Interactions of gulf cordgrass, Spartina spartinae (Trin.) Merr. ex Hitchc., habitat with ixodids on the South Texas coastal plain
Environmental Systems Research volume 12, Article number: 28 (2023)
Many ixodid species on the South Texas coastal plain can transmit pathogens to their hosts. Substantial areas are dominated by gulf cordgrass, Spartina spartinae (Trin.) Merr. ex Hitch. The S. spartinae habitat was examined in terms of abiotic and biotic factors that likely impinge upon ixodids using the plant for questing. Natural enemies, temperature, humidity, and plant structure were investigated as possible mortality factors and improving ixodid survival. Spartina spartinae (soil and foliage) harbored few natural enemies of ixodids, and soil salinity was nonlethal. Conditions were cooler and more humid inside S. spartinae clumps than in the canopies; hence, questing ixodids can rehydrate inside the clump when threatened by desiccation. Leaves were mostly “V” shaped in cooler months and, during warm months, the tightly folded leaf slot predominated, into which larvae crawled. Immature ixodids were more abundant in the concave side of the “V”-shaped leaves than on the exposed convex side. Larvae sought refuge from potential lethal ambient air conditions by entering tight warm season leaf folds. The leaf “V” and fold offer opportunities for rehydration on the leaf without moving to the clump’s base. In addition to five other species collected on the grass, a relatively heavy larval southern cattle fever tick, Rhipicephalus (Boophilus) microplus (Canestrini), population was detected, suggesting that abundances in the South Texas cattle fever tick eradication quarantine zone are increasing and might spread further into Texas.
The subtropical South Texas, USA, coastal plain of Cameron and Willacy counties provides a corridor for wild animals that host multiple ixodid (Ixodida: Ixodidae) species, extending from the Rio Grande to large cattle ranches in Willacy County and farther northward. Some of the ixodids transmit important disease agents that infect humans, livestock, and wildlife (Showler and Pérez de León 2020). Soil salinity caused by occasional shallow flooding with hypersaline Lower Laguna Madre bay water (due to wind tides and storm surges) has a crucial role in the region’s distribution of ixodids that involves the ovicidal action of hypersaline water and egg predation by halophilic mud flat fiddler crabs, Uca rapax Smith (Showler et al. 2019b). While dense stands of sea ox eye daisy, Borrichia frutescens (L.) DC, are indicative of saline habitat in the region and U. rapax populations (Showler et al. 2019b), direct interactions of ixodids with predominant plant species on the wildlife corridor have not been described.
The roles of plants appear to be important to ixodid ecology, with some plant types and plant species possibly being more influential than others. Areas of brush and low trees in Oklahoma, USA, for example, harbored ≥3-fold more ixodid adults and 15-fold more nymphs than surrounding areas dominated by lower-growing grasses and other plants (Hair and Howell 1970). In Arkansas, USA, 69.6% of the lone star tick, Amblyomma americanum (L.) occurred in brushy habitat compared to 30.4% in grassland (Lancaster 1957), and other reports suggest that survival of the southern cattle fever tick, Rhipicephalus (Boophilus) microplus (Canestrini), and the cattle fever tick, Rhipicephalus (Boophilus) annulatus Say, was favored more by honey mesquite-, Prosopis gladulosa Torr., shaded and mixed brush vegetation than by open grass and by grass mixed with brush (Davey et al. 1994; Teel et al. 1997). Although ixodids use plants as elevated sites from which to acquire passing host animals, a behavior known as “questing”, interactions with plants have not been assessed sufficiently to elucidate complexities.
Aside from the relatively diverse mesquite-thorn scrub habitat and saline areas mainly occupied by B. frutescens, some parts of the South Texas coastal plain are dominated by gulf cordgrass, Spartina spartinae (Trin.) Merr. ex Hitchc. (Showler et al. 2019b). Spartina spartinae is a widely distributed perennial bunchgrass on coastal areas of the Western Hemisphere (Bertness 1991), often excluding other plant species (Oefinger and Scifres 1977). Spartina spartinae grows in clumps with a ≈1 m diam canopy of thin leaf blades that extend upward and outward, like a fountain, from a dense base where the leaves are mostly vertical. While highly productive (Garza et al. 1994), S. spartinae is coarse and relatively unpalatable to livestock (Angell et al. 1986). The purpose of this study was to identify interactions of ixodids with the South Texas S. spartinae habitat that likely affect the parasites’ survival, including prevalence of predatory arthropods, ambient abiotic conditions, and, in particular, plant structure and leaf shape. Data collected was used to describe relationships between potentially relevant ecological compartments that, together, inform on a habitat-based ixodid environmental system. Determination of ecological interactions enhances understanding of how medially and agriculturally important arthropods survive and proliferate in some environmental systems.
