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. Author manuscript; available in PMC: 2025 Apr 28.
Published in final edited form as: Southwest Entomol. 2024 Dec 16;49(4):1414–1422. doi: 10.3958/059.049.0421

Tick Utilization of Eastern Redcedar1 Encroached Areas at the Individual Tree Scale in Oklahoma

Utilización de Garrapatas en Áreas Invadidas por el Cedro Rojo Oriental1 a Escala de Árbol Individual en Oklahoma

Olivia Horton 2, Jozlyn Propst 3, Scott R Loss 2, Bruce H Noden 3,*
PMCID: PMC12036698  NIHMSID: NIHMS2071500  PMID: 40297325

Abstract

The Great Plains region is experiencing a biome-level conversion as grasslands are being rapidly encroached by eastern redcedar (Juniperus virginiana L.; ERC) which, in turn, causes abiotic and biotic changes throughout the region. These changes brought about by ERC encroachment are providing habitat for ticks and mosquitoes that increase the risk for vector-borne diseases. This study evaluated the influence of ERC on the abundance of ticks at the tree level by matching CO2 traps under individual ERC trees with traps in nearby grass patches at seven sites across central and western Oklahoma. From 3,654 ticks collected, significantly more adult and nymphal Amblyomma americanum (L.) and adult Dermacentor variabilis (Say) were collected under the individual ERC trees compared to the adjacent grass patches. Along with growing evidence that larger-scale ERC encroachment increases the abundance of ticks, this finding suggests that even single ERC trees within an encroached area provide sufficient habitat for A. americanum and D. variabilis. This study also contributes novel information about the fine-scale effects of this invasive encroaching tree species on the ecology of vector-borne disease systems.

Keywords: Lone star ticks, American dog tick, encroachment

Resumen:

La región de las Grandes Llanuras está experimentando una conversión a nivel de bioma a medida que los pastizales están siendo invadidos rápidamente por el cedro rojo oriental (Juniperus virginiana L.;CRO), lo que, a su vez, provoca cambios abióticos y bióticos en toda la región. Los cambios provocados por la invasión del CRO están proporcionando un hábitat para garrapatas y mosquitos que aumentan el riesgo de enfermedades transmitidas por vectores. Este estudio evaluó la influencia de CRO en la abundancia de garrapatas a nivel de árbol individual haciendo coincidir trampas de CO2 debajo de CRO individuales con trampas en parches de pasto cercanos en siete sitios en el centro y oeste de Oklahoma. De 3,654 garrapatas recolectadas, Amblyomma americanum (L.) adultos y ninfas y adultos de Dermacentor variabilis (Say) fueron los más abundantes debajo de CRO individuales en comparación con parches de pasto adyacentes. Junto con la creciente evidencia de que la invasión de CRO a mayor escala aumenta la abundancia de garrapatas, este hallazgo sugiere que incluso un solo árbol de CRO dentro de un área invadida proporciona un hábitat suficiente para A. americanum, y D. variabilis. Este estudio también aporta información novedosa sobre los efectos a pequeña escala de esta especie de árbol invasor en la ecología de los sistemas de enfermedades transmitidas por vectores.

Introduction

Tick-transmitted pathogens account for 95% of human vector-borne disease cases in the United States (Rosenberg et al. 2018). Tick-borne diseases can only occur when competent tick species interact with susceptible hosts in specific environments (i.e., the ‘nidus of infection’ (Reisen 2010)). Anthropogenic factors, like changes in land cover, alter environments in ways that greatly affect this nidus, including by facilitating the expansion of vectors (Reisen 2010, Noden and Dubie 2017). One of the factors that contributes to the expansion of vector populations in the United States is woody plant encroachment (WPE) (Loss et al. 2022), which is the expansion of woody plants into grasslands and urban green spaces (Engle et al. 2008, Nackley et al. 2017).

