications have been successfully used to control blacklegged tick adults and nymphs, but high cost and regulatory considerations make this approach impractical in all but unique situations.