Materials and methods
Field experiments were performed on the South Texas coastal plain in Cameron and Willacy counties, Texas, in S. spartinae and mesquite-thorn scrub habitats. That information was collected in order to characterize key abiotic and biotic factors that likely affect ixodid survival in the South Texas S. spartinae habitat. Field data was collected from Jan to Aug 2018, and one outdoor experiment in Aug 2019 was conducted at the U.S. Department of Agriculture – Agricultural Research Service’s Knipling-Bushland United States Livestock Insects Research Laboratory (KBUSLIRL) in Kerrville, Kerr County, Texas, USA. Ixodid samples from S. spartinae plants were collected in 2022; the interval between those samples and earlier parts of the study was due to employer-mandated travel restrictions associated with COVID-19.
Soil samples were collected from the top 7.6 cm using a 20,502 Pro steel weeder, narrow-blade shovel (Radius Garden, Ann Arbor, Michigan, USA). At each of 12 sampling sites, six in S. spartinae-dominated habitat and six in mesquite-thorn scrub habitat (Table 1), soil was taken from five places 5–10 m apart from each other to obtain a total of ≈3.8 L of soil in a plastic bag; the bag was shaken vigorously for 30 s to mix the soil. Electrical conductivity, expressed as mmhos/cm and used as a measure of soil salinity, was determined by the Texas Plant and Soil Lab, Edinburg, Hidalgo County, Texas.
Plant habitat composition
A 0.25 m2 quadrat was tossed three times in random directions at each of six S. spartinae-dominated areas and at each of six mesquite-thorn scrub areas (two of the samples from each habitat were taken at Port Mansfield and four at Holly Beach) (Showler et al. 2019b) (Table 1). Numbers of plants of each species were counted and percentage ground cover was visually estimated within the quadrats 11–14 Jun 2018.
Temperature and relative humidity of S. spartinae clumps
Temperature and relative humidity were measured using an MS6508 digital temperature-humidity meter (Peakmeter, Shenzhen, Guangdong, China) within the basal clump of 10 randomly-selected S. spartinae plants (≈5 cm above the soil surface) and in the open canopies (≈20 cm below the top of the plant) within 500 m of N 26° 08.20’ W97° 17.41’ in Cameron County. Wind speed was taken once 30 cm above each of the S. spartinae canopies. These measurements and the S. spartinae leaf shape observations were recorded at 1100 to 1400 h on 15 Feb, 24 Apr, and 12 Jun 2018.
Six S. spartinae leaves from each of 10 different randomly selected plants in Cameron and Willacy counties were excised with scissors at their bases and visually examined to determine whether the leaf was open or folded, 29 Jan, 25 Apr, 12 Jun, and 25 Oct 2018. The breadths of the “V” in 60 randomly selected “open” leaves, and of the slot in 60 randomly selected “folded” leaves at the midpoint of each, were measured using a digital caliper (resolution 0.01 mm, model 61,585, Pittsburgh Pro, Camarillo, California, USA). Leaves for determining the breadth of leaf “V”s and slots were obtained from five S. spartinae locations (two samples per location) in Cameron and Willacy counties.
Larval and nymphal questing site preferences on individual S. spartinae leaves
We released 50 larval and 50 nymphal A. americanum on ten adjacent S. spartinae plants in Cameron County. The ixodids were reared on Bos taurus L. calves in a closed colony of wild-caught (Aug 2006) A. americanum maintained at the USDA-ARS Knipling-Bushland United States Livestock Insects Research laboratory in Kerrville, Texas (USDA-ARS 2012), for ≥20 generations. Larvae and nymphs used in this study were 6–12 d old after hatching and molting, respectively. At the end of the field observations described below, the ten S. spartinae plants were treated with silica gel-based desiccant dust (CimeXa, Rockwell Labs, Kansas City, Missouri, USA) using a hand-held manual duster (Dustin-Mizer, Earthduster, Fayetteville, Arkansas, USA), which has been shown to be effective for controlling gulf coast ticks, Amblyomma maculatum Koch, on S. spartinae in the same area and during relatively high wind conditions (Showler et al. 2018). After 1 d, visual inspection of the plants, followed by flag sampling, indicated that the ixodids had been eliminated.
