Proportion of adult lone star ticks (Amblyomma americanum) questing in a tick population.
Ticks quest for hosts by waiting on the tips of vegetation at variable heights. Only a portion of the ticks in a given population are questing at any one time as ticks must periodically leave their questing sites to move to the litter zone to rehydrate by active water sorption (Needham and Teel 1986). Since ticks quest for hosts, tick population densities can be estimated by using tick traps baited with dry ice, dragging with a white flannel cloth, or walking surveys (Falco and Fish 1989, Solberg et al. 1992, Schulze et al. 1997). Drag cloth sampling, which is relatively inexpensive and easy to standardize by distance or duration, has been shown to catch twice as many adult Amblyomma americanum as walking surveys (Schulze et al. 1997). However, drag cloth sampling only measures the density of questing ticks and not the true population density (Falco and Fish 1988, Solberg et al. 1992). Field studies with the European castor bean tick, Ixodes ricinus, have shown that approximately 24% of adults are questing at any one time; and studies with caged lone star ticks, Amblyomma americanum, in Oklahoma found that the percentage questing varied from 0 in mid-April to 90 during May and early June (Semtner and Hair 1973). This study was designed to assess the proportion of adult lone star ticks questing in a known tick population in central Mississippi and to relate that activity to factors such as site, temperature, humidity, and population density of ticks.
MATERIAL AND METHODS
For this study, a predetermined number of ticks were released into each of 9 wooded/grassy plots known to be free of lone star ticks, as determined by extensive drag cloth sampling in the plots prior to the study. Plots were located in central Mississippi and were 15m x 3m (approximately 50 ft x 10 ft); each contained a nature trail through them. Note: one of the 9 plots was exactly doubled in size for an additional experiment. All plots were thoroughly sampled by drag cloth twice prior to the study; not one lone star tick was collected. Adult A. americanum were collected by drag cloth from state-owned property in two parks, held in vials with moistened pieces of cloth, and released within 24 hours in the plots (also state-owned property--the former Lake Dockery area) near the author's home in Hinds County, MS. The study was blinded in that the person who released the ticks in the plots was different from the one who sampled them, and the collector did not know the plot assignments. The entire experiment--all tick captures, releases, and recaptures--was performed during mid-May 2007 because previous studies have shown May through early June to be the peak of adult lone star activity in Mississippi (Goddard 1997, Goddard and Layton 2006). In addition, all sampling was performed within 3 days after tick release to minimize any loss of ticks due to migration, death, or host acquisition.
Plots were randomly assigned as follows: 2 controls (no ticks), 2 with 12 ticks each released in them, 2 with 25 each, and 2 with 50 ticks each. The single, double-sized plot received 25 ticks. Plots were sampled with a 1[m.sup.2] drag cloth at 24, 48, and 72 hour intervals after tick release. Temperature and humidity were recorded at each sampling interval. To reduce variability, drag sampling was begun at 3:00 p.m. each day since previous research has shown that peak numbers of lone star tick questing varies by time of day (Schulze et al. 2001, Schulze and Jordan 2003). Four swaths were made through each plot each day, completely covering the area. The drag cloth was examined every 7.5m. Any ticks seen on the cloth were field-identified and released randomly in the previous 7.5m path. At the end of the study, ticks were exterminated by spraying all plots with a synthetic pyrethroid (deltamethrin) according to label instructions using a 4-gallon backpack sprayer with the nozzle set on "fan spray."
Statistical analysis. Data were analyzed using SAS software (SAS 2004). A two-factor, repeated measures design was used to compare site and time of day. A logistic regression was used to estimate probability of tick questing based on the known tick population. Environmental factors such as humidity and temperature were included as predictors for tick questing.
No lone star ticks were collected in surveys of the plots before the study began, nor in control plots during the study. Accordingly, we feel confident that all lone star ticks collected during our experiments were ticks we had released. Overall, out of the 199 ticks released, 29 (14.5%) were recaptured at 24 hours, 46 (23.1%) were recaptured at 48 hours, and 36 (18.0%) were recaptured at 72 hours (Table 1). Note that all ticks collected were released back into the plots each time and thus available for recapture upon subsequent sampling events. Size of the plot made little difference in number of ticks collected. The double-sized plot containing 25 ticks produced sampling results similar to the smaller plots containing the same number of ticks (Table 1).