In the southern Great Plains, the rapid expansion of eastern redcedar (Juniperus virginiana L.; ERC) (Wang et al. 2018, Zou et al. 2018) has caused ecosystems to change from grasslands to woodlands, with dramatic associated changes in abiotic conditions (e.g., temperature and humidity) (Caterina et al. 2014, Acharya et al. 2017) and populations of plants, arthropods, and wildlife (Coppedge et al. 2001, Horncastle et al. 2005, Noden et al. 2021a,b). Initial studies determined that A. americanum (L.), a tick species historically associated with humid, wet areas of eastern Oklahoma, has expanded into arid, dry areas in western Oklahoma due to the favorable environment created by the encroachment of ERC (Barrett et al. 2015, Noden and Dubie 2017). A subsequent study characterized how ERC influences tick populations across a larger spatial scale, specifically, a 25 ha area experiencing varying levels of ERC encroachment (Noden et al. 2021a). That study, like most involving ticks within a given vegetation type, evaluated the abundance of ticks at a coarse scale to better predict the presence of ticks within ERC-dominated landscapes.

There is a need for fine-scale studies on ticks and vegetation cover in order to link the small-scale processes that drive tick movements, distributions, and host-seeking behaviors with larger scale, emergent patterns of prevalence and risk of tick-borne disease. This fine-scale research is also needed to provide management insight into whether the removal of individual ERC trees at early stages of encroachment can reduce the risk of encountering ticks and tick-borne pathogens. Although ERC is known to create micro-climate changes at the level of individual trees (Zou et al. 2015, Acharya et al. 2017), no study has explored whether individual ERC trees influence tick populations. This study addressed this research need by documenting micro-level variation of tick abundance between lone ERC trees and adjacent grassy patches within larger sites that are broadly experiencing ERC encroachment. Our hypothesis was that there would be more ticks under individual ERC trees compared to adjacent grass patches.

Materials and Methods

Collection sites:

We collected ticks at seven sites across western and central Oklahoma that were defined for a 2-year, NIH-funded study of ERC effects on ticks at a larger, site scale. We first identified publicly accessible areas in central and western Oklahoma experiencing ERC encroachment ranging from open grasslands to closed-canopy ERC forests. Then, we identified study sites (each site >2 ha) using ArcGIS 10.1 (Environmental Systems Research Institute, Redlands, California) and the Oklahoma Ecological Systems Mapping (OESM) data layer, a 10x10-m resolution land cover layer that covers all of Oklahoma and includes specific land cover types for which eastern redcedar is a dominant species (Diamond et al. 2015). We ground-truthed all the candidate sites, excluding areas that lacked adequate road access. In total, 28 sites in 4 different stages of encroachment were identified for the broader study, and site boundaries were georeferenced. For this study, we selected seven sites that were all in an early stage of ERC encroachment, specifically, grasslands with young eastern redcedar stands (4-6 yr old; 1- 2 m tall) with scattered small trees and extensive intervening grass cover (Engle and Kulbeth 1992). We focused on this early stage of ERC encroachment because, as part of our related studies, we found that these sites had a much higher abundance of ticks, especially of A. americanum, compared to open grasslands and to sites in later stages of ERC encroachment (Propst and Taylor, unpublished data). The sites included Boiling Springs State Park, Canton WMA, American Horse Lake, Fort Cobb State Park (West and East), OSU Research Range and OSU Lake Carl Blackwell.

Sampling Protocol:

Between May 30 and July 14, 2023, ticks were collected at each site on three different days, at least two weeks apart, to capture the temporal variation that may occur across the region. For each site visit, three individual ERC trees were selected, each >100 m apart from each other and with no other ERC trees within 30 m. For each tree, we selected a matched trap location 20 m from the focal tree and from any other ERC tree in an open grassy patch. We placed CO2 traps (using dry ice) (Noden et al. 2021a) at each of the three paired locations for a total of six traps per site per collection period. Selected trees and grass locations at each site changed for each of the three trapping days. 90 minutes after placing the traps, all the ticks collected were placed into specifically labelled vials for each trap. In the lab, the ticks were identified using established keys (Keirans and Litwak 1989, Keirans and Durden 1998, Dubie et al. 2017) and stored in vials by species, sex, date, and trap.