The S. spartinae plants were growing in an area that local hunters indicated was frequented by wild ungulates, particularly white-tailed deer. Odocoileus virginianus Zimmerman, and feral nilgai antelope (originally from South Asia), Bosephalus tragocamelus (Pallas). Ten individual living and 10 dead S. spartinae leaves were examined from each of 10 selected infested plants and visually examined. Each leaf was characterized as either being “open”, with a cross-sectional “V” shape, or as being “folded” with a narrow slot. Dead leaves were all “closed” (tightly furled, lumen diam < 0.25 mm) excluding tattered distal tips. Ixodid larvae and nymphs on each leaf were counted and their positions in terms of distance from the distal end of the leaf, and whether the ixodid was inside the “V” or on the opposite convex side, were recorded. To determine relative positions on the leaves, we sampled leaves until 10 larval and 10 nymphal ixodids were found on each of the 10 selected plants. Ad hoc observations of A. americanum behavior on folded leaves were also recorded. Fifty dead leaves were cut in half lengthwise with a razor and the interiors were visually inspected for ixodids.
In a second experiment conducted on 6 Aug 2019, thirty 50 cm lengths of S. spartinae leaves with the slot shape were excised from whole plants growing at lat. N 26° 08.359’, long. W 97° 17.709’, Cameron County, Texas, alongside the mainland shore of Lower Laguna Madre. The basal, cut ends of the leaves were inserted into a 15 cm deep plastic bag containing water, and the mouth of the bag was cinched around the protruding stalks using a rubber band. The bag, kept upright, was transported to KBUSLIRL. On 8 Aug at 1200 h, 36.7 °C, 80% relative humidity, under a cloudless sky, the leaves (still green) were removed from the water in the plastic bag and the base of each of 10 leaves was embedded 1 cm deep in plastilina modeling clay (Sargent Art, Hazleton, Pennsylvania, USA); the clay had been stuck to a 1 m × 5 cm × 10 cm length of wood for support. The wood, with the clay and leaves attached, was placed outdoors at KBUSLIRL in direct sunlight. The long and supple leaves arced from bases at ≈45° angles, the slot sides ventral, all conforming to the growth habit of S. spartinae under natural conditions. Five larval A. americanum were transferred, using a #5 1.6 cm camel hair paint brush (Charles Leonard, Glendale, New York, USA), to the approximate middle of each leaf. Larvae were observed during the next hour. At the end of the hour, numbers of larvae on the external surface of each leaf were recorded and removed with the paint brush. The interiors of the leaf slots were then examined using a 10× magnification hand lens and larvae within were counted.
Flag sampling natural ixodid populations
Ixodids were collected from vegetation using a 48 cm × 100 cm (w × l) flag comprised of white 100% cotton flannel (Fabric, Kennesaw, Georgia, USA) attached to a 1.4-m-long wooden handle. The flag was manually dragged at walking speed across low-growing vegetation in 50-m-long transects 12–15 Feb, 23–25 Apr, and 11–13 Jun 2018. The flag was examined for ixodids every 10 m (hence, five subsamples per transect) and larvae, nymphs, and adults were counted. Six locations dominated by S. spartinae and six locations dominated by mixed mesquite-thorn scrub (Table 1) were sampled using 28 transects in each habitat (at least four transects per location) in February, and 17 transects in each habitat (at least two transects per location) in April and in June.
Ixodids were also flag-collected from S. spartinae in a 300 m2 area where S. spartina was prevalent, around 26˚ 08’ 09” N, 97˚ 10’ 42” W, Laguna Atascosa Wildlife Refuge, Cameron County, Texas, 23–25 May and 20–23 Jun, 2022. Sampling was conducted until ≥100 ixodids were collected on each day and stored in 70% alcohol-filled glass vials. The vials of ixodids were sent to the United States Department of Agriculture – Animal and Plant Health Inspection Service’s National Veterinary Services Laboratory in Ames, Iowa, USA, for species identification of larvae, nymphs, and adults, and the sex of each adult. The samples were collected to determine which ixodid genera and species were questing on S. spartinae.
Natural enemies of ixodids
Uca rapax populations and egg predation
At each of six locations in Cameron and Willacy counties (Table 1), five 10-m-long walking transects were conducted during which numbers of U. rapax holes were counted within 1.5 m on both sides of each transect. In S. spartinae habitats the person sampling had to walk between S. spartinae clumps instead of in a straight line.
At each of 12 locations in the same two counties (Table 1), a 470 ml plastic cup (10.5 cm × 9.9 cm × 7 cm, h × top diam × bottom diam) (Ball, Fishers, Indiana, USA) was buried flush to the lip 10–30 m apart at 1000 h to serve as a pitfall trap. Numbers of U. rapax, ants, and other arthropods collected in each were recorded 48 h after the traps were deployed.
