Tick Defense: Evaluating Repellent Effectiveness Against Host-Seeking Ticks at ACLT
Updated: Oct 28
By Addie Brown, former Stewardship Coordinator
As many ACLT visitors know, spending time outdoors often comes with an unwelcome companion – ticks. Whether you’re hiking ACLT trails, volunteering at events, or stewarding the landscape, ticks are simply something we all must contend with. This past summer, I conducted a second tick surveillance study to build on the findings of ACLT’s first formal tick surveillance study, which I carried out during the spring and summer of 2025. The 2025 study established the sampling protocols used in this year’s survey and provided valuable insight into tick populations across ACLT properties.
To understand the goals of this year’s survey, it’s helpful to first look back at the original 2025 study. Similar to many of ACLT’s other ecological surveys–including tree diversity, herbaceous vegetation, fish, and benthic macroinvertebrate surveys–I wanted to better understand the tick populations present on ACLT lands, particularly near public hiking trails. Specifically, I was interested in determining whether tick abundance varied among different habitat types. Four habitat types were selected for sampling: meadows, low-lying floodplains, forests with dense understory vegetation, and forests characterized primarily by leaf litter. Throughout the 2025 sampling season, I conducted 24 surveys across eight selected sites, completing three surveys at each of the two sites within each habitat type. Sampling was completed using a flag made of flannel cloth (approximately 1 square meter) attached to a tobacco stake. The flag was dragged along a 750-meter transect through each habitat type. This method, which follows tick surveillance protocols established by the Centers for Disease Control and Prevention (CDC), takes advantage of a tick behavior known as questing. While questing, ticks climb onto vegetation and extend their front legs outward, allowing them to attach to passing hosts. As the cloth was moved through the habitat, it mimicked the movement of a potential host, causing questing ticks to attach to the flag. Every 15 meters, the flag was inspected, and any ticks present were collected and identified by species and life stage. In total, 860 ticks were collected across the 24 surveys. Overall, habitats dominated by leaf litter supported the highest tick abundance compared to the other habitat types sampled. From the sample of 860 ticks, 64 were randomly selected and tested for pathogens associated with common tick-borne illnesses. Of those tested, only six individuals (9.4%) tested positive for pathogens. For more details about the 2025 tick survey, please refer to the article titled “Ticks, Ticks, and More Ticks: A Survey of Tick Populations Near ACLT Trails” in ACLT’s fall 2025 newsletter.
The 2025 survey provided important information about the types of ticks that staff, volunteers, and visitors may encounter while exploring ACLT lands. It helped us better understand tick abundance, habitat preferences, and the potential presence of tick-borne pathogens. However, understanding tick populations is only part of the solution. The next question was: how can we protect ourselves while working and enjoying the outdoors? To address this question, I conducted a follow-up survey in June 2026 using grant funding from the Cove Point Natural Heritage Trust. This study aimed to evaluate the effectiveness of commonly used tick repellents compared to an untreated control under field conditions. Because leaf litter habitats accounted for approximately 65% of all ticks collected during the 2025 survey, the 2026 survey focused exclusively on this habitat type. The same 750-meter transects
established during the previous year were used, with one site located near the Holly Hill trail on ACLT’s northside and the other located near the Ridge Loop trail on the southside. At each site, the 750-meter transect was divided into five 150-meter sub-transects, with each representing one of five treatment groups: DEET, Picaridin, lemon eucalyptus oil, Herbal Armor (a blend of natural essential oils), and the untreated control group. At the beginning of each sampling event, treatments were randomly assigned to the sub-transects to minimize potential spatial bias. Five flags were used. Each repellent was applied to its designated flag according to manufacturer instructions, and each flag was only used with its assigned treatment to prevent cross-contamination. Similar to the 2025 survey, the flags were dragged steadily through their transect and checked every 15 meters for ticks. All ticks collected were stored in vials for later identification and pathogen testing. This process was repeated at both sites five times, allowing each treatment flag to be used on each of the five sub-transects. A total of ten 750-meter surveys were completed, resulting in the collection of 91 ticks. In addition to treatment type and total tick counts, weather conditions were recorded at the beginning of each sampling event. Tick abundance was compared with air temperature and relative humidity to evaluate whether weather conditions were associated with tick activity. Although temperature and humidity varied among survey days, neither showed a strong relationship with the number of ticks collected. These results suggest that weather conditions alone did not appear to explain differences in tick activity during the study period. Following sampling, I identified all ticks by species and life stage, allowing me to examine differences in tick composition and abundance across the two sites. Nymphs–the life stage between larva and adult–were the most frequently collected life stage, making up 65% of the samples. The Ridge Loop trail site had a higher overall tick abundance, with 59 ticks collected compared to the 32 ticks collected at the Holly Hill trail site. Following identification, I sent a subsample of 60 ticks (30 from each site) to a laboratory in Virginia for pathogen testing.