Analysis:

Because the collection sites were paired replicates, we used paired t-tests (SAS JMP®, Version 18.2 Pro, SAS Institute Inc., Cary, NC, 1989-2024) to evaluate if tick abundance differed between the traps located in the ERC trees and grass patches. We conducted separate analyses for A. americanum adults, A. americanum nymphs, and D. variabilis (Say) adults. For each analysis, replicates were individual visits to each trap pair (3 visits x 7 sites x 3 trap pairs = 63 replicates). Prior to the analysis, we square-root transformed the tick abundance data to meet normality assumptions. No analysis was conducted for A. maculatum Koch due to the small sample sizes.

Results

A total of 3,654 ticks were collected across the three sampling periods and seven sites between May and July of 2023 (Table 1). Of the ticks collected, 1,007 (27.6%) were adults, and 2,647 (72.4%) were nymphs. By species, 3,630 (99.4%) were A. americanum; 16 (0.4%) were D. variabilis, and 8 (0.2%) were A. maculatum. The vast majority of both adult and nymphal ticks (96.4%) were collected under the ERC trees with the remaining ticks (3.6%) collected in grass patches. The most abundant species collected under ERC were A. americanum (96.5%), while all D. variabilis were collected under ERC. Of the A. americanum collected, more nymphs (98.2%) than adults (91.9%) were collected under ERC. The collections of A. maculatum were evenly split between ERC (50%) and grass (50%).

Table 1.

Total ticks collected by species and life stage from paired traps located under individual eastern redcedar trees and in adjacent grass patches in central and western Oklahoma, May to July 2023.

Cuadro 1. Total de garrapatas recolectadas por especie y etapa de desarrollo en trampas pareadas ubicadas debajo de árboles individuales de cedro rojo oriental y en parches de pasto adyacentes en el centro y oeste de Oklahoma, de mayo a julio de 2023.

Species and life stage ERC Grass Total
A. americanum adults 903 (91.9%) 80 (8.1%) 983
A. americanum nymphs 2599 (98.2%) 48 (1.8%) 2647
D. variabilis adults 16 (100%) 0 (0%) 16
A. maculatum adults 4 (50%) 4 (50%) 8
3522 (96.4%) 132 (3.6%) 3654
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Based on results of these statistical analyses, there were significantly more adult and nymph A. americanum and adult D. variabilis collected in traps under the ERC trees compared with matched traps in the nearby grass patches (Table 2).

Table 2.

Total and mean (± SE) number of ticks collected by species and life stage from 63 paired traps located under individual eastern redcedar trees and in adjacent grass patches in central and western Oklahoma, May to July 2023.

Cuadro 2. Total y media (± D.E.) de garrapatas recolectadas por especie y etapa de desarrollo en 63 trampas pareadas ubicadas debajo de árboles individuales de cedro rojo oriental y en parches de pasto adyacentes en el centro y oeste de Oklahoma, de mayo a julio de 2023.

Species Life
stage		 Total
collected		 Habitat Mean ± SE t-ratio DF P
A. americanum nymph 48 Grass 0.7 ± 0.2 4.21 62 <0.0001
2,599 ERC 39.4 ± 14.5
A. americanum adult 80 Grass 1.2 ± 0.3 5.26 62 <0.0001
903 ERC 13.7 ± 4.8
D. variabilis adult 0 Grass 0.0 ± 0.0 3.24 62 0.0018
16 ERC 0.2 ± 0.9
A. maculatum adult 4 Grass 0.06 ± 0.03
4 ERC 0.06 ± 0.03
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Discussion

We collected significantly more A. americanum and D. variabilis ticks under individual ERC trees compared to adjacent grass patches across seven sites in Western and Central Oklahoma that were in an early stage of ERC encroachment. Because of their size and preferences for specific abiotic and habitat conditions, ticks tend to be localized in specific types of vegetation (Sonenshine 2018, Nicholson et al. 2019). Because of this, most tick habitat studies are conducted at coarse scales to increase understanding of the ecological, environmental, and host-related factors that impact tick abundance. However, in the context of the sweeping ecosystem changes brought about by ERC encroachment in the Great Plains region (Zou et al. 2018), the documentation of a greater abundance of ticks under the individual ERC trees as compared with the adjacent grassy patches has important implications for public health and the management of ERC and also enhances the understanding of tick-borne disease ecology in this region.