Six marker flags were deployed > 10 m apart from one another in S. spartinae habitats at each of six locations (Table 1) 12–15 Feb, 23–25 Apr, and 11–13 Jun 2018. At 1800 h a living 1 cm3A. americanum egg mass was placed at the base of each flag. At 0830 h on the following morning, the base of each flag was inspected for the presence of eggs. The eggs were deployed overnight because U. rapax mostly forages between dusk and dawn (Showler et al. 2019b).
Other soil-associated natural enemies
Three pitfall traps were deployed at each of two S. spartinae habitats in Willacy County and four in Cameron County (Table 1); each sampling site was separated by ≥0.5 km. The pitfall traps were comprised of a 946.4 ml plastic drinking cup (17.8 cm × 10.9 cm × 9.8 cm, h × top diam × bottom diam) (Dart Solo, Dallas, Texas, USA) buried flush to its lip in the soil and filled halfway with ethylene glycol. A 22.9 cm diam paper plate (Harvest Pack, St. Paul, Minnesota, USA) was pierced 5 cm apart on each of two opposite sides and a 3.2 cm × 30.4 cm Ohuhu galvanized iron ground staple (Union City, California, USA) was fitted through each pair of holes. The legs of the staple were inserted in the soil on each side of the pitfall forming a shelter ≈4 cm overhead to slow evaporation and to protect the trap from being filled with debris. The traps were deployed on 2 May 2018 and collected 2 wk later on 16 May. The contents were poured onto a hand-held metal sieve (50 mesh/cm2) (TCP Global, San Diego, California, USA) and arthropods were counted and identified to family.
Three sets of five 180° sweeps of a sweep net (38.1 cm diam, 1-m-long handle; 7600 Standard Series insect net with standard aerial bag, Bio Quip, Rancho Dominguez, California, USA) through the canopies of S. spartinae plants were conducted at each of two S. spartinae habitats in Willacy County and four in Cameron County (Table 1) 23–25 Apr 2018. Collected arthropods were counted and identified to family.
The two-sample t test (Analytical Software 2008) was used on data for temperature, relative humidity, soil salinity, numbers of U. rapax holes and U. rapax captured in pitfalls, and tick positions on S. spartinae leaves. One-way ANOVA was used for seasonal leaf shape data, means separated using Tukey’s HSD (Analytical Software 2008). Percentage data was arcsin-square root-transformed before analysis (Analytical Software 2008). Because normality and homogeneity of variance assumptions were not violated, data were not log(x + 1)-transformed.
Soils collected from S. spartinae habitat did not differ in terms of salinity from soils collected in mesquite-thorn scrub habitat (Showler and Osbrink 2020). Soil salinity in S. spartinae habitat was 0.86 ± 0.15 mmhos/cm, and 0.79± 0.17 mmhos/cm in mesquite-thorn scrub habitat.
Plant habitat composition
In S. spartinae-dominated habitat, S. spartinae comprised 80–100% of the vegetational cover and in some places S. spartinae completely covered the soil surface (Table 2). Other plant species were less abundant (< 5% ground cover) (Table 2). Most S. spartinae grew fully exposed to the sun interspersed by scattered P. glandulosa trees and small (< 20 m2) patches of thorn scrub (Table 2 for broadleaf species). Grasses in mesquite-thorn scrub habitat were mostly Vasey’s grass, Paspalum urvillei Steud.; dallis grass, Paspalum dilatatum Poir., and little bluestem, Schizachyrium scoparium (Michx.) Nash (Table 2).
Temperature and relative humidity of S. spartinae clumps
The temperature inside S. spartinae clumps in the winter was 4.5 ˚C lower than in the canopy (t = 5.01, df = 1, 18, P = 0.0001) (Fig. 1). Relative humidity was 32.4% greater in the interiors of S. spartinae clumps than in the canopy (t = 7.71, df = 1, 18, P < 0.0001) (Fig. 2). During spring, the temperature inside S. spartinae clumps was 3.5 ˚C lower than in the canopy (t = 3.30, df = 1, 18, P = 0.0040) (Fig. 1). Relative humidity was 14.8% greater in the interiors of S. spartinae clumps than in the canopy periphery (t = 3.24, df = 1, 18, P = 0.0045) (Fig. 2). In summer the temperature inside S. spartinae clumps was 3.1 ˚C lower than in the canopy (t = 3.56, df = 1, 18, P = 0.0022) (Fig. 1) and relative humidity was 32.1% greater in the interiors of S. spartinae clumps than in the canopy (t = 9.51, df = 1, 18, P < 0.0001) (Fig. 2).
