Impact of Controlled Burns on the Abundance of Ixodes scapularis (Acari: Ixodidae)

Abstract

Information on the effect of vegetative destruction by controlled burns in reducing the abundance of the blacklegged tick, Ixodes scapularis Say, the vector for the agents of Lyme disease, human babesiosis, and human granulocytic ehrlichiosis, is limited. Therefore, the abundance of nymphal, larval, and adult I. scapularis was monitored by dragging the vegetation at 2 separate 4-ha tracts in Cockaponset State Forest in Connecticut following a single controlled burn on 15 April or 21 May 1992. The burn at the April burn site was rated as light to moderate with a flame height of 0.3 m and consumed approximately 67% of the surface leaf litter. The burn at the May burn site was rated moderate to severe with a flame height of 0.6-0.9 m., which consumed vegetation < 5 cm in diameter and approximately 100% of the surface leaf litter. The impact of the burn was strongly influenced by the intensity and timing of the burn. Burning of the vegetation resulted in a reduction of the abundance of nymphal I. scapularis by 74% at the moderately burned site and 97% at the severely burned site, compared with adjacent unburned woodland. No larvae were recovered later in the summer from the severely burned tract. However, judging by the comparable abundance of adult I. scapularis in the fall at the burned and unburned woodlands, the effect of the burns was temporary. Burning the vegetation for the control of I. scapularis appears limited in effect and could be applied only on a large scale in areas with little or no human habitations.

Full text

Impact of Controlled Burns on the Abundance of Ixodes scapularis
(Acari: Ixodidae)
KIRBY C. STAFFORD III, JEFFREY S. WARD, AND LOUIS A. MAGNARELLI
Connecticut Agricultural Experiment Station, 123 Huntington Street, Box 1106, New Haven, CT 06504
J. Med. Entomol. 35(4): 510Ð513 (1998)
ABSTRACT Information on the effect of vegetative destruction by controlled burns in reducing
the abundance of the blacklegged tick, Ixodes scapularis Say, the vector for the agents of Lyme
disease, human babesiosis, and human granulocytic ehrlichiosis, is limited. Therefore, the abundance
of nymphal, larval, and adult I. scapularis was monitored by dragging the vegetation at 2 separate
4-ha tracts in Cockaponset State Forest in Connecticut following a single controlled burn on 15 April
or 21 May 1992. The burn at the April burn site was rated as light to moderate with a ßame height
of 0.3 m and consumed '67% of the surface leaf litter. The burn at the May burn site was rated
moderate to severe with a ßame height of 0.6 Ð0.9 m., which consumed vegetation ,5 cm in diameter
and '100% of the surface leaf litter. The impact of the burn was strongly inßuenced by the intensity
and timing of the burn. Burning of the vegetation resulted in a reduction of the abundance of
nymphal I. scapularis by 74% at the moderately burned site and 97% at the severely burned site,
compared with adjacent unburned woodland. No larvae were recovered later in the summer from
the severely burned tract. However, judging by the comparable abundance of adult I. scapularis in
the fall at the burned and unburned woodlands, the effect of the burns was temporary. Burning the
vegetation for the control of I. scapularis appears limited in effect and could be applied only on a
large scale in areas with little or no human habitations.
KEY WORDS Ixodes scapularis, Lyme disease, ticks, tick control, burning, habitat
THE TICK VECTOR in the northeastern United States for
Borrelia burgdorferi Johnson, Schmid, Hyde, Steiger-
walt & Brenner and Babesia microti Franca, the causal
agents of Lyme disease and human babesiosis, respec-
tively, is the blacklegged tick, Ixodes scapularis Say
(Spielman et al. 1979, Steere and Malawista 1979,
Burgdorfer et al. 1982, Johnson et al. 1984, Piesman et
al. 1986). Since surveillance for Lyme disease was
begun by the Centers for Disease Control and Pre-
vention (CDC) in 1982, the number of Lyme disease
cases has increased from 491 to 16,461 cases in 1996
(CDC 1997). Much of the research to reduce the
incidence of Lyme disease has focused on vector con-
trol. Tick control methods for I. scapularis have in-
cluded the application of insecticides to the vegetation
(Schulze et al. 1987, 1991; Stafford 1991a; Solberg et al.
1992); a host-targeted insecticide (Daniels et al. 1991;
Deblinger and Rimmer 1991; Stafford 1991b, 1992);
reduction or exclusion of white-tailed deer,
Odocoileus virginianus (Zimmerman) (Wilson et al.
1984, Wilson et al. 1988, Daniels et al. 1993, Stafford
1993); and modiÞcation of the environment by burn-
ing or mowing (Wilson 1986, Mather et al. 1993).
Controlled burning of vegetation has been shown to
reduce the abundance of Dermacentor albipictus
(Packard), D. variabilis (Say), and Amblyomma
americanum (L.) (Smith et al. 1946, Hoch et al. 1972,
Drew et al. 1985). Little information is available on the
impact of controlled burns on the blacklegged tick. A
reduction in the density of adult I. scapularis was
observed after burning the vegetation in northern
Florida and coastal Massachusetts (Rogers 1953, Wil-
son 1986). Fewer nymphs of I. scapularis were recov-
ered from a burned woodlot on Shelter Island com-
pared with an unburned portion (Mather et al. 1993),
although the risk of Lyme disease was supposedly
unchanged following the burn because of a differen-
tial in infection of the nymphs with B. burgdoferi
between the burned and unburned woods. This study
reports the impact of 2 woodland burns, 1 moderate
and 1 severe in intensity, on the abundance of I.
scapularis nymphs, larvae, and adults following the
burn.
Materials and Methods
The study was conducted in Cockaponset State For-
est in the town of Chester, CT. The burns were con-
ducted at 2 locations of 4 ha each by personnel from
the Connecticut Department of Environmental Pro-
tection. Both sites were mesic with slopes of ,5%. The
woodland at the 1st burn site (Truck Road) was com-
posed of mixed oak, Quercus spp., and hickory, Carya
spp., overstory with a subcanopy of maple, Acer spp.,
birch, Betula spp., and American beech, Fagus gran-
difolia Ehrh. Sweet pepperbush, Clethra alnifolia L.,
blueberries, Vaccinium spp., and beaked hazelnut,
Corylus cornuta Marsh, dominated the shrub layer.
There was a stand of eastern hemlock, Tsuga canaden-
sis (L.), in 1 portion of the burn site. The 2nd burn site
0022-2585/98/0510Ð0513$02.00/0 q1998 Entomological Society of America