The percentage of ticks questing (PTQ) varied among plots. Some of the plots were as close as 35 feet to each other. However, even though they were this close together, plots sampled on the same day produced different PTQ values. The mean proportions at 3 different sites by 3 different collection times ranged from 13% to 30% (Table 2). When analyzed by ANOVA, there were no significant differences in PTQ among sites or times (Table 3).
Since the plots were all sampled within 1 hour, weather conditions were essentially the same for all plots on the same day. Accordingly, there was little relationship between temperature and humidity and PTQ in this study (Table 4). However, other studies have demonstrated that questing is regulated by a combination of factors including relative humidity, temperature, and photoperiod (Semtner and Hair 1973). Ticks likely have a range of temperature and relative humidity levels required for them to function normally, so sampling temperatures falling outside of this range might force the PTQ to be extremely small to nonexistent. However, our data suggest that temperature or humidity changes within the normal operating range yield small, unrelated changes to the PTQ.
A logistic regression on this data set showed no significant relationship between tick populations and the fraction questing (Table 4). The predicted probability of tick questing based on number of ticks released at each site was less than 25%. This should not be construed to mean the absolute number of ticks questing is not related to the absolute number released. This only addresses the probability (proportion) of ticks questing. Further research with larger sample sizes is needed to elucidate the relationship between tick population and questing.
Dr. Brad Biggerstaff, Centers for Disease Control, provided helpful comments during preparation of the manuscript. This study was funded by a grant from the Bayer Corporation and we are especially grateful to Nick Hamon and Chip Anderson for their support. This article has been approved for publication as Journal Article No. J-11511 of the Mississippi Agriculture and Forestry Experiment Station, Mississippi State University.
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Jerome Goddard, (1) Jerome Goddard II, (2) and Xueyuan Wang (3)
(1) Department of Entomology and Plant Pathology, Mississippi State University, Mississippi State, MS 39762,
(2) Department of Mathematics, Mississippi State University, Mississippi State, MS 39762, (3) Department of Community Health Science, University of Southern Mississippi, Hattiesburg, MS
Corresponding Author: Jerome Goddard
Table 1. Lone star ticks collected in plots before and after release. Plot # Ticks pre- # Ticks # Ticks sample 24 hrs 48 hrs Control Rep 1 0 0 0 Control Rep 2 0 0 0 Plot with 12 ticks, Rep 1 0 3 1 Plot with 12 ticks, Rep 2 0 1 3 Plot with 25 ticks, Rep 1 0 4 5 Plot with 25 ticks, Rep 2 0 6 3 Plot with 50 ticks, Rep 1 0 9 14 Plot with 50 ticks, Rep 2 0 4 16 Plot with 25 ticks (2x size plot) 0 2 4 Plot #Ticks Avg. % 72 hrs questing Control Rep 1 0 -- Control Rep 2 0 -- Plot with 12 ticks, Rep 1 3 19 Plot with 12 ticks, Rep 2 1 14 Plot with 25 ticks, Rep 1 8 23 Plot with 25 ticks, Rep 2 3 16 Plot with 50 ticks, Rep 1 9 21 Plot with 50 ticks, Rep 2 7 18 Plot with 25 ticks (2x size plot) 5 15 Table 2. Percentages of tick questing by site and time. Collection Collection Collection at 24 hr at 48 hr at 72 hr Plots with 12 ticks 17% 17% 17% Plots with 25 ticks 20% 16% 22% Plots with 50 ticks 13% 30% 16% Table 3. Tick questing proportions by site and time Variable F value P Site 0.38 0.7151 Time 0.28 0.7651 Time * site 0.75 0.5916 Table 4. Probability of tick questing and predictor variables. Weighted Standard value B error P Exponent (B) Tick population 0.010 0.010 0.303 1.010 Humidity 0.005 0.015 0.741 1.005 Temperature 0.037 0.029 0.201 1.038
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|Author:||Goddard, Jerome; Goddard, Jerome, II; Wang, Xueyuan|
|Publication:||Journal of the Mississippi Academy of Sciences|
|Date:||Jul 1, 2009|
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