Living S. spartinae leaves were 64.7 ± 4.9 cm long. More than 98% of them were open or folded along ≥80% of their lengths. Excluding the wind-tattered distal 2–3 cm, dead dry leaves were all closed. In late January 2018, 2.8% ± 2.8 of the S. spartinae leaves were “folded”, forming a 1.01 ± 0.01-mm-wide groove or slot in the leaf instead of the 3.94 ± 0.08-mm-wide “V”. By late April, mid-June, and late October, 86.1% ± 5.1, 63.9% ± 5.1, and 57.5% ± 3.7, respectively, leaves were folded. Seasonal differences in numbers of folded leaves were detected, with 30.8-, 22.8- and 20.5-fold more in October, April, and June, respectively, than in January (F = 50.27, df = 3, 23, P < 0.0001).
Larvae and nymphs on individual S. spartinae leaves
Amblyomma americanum larvae and nymphs on living S. spartinae leaves were observed 20.8 ± 1.8 cm and 21.7 ± 1.6 cm from the distal end, respectively. Approximately 44% ± 5.4 and 43% ± 4.2 of larvae and nymphs, respectively, were within 10 cm of the leaf tips. Larval A. americanum were 1.7-fold more abundant (t = 3.67, df = 1, 18, P = 0.0018) within the “V” of open leaves than nymphs (Fig. 3). Percentages of larvae within 10 cm of the distal end were ≈1.8-fold more likely to be within the “V” than on the opposite side of the leaf (t = 2.93, df = 1, 18, P = 0.0089) (Fig. 4). Percentages of nymphs were ≈1.4-fold more abundant than larvae (t = 2.73, df = 1, 18, P = 0.0139) on the opposite side of the “V” (Fig. 5). Larvae and nymphs were, on (pooled) average, 16.0 ± 3.3 cm from the leaf tip on dead (closed) leaves, and 2.5- (t = 2.45, df = 1, 18, P = 0.0250) and 2.2-fold (t = 2.13, df = 1, 18, P = 0.0472), respectively, more numerous on living leaves than on dead leaves (Fig. 6). No ixodids were observed inside the lumens of dry, dead S. spartinae leaves.
In the second experiment, four of the A. americanum larvae were observed entering S. spartinae leaf slots. While 3.5 ± 0.3 (70%) larvae were found on the external leaf surface 1 h after the larvae were released on the leaves, 1.2 ± 0.3 larvae (24%) had taken refuge within the leaf slots. Most larvae, whether on the external leaf surface or in the leaf slot, were at the part of the leaf arc that was higher than the free-hanging distal end and the basal part of the leaf (due to negative geotaxic ixodid behavior). A total of three larvae had fallen from the leaves during the experiment. The mean number of larvae on the external leave surface was 2.9-fold greater than the number that was inside the leaf slot (t = 4.87, df = 1, 18, P = 0.0001).
Flag sampling natural ixodid populations
In mid-February and in late April 2018, questing flag-sampled larval, nymphal, and total ixodids were relatively common (Table 3). Whether absolute numbers of larvae, or clustered larval populations from an egg mass hatch were counted as one larva (to avoid introducing substantial variation), differences between questing larval abundances on S. spartinae and mesquite-thorn scrub were not detected. In mid-June, differences were also not detected for questing larval, adult, and total ixodids, but nymphs were 5.8-fold more abundant in mesquite-thorn scrub than on S. spartinae (t = 2.10, df = 1, 18, P = 0.0439) (Table 3).
Ixodid samples collected from S. spartinae 20–23 Jun 2022 contained individuals, including larvae (91%), nymphs (5.5%), and adults (3.5%) of six species: A. maculatum (0.1%); the northern cayenne tick, Amblyomma mixtum Koch (0.1%); the false cayenne tick, Amblyomma tenellum Koch (8.5%); the tropical horse tick, Anocentor nitens (Neumann) (1.9%); the American dog tick, Dermacentor variabilis (Say) (0.3%); and R. (B.) microplus (89.1%) (Table 4). Anocentor nitens and R. (B.) microplus are one-host ixodids, hence, only larvae were detected on the vegetation (Table 4). Rhipicephalus (B.) microplus larvae constituted 97.9% of the total number of larvae (Table 4).
Natural enemies of ixodids
Uca rapax populations and egg predation
Numbers of U. rapax holes were not different between the S. spartinae and mesquite-thorn scrub habitats, with a pooled average of 0.34 ± 0.17 holes/m2. Numbers of pitfall-collected U. rapax were also not different, with a pooled mean of 0.20 ± 0.13/trap.