All the collection sites in the current study were in an early stage of ERC encroachment characterized by many small, scattered, and isolated ERC trees within areas that still retained substantial grass cover. ERC encroachment into the grasslands occurs rapidly; in Oklahoma, ERC has expanded in coverage by 40 km2 annually, and the tree height can increase up to 0.5 m annually (Engle and Kulbeth 1992, Wang et al. 2018). Furthermore, within 40 years, ERC-encroached grasslands can become completely forested (Briggs et al. 2002). The ecological changes brought about by ERC generate vegetation and abiotic conditions that can sustain high numbers of ticks, even in areas experiencing the earliest stages of encroachment, such as the sites in this study (Noden et al., 2021a, Propst and Taylor, unpublished data). Our study further demonstrates that there is micro-level variation in the tick numbers within such sites, which likely reflects that the abiotic environment under ERC trees, even lone trees, is more favorable for ticks than the intervening grass patches, even grass patches with many other isolated ERC trees encroaching into them. Although the abiotic changes brought about by ERC encroachment require further research, the area underneath the canopy of ERC trees appears to provide adequate leaf litter and relatively high levels of humidity compared to more open environments (Zou et al. 2015, Acharya et al. 2017), conditions that allow ticks to persist for long periods without desiccation. Although ERC intercepts a large amount of precipitation before it ever reaches the ground, the ERC trees growing in open grassland can funnel a high proportion of precipitation to the base of the tree where the litter layer accumulates water (Zou et al. 2015). Specifically, this litter at the base of ERC trees can be up to several centimeters deep and retain up to 8% of precipitation (Acharya et al. 2017). The ticks prevent desiccation by burrowing into the leaf litter (Goddard et al. 2024), and this likely contributes to the higher number of ticks under the ERC trees compared to the surrounding grassland, where lower litter biomass may limit the survival of ticks due to water loss by direct sunlight (Stachurski et al. 2010). In addition to changing abiotic conditions, the ERC effects on movement, foraging, and habitat use of wildlife, including potential blood meal hosts like birds and deer, may cause these potential hosts to be more clumped under the ERC trees compared to the grass patches (Coppedge et al. 2001, Horncastle et al. 2005). This could occur due to the ERC’s provision of food, shelter, and nesting substrates, and could contribute to the disproportionate dropping of ticks off wildlife under or near the ERC trees. All these potential mechanisms that underlie the patterns we observed require further research as they were not addressed in the current study.

The widespread expansion of ERC into grasslands is causing a biome-level transition of vegetation cover in the Great Plains of the United States (Twidwell et al. 2021). While this ERC expansion may be contributing to an increased risk for diseases transmitted by mosquitoes (Noden et al. 2021b, Maichak et al. 2022) and ticks (Noden et al. 2021a), our study indicates that individual ERC trees, even within encroached areas, are influencing local tick abundance, likely through micro-habitat/climate changes. As ERC effects appear important at several spatial scales, studies are needed to evaluate which scale or scales of ERC clumping have the most effect on tick abundance, including individual isolated trees, clumps of multiple trees, copses of larger tree patches, or extensive tree forests that cover hectares and square miles. Recommendations already exist that demonstrate the most effective and efficient way to prevent ERC encroachment is to address it before it starts by using prescribed burning in open grasslands and to remove ERC at the early stages of encroachment. This is due to the logistic ease and cost-effectiveness of using fire or mechanical removal at early encroachment stages compared to areas that experience later stages of encroachment that cannot be safely burned or cheaply cut (Twidwell et al. 2021). The fact that tick abundance increases under lone ERC trees and in the early stages of ERC encroachment suggests that another reason to focus on stopping encroachment before it becomes too advanced is to avoid and reduce the potential public health impacts of this encroaching woody tree species.

Acknowledgements

We would like to thank Irene Eubanks and Tucker Taylor for their assistance with tick collection and identification. Funding was provided by the National Institutes of Health (R03-5R03AI163283-02), the Tick Rearing Facility (OKL-0336), the OSU 2024 President’s Fellows Faculty Research Award and the U.S. Department of Agriculture National Institute of Food and Agriculture through the Oklahoma Agricultural Experiment Station from a Multistate project (OKL-03186), and Hatch Grant (#OKL-03150).

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