This year’s survey saw an increase in the number of positive pathogen results. Of the 60 randomly selected ticks tested, 31 (approximately 52%) were positive for pathogens that can cause common tick-borne illnesses. The Holly Hill trail site had 14 pathogen-positive ticks, while the Ridge Loop trail site had 17 pathogen-positive ticks. By life stage, eight adults, 22 nymphs, and one larva tested positive. Two ticks tested positive for Ehrlichia ewingii, a species of bacteria that attacks white blood cells. These bacteria cause ehrlichiosis in humans, which can present as flu-like symptoms, including fever, chills, and fatigue. Additionally, two ticks tested positive for Borrelia, a group of bacteria that can cause Lyme disease and relapsing fever in humans. Because the exact species of Borrelia was not determined, it is unclear which illness these ticks may have been capable of transmitting. Like ehrlichiosis, Lyme disease and relapsing fever can present with flu-like symptoms. In some cases, Lyme disease may also cause the characteristic bull’s-eye rash associated with the tick bite. Finally 29 ticks (including the two that were positive for Ehrlichia ewingii) tested positive for Rickettsia, a group of bacteria that cause spotted fever illnesses. The high number of Rickettsia-positive results may be attributed to the large number of lone star ticks included in the sample, as lone star ticks are the main vector for several Rickettsia species. Rickettsial diseases can cause mild-flu-like symptoms and, in some cases, a dark scab, or eschar, at the site of the tick bite. If you have been bitten by a tick, it is recommended that you save the tick when possible. Keeping the tick for later identification may be useful if symptoms develop and medical intervention is needed.
Of the 91 ticks collected during this year’s survey, lone star ticks (Amblyomma americanum) were the most common species found at both sites, accounting for approximately 76% of all individuals collected. Blacklegged ticks (Ixodes scapularis) were the next most common species, accounting for approximately 14% of the individuals collected. Several less common species were also identified, including three other species in the Ixodes genus: Ixodes keiransi, which does not have a formal common name; Ixodes brunneus, commonly referred to as the bird tick; and Ixodes dentatus, commonly referred to as the rabbit tick. In addition to these species, one Gulf Coast tick (Amblyomma maculatum) and three Asian longhorn ticks (Haemaphysalis longicornis) were collected during this year's survey. These species are less common than the other tick species found in Maryland, such as lone star and blacklegged ticks, but are becoming increasingly prevalent as their ranges expand. The Gulf Coast tick is native to grasslands along the Gulf of Mexico and other areas of the southern Atlantic coast. However, changing climate conditions, shifts in the distribution of wildlife hosts, and changes to landscapes and habitats have created new opportunities for this species to expand into the Midwest and northward along the East Coast. Although the Gulf Coast tick collected during this study did not test positive for any pathogens, the species is capable of transmitting pathogens associated with several illnesses. Gulf Coast ticks closely resemble American Dog ticks (Dermacentor variabilis), but the Gulf Coast ticks can be distinguished by their longer mouthparts.
The Asian longhorned tick was another unexpected find, as this species is native to East Asia. It was first identified in New Jersey in 2017 and has since spread throughout much of the United States. Although this species has not yet been documented as a significant vector of common tick-borne illness in humans in the United States, it poses a significant threat to livestock, wildlife, and pets. Female Asian longhorned ticks are capable of reproducing asexually through a process called parthenogenesis. This allows populations to increase rapidly, as a single female can lay up to 1,000 eggs. Rapid population growth can lead to infestations, particularly in pastures where cattle reside. The species is of particular concern because it can transmit bovine theileriosis, a disease caused by a parasite that targets the blood cells of cattle. Infection can result in severe anemia and, in some cases, death, particularly among calves. While the presence of the Asian longhorned tick is concerning, it highlights the greater species diversity encountered during this year’s survey compared to 2025.
Overall, the primary objective of the study was to evaluate the effectiveness of different repellent treatments by comparing the number of ticks collected under each treatment. The study also aimed to provide the ACLT community with locally relevant data to help inform decisions about tick prevention while spending time in our natural spaces. The results showed differences in the number of ticks collected across various repellent treatments, providing insight into their relative effectiveness. Among the repellents tested, DEET and Picaridin had the fewest ticks encountered, with only seven ticks collected for each treatment. In contrast, the untreated control flag collected the highest number of ticks, accounting for 56 of the 91 total ticks collected. The table below provides a breakdown of how each repellent performed at each site.
Repellent Treatment | Holly Hill Trail Site | Ridge Loop Trail Site | Total Ticks Collected |
DEET | 4 | 3 | 7 |
Picaridin | 1 | 6 | 7 |
Lemon eucalyptus oil | 3 | 6 | 9 |
Herbal Armor | 5 | 7 | 12 |
Untreated Control | 19 | 37 | 56 |
Total | 32 | 59 | 91 |
Table 1. Number of ticks collected by repellent treatment and sampling site.
By evaluating the effectiveness of commonly used tick repellents, this study provides insight into how well these products perform under real field conditions. While differences were observed among treatments, every repellent tested resulted in fewer ticks collected compared to the untreated control flag. Whether you choose a natural insect repellent or a product containing synthetic chemical ingredients, using a repellent is an important first line of defense against ticks. Although this study cannot identify one single “best” repellent, it does provide valuable guidance on how we can better protect ourselves while stewarding and enjoying the natural landscapes that make ACLT so special.
For more information on preventing tick bites, please visit the CDC’s website at https://www.cdc.gov/ticks/prevention/index.html. You can also access helpful resources from the U.S. Environmental Protection Agency on insect repellent use at https://www.epa.gov/insect-repellents.
References
Centers for Disease Control and Prevention. (n.d.). Where ticks live. https://www.cdc.gov/ticks/about/where-ticks-live.html
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