Negligible numbers (≤1) of A. americanum egg masses were removed in S. spartinae-dominated habitats during the night regardless of the season. The egg masses were all intact and had not been disturbed.
Other soil-associated natural enemies
Ground-associated arthropods were sparse in S. spartinae habitats with ≤1.2 individuals of each taxa (Table 4) collected over a 2-wk period. Only blattellids (wood cockroaches) were more abundant at 5.5 ± 3.4/trap. Predators included U. rapax, scorpions, spiders, formicids, elaterids, and reduviids (Table 5).
Sweep net collected arthropods were sparse, usually ≤4 per sample, and mostly small (≤1 mm long) (Table 6). The most common arthropods were cicadellids and dipterans. Predators were limited to negligible numbers (< 3/trap) of linyphiid and salticid spiders, and one reduviid.
Terrestrial plant communities on the South Texas coastal plain generally conform to one of three vegetational habitats. One habitat is characterized by B. frutescens that grows in dense stands on saline (6.8–91.0 mmhos/cm) soils created by occasional wind tides and infrequent storm surges from the hypersaline Lower Laguna Madre (Showler et al. 2019b; Showler and Pérez de León 2020). Hypersaline bay water is lethal to ixodid eggs but saline soil is not particularly ovicidal (Showler et al. 2019b). On the other hand, saline soil is associated with large, dense populations of U. rapax, an efficient ixodid egg predator (Showler et al. 2019b). Relative to mesquite-thorn scrub, ixodid numbers in dense stands of B. frutescens are negligible (Showler et al. 2019b). The second major vegetational habitat is comprised of mesquite-thorn scrub typical of low salinity soil on the coastal plain and the third is dominated by S. spartinae (Showler et al. 2019b), also on low salinity soil. Other habitats (e.g., sand dunes, marsh, hardwood forest) are spatially limited relative to areas of mesquite-thorn scrub (grasses are a component) and S. spartinae-dominated areas.
The relatively low soil salinity in the S. spartinae habitat indicates that salinity is not an ixodid mortality factor (Showler et al. 2019b). Uca rapax is an effective ixodid egg predator in typically saline, B. frutescens-dominated habitats (Showler et al. 2019b), but because soil salinity was low in S. spartinae habitats, so, too, were U. rapax populations. Hence, U. rapax predation on ixodid eggs in mesquite-thorn scrub habitats and in S. spartinae-dominated habitats was negligible. Sweep net and pitfall arthropod sampling yielded few predators and did not suggest the existence of predatory species and populations that might appreciably suppress ixodid abundances. Predatory ants have been implicated with the suppression of ixodid populations (Fleetwood et al. 1984; Sutherst et al. 2000), but recent research demonstrated that ants, including S. invicta, of the South Texas coastal plain wildlife corridor, West Texas (Brewster County), and the Texas Hill Country (Kerr County), do not attack A. americanum eggs, larvae, nymphs, unfed adults, and blood-engorged adults (Showler et al. 2019a, b). Some ixodid genera (i.e., Amblyomma, Dermacentor, and Rhipicephalus) emit an allomone that masks them from being recognized by ants as food (Yoder et al. 1992, 2009; Yoder and Domingus 2003; Showler et al. 2019a). Pressure from natural enemies in S. spartinae habitat does not appear to strongly affect ixodid survival.
Ixodid populations tend to increase during and after rainy periods and to decrease in dry times (McCulloch and Lewis 1968; Rawlins 1979; Daynes and Gutierrez 1980; Cardozo et al. 1984). As a group, ixodids can survive longer than many other arthropods without drinking water because they can also absorb moisture from the air (Needham and Teel 1991; Perret et al. 2003). While ambient temperature and humidity are important to ixodid (especially the larval stage) survival (Davey et al. 1991, 1994; Estrada-Peña 2001; Corson et al. 2004; Estrada-Peña and Vanzal 2006), localized factors can affect it by modifying temperature and humidity (Wilkinson and Wilson 1959; Cherny and de la Cruz 1971; Branagan 1978; Sutherst 1983; Sutherst et al. 1983; Utech et al. 1983; Sutherst and Maywald 1985; Perret et al. 2003). Ixodids interact with plants in a variety of ways, particularly by climbing vegetation to quest for passing hosts (Garris and Popham 1990; Schulze et al. 2001a; Perret et al. 2003; Randolph 2004). Anomalies related to ixodid populations and levels of activity suggest that microclimatic factors related to vegetation influence ixodid survival (Hoogstraal 1956; Branagan 1978; Perret et al. 2003); at the drier margins of ixodid habitats, for example, relative humidity within plant transpirational microclimates can be important to survival (Branagan 1978; Perret et al. 2003). The brown ear tick, Rhipicephalus appendiculatus Neumann, for instance, is usually confined to areas that offer particularly favorable conditions, such as dense grass cover (Yeoman and Walker 1967), and Yeoman (1967) correlated reduced grass cover with declining ixodid abundances because sparse grass cover exposes ixodids to lethal desiccating climatic factors. Phillips et al. (2014) suggested that R. microplus and R. annulatus were more likely to be found at the edges of thorn thicket and P. glandulosa patches than in adjacent open grassy areas because the former habitats are more favorable and because hosts often take refuge in dense vegetative cover to avoid desiccation. Survival of R. microplus was 61.5–76.9% lower in open habitats dominated by buffelgrass, Pennisetum ciliaris (L.) Link, than in P. ciliaris under P. glandulosa canopy (Davey et al. 1994). Questing A. americanum are also particularly abundant along shady woodland cattle and deer trails and in shady places in pastures (Harris and Burns 1972; Goddard 1992; Schulze et al. 2001b). Further, plant morphology and plant surface moisture can affect larval ixodid clustering (Sutherst 1983) and longevity (Garris and Popham 1990; Perret et al. 2003).
Areas dominated by S. spartinae tend to have little or no tree overstory shade that would provide reduced temperatures and increased relative humidity levels favoring ixodids more than conditions of direct sunlight (Perret et al. 2003). Spartina spartinae plants, however, themselves provision lower temperatures and greater relative humidities within the dense basal clump relative to the canopy where questing occurs. The lower numbers of questing nymphs collected from S. spartinae compared to mesquite-thorn scrub in the summer suggest that ambient daytime conditions on the sun-exposed S. spartinae canopy (not necessarily inside the clump) can be lethal. Ixodids likely find refuge in the clump’s interior during unfavorably hot and dry conditions. Using an automated video tracking system that continuously recorded movements of immature sheep ticks, Ixodes ricinus L., Perret et al. (2003) found that the parasites ascended plants to quest and intermittently descended to microhabitats on the same plant and on the soil where standing and airborne moisture (humidity) allow for rehydration before they ascended again to continue questing.
Ixodid questing involves expenditure of energy and water loss in search of nourishment from hosts (Perret et al. 2003). Larvae, however, can survive off-host for 8–9 mo in hot semi-arid environments before they expire from starvation (Hitchcock 1955; Utech et al. 1983; Needham and Teel 1991; Perret et al. 2003). Individual S. spartinae clumps provided substrates for questing and for intermittent (quiescent) periods of rehydration in protected microenvironments. While temperature and relative humidity conditions in the basal parts of the S. spartinae clumps were significantly different from the more exposed canopy, and potentially able to protect ixodids from desiccation, the shapes of living S. spartinae leaf blades also might affect ixodid survival. During cool months, S. spartinae leaf blades present a long concave surface that likely conserves humidity on the concave side, and morning dew was retained longer in the relatively open “V” and in the slot-shaped “folded” leaves than on the more exposed, convex side of the leaves. Conservation of humidity and moisture in the concave side is suggested by the larger percentage of A. americanum larvae observed in the “V” than on the convex side, even though the concave side of the leaves usually faced up. The body surface area:volume ratio is smaller in nymphs than in larvae and less conducive to integumentary water loss, hence, nymphs might rely less than larvae on being in the convex side of the leaf blade “V” or in the slot-like fold for survival. Questing larval A. americanum were most abundant in the distal 10 cm of the open leaf “V” where it is possible that relative humidity was greater than on the convex side. The greater numbers of A. americanum larvae and nymphs on living than on dead S. spartinae leaves were likely because of the humidity- and dew-conserving “V” shape of many living leaves in contrast with the exposed outer surface of the dead, dry, and completely furled leaves. Living leaves were also more erect (> 45° angle relative to the soil surface) than dead leaves (most < 45° relative to the soil surface) which were likely less preferred by negatively geotaxic ixodids (Kroeger et al. 2013; Romaschenko et al. 2013). Dead leaves, being dry, were also unlikely to conserve as much moisture as living leaves that contain water.
During most of the year, when the weather is warm, S. spartinae leaf shape changes from the “V” to a narrow slot. While field sampling, two instances occurred where larval ixodids (species not determined), sometimes > 10 individuals, within a leaf slot dropped from the slot onto an exposed human forearm that was inadvertently brushed against the leaves. The experiment conducted at KBUSLIRL demonstrated that larval ixodids actively enter S. spartinae leaf slots under conditions of direct sunlight and warm temperatures. Although more A. americanum larvae in the experiment were observed on the exterior surfaces of S. spartinae leaves than inside the leaf slots, it is nevertheless important that a substantial percentage of the larvae entered the leaf slots. The narrow leaf slots likely provide favorable conditions for ixodid larvae at their questing sites, particularly during desiccating ambient conditions. It was expected that a proportion of the larvae would be questing while a minority moved into the protected leaf slot to rehydrate.
Spartina spartinae-dominated habitats on the wildlife corridor generally have two spatial configurations. In many areas S. spartinae clumps are not densely packed together, with bare and lightly vegetated areas surrounding each clump or small group of clumps. The “loose” configuration of clumps affords ungulate hosts numerous choices of paths to traverse and a herd can disperse among the clumps while retaining cohesion. Alternatively, in areas where S. spartinae clumps are dense, tightly abutting one another, the soil surface is often > 90% obscured by matted leaves; ungulate trails are fewer and more clearly defined than in the “loose” configuration. Hence, in dense S. spartinae stands, movement of ungulates is largely confined to established trails beyond which some ixodid species populations might be lower. Because ixodid abundance is substantially influenced by host frequency (Branagan 1978), distribution of ixodids that are restricted to ungulate hosts, such as R. microplus (a one-host parasite) larvae, in relatively widely spaced S. spartinae habitat might be more uniform than where S. spartinae clumps are sufficiently dense to confine ungulates to well-defined trails. Ixodid species parasitizing small hosts that can easily move through dense S. spartinae habitat, however, likely have less restricted distributions.
Spartina spartinae occupies substantial areas of the South Texas coastal plain wildlife corridor. Ixodid populations are not appreciably different between S. spartinae and mesquite-thorn scrub habitats likely because neither habitat involves high soil salinity that favors U. rapax egg predation, and the mostly nonshaded S. spartinae offers refuges from desiccation for reabsorbing water within the leaves and in the interiors of plant bases between periods of questing. Our study demonstrated that multiple ixodid species, including larvae, nymphs, and adults, occur on S. spartinae. It is likely that any ixodid species in that habitat will quest on S. spartinae, as well as other plant species. Surveillance for ixodids should include S. spartinae habitats while recognizing that quiescent ixodids are not detected; only actively questing individuals collected using flags and other methods that select for questing ixodids, are detected. Basing ixodid population estimates on flag sampling (and other means of exclusively sampling for questing individuals) is likely to skew data by not including quiescent cohorts. Our finding indicate that ixodids, on S. spartinae and possibly other plant species, can reabsorb water in more than one place on the plant where they are not questing and available to be sampled using flagging (Perret et al. 2003).
Located in the Cattle Fever Tick Quarantine Zone in South Texas, this study indicated that R. (B.) microplus larvae occurred as a relatively heavy infestation on vegetation in contrast to single specimens collected on vegetation at two locations in the same quarantine zone in 2018 (Osbrink et al. 2020). The large numbers of R. (B.) microplus larvae on vegetation is the result of substantial reproduction in the Texas quarantine zone. Vigilance outside of the quarantine zone warrants intensification (Showler and Peréz de León 2020, Showler et al. 2021). For surveillance involving populations on vegetation, S. spartinae habitat should be given as much attention as other habitats.
This study demonstrates that S. spartinae habitats on the South Texas coastal plain do not harbor soil-borne and foliar predatory arthropods in sufficient abundances to govern ixodid population levels. Soil and water salinity, both of which are lethal particularly to ixodid eggs (Showler et al. 2019b), are also not substantial mortality factors in the S. spartinae habitat. High ambient temperature and low relative humidity can each, and in combination, induce lethal desiccation to off-host ixodids. The S. spartinae plant offers two possible refuges in which ixodids can undergo rehydration quiescence: in the basal clamp and within concave parts of leaf blades.
Relating pertinent ecological compartments has provided clarity in terms of how a medically and agriculturally important arthropods can survive in environmental systems, using the cordgrass habitat as an example, that would otherwise be lethal. Understanding of environmental systems by determining abiotic and biotic ecological compartments can contribute to new opportunities to manage such pests.
All data generated or analyzed during this study are included in this published article.
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Thanks to Wayne Ryan for rearing A. americanum and to William Brady for assistance in the field, and James Mertins for identification of ixodids collected from gulf cordgrass.
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Showler, A.T. Interactions of gulf cordgrass, Spartina spartinae (Trin.) Merr. ex Hitchc., habitat with ixodids on the South Texas coastal plain. Environ Syst Res 12, 28 (2023). https://doi.org/10.1186/s40068-023-00311-w
- Cattle fever tick
- Relative humidity
- Rhipicephalus microplus