(Spruce Ledge) was dominated by oaks and Lirioden-
dron tulipifera L., with a subcanopy of maple, birch,
beech, and sassafras, Sassafras albidum (Nutt.). The
shrub layer was composed primarily of pepperbush,
blueberries, and hophornbeam, Ostrya virginiana
(Mil.). Unburned woodlands adjacent to the 1st burn
site and separated from that site by a Þre lane was used
as the 1st control site. Woodlands in East Haddam, CT,
were used for a 2nd control site. This privately owned
44-ha tract is a mixture of woodlands and open pasture.
The woods were dominated by oaks and maples with
a shrub layer dominated by barberry, Berberis spp. The
Truck Road site was burned on 15 April 1992 with a
light to moderate intensity with ßame height of '0.3
m and a rate of spread of 0.3Ð0.6 m/min. The Þre
produced an vegetative diameter LD
50
of 1.3 cm and
cleared '67% of the surface litter. The Spruce Ledge
site was burned on 21 May 1992 with a ßame height of
0.6Ð0.9 m and moderate to severe intensity. The rate
of spread is unknown, but the resulting Þre consumed
'100% of the surface leaf litter, and the vegetation
diameter LD
50
was 5.1 cm.
The abundance of host-seeking I. scapularis was
determined by dragging a 1.0-m
2
piece of white ßannel
cloth over the vegetation at plots (10 by 10 m) estab-
lished at 6 locations in each of the burn and control
sites in Cockaponset State Forest. Two plots (10 by
25 m) were sampled in the woodlands in East Haddam.
Host-seeking I. scapularis were sampled twice each
month from May through October. Any ticks found on
the drag cloth were removed, placed in vials with a
blade of grass, and returned to the laboratory for
identiÞcation and testing for B. burgdorferi. The pres-
ence of B. burgdorferi in host-seeking nymphs was
determined by indirect ßuorescent antibody (IFA)
staining of tick midgut tissues with murine monoclonal
antibody (H5332) directed to outer surface protein A
(Osp A) and ßuorescein-conjugated antibodies as pre-
viously described (Magnarelli et al. 1987).
Tick abundance was tabulated for the period of
activity for each stage (i.e., August for larvae, MayÐ
August for nymphs, and October and November for
adults). The number of ticks per 100 m
2
for the 2
burned and 2 unburned areas was compared by the
nonparametric KruskalÐWallis 1-way analysis of vari-
ance (ANOVA) on ranks using SigmaStat 2.0 (Jandel
1995) (Fox et al. 1995). DunnÕs method using SigmaS-
tat was used for pairwise multiple comparisons of the
4 treatment groups.
Results and Discussion
Burning of the vegetation resulted in a reduction in
the abundance of nymphal I. scapularis of 74 and 97%
at the April moderate burn and May severe burn sites,
respectively. However, judging by the presence of
adult I. scapularis at all 4 sites in the fall, the impact was
temporary (Fig. 1). A summary of the abundance of I.
scapularis larvae, nymphs, and adults for each of the
burn and unburned sites from May through October
is shown in Table 1. The abundance of nymphal I.
scapularis at the burned properties during the summer
following the burn was signiÞcantly lower than that in
unburned woodland (H535.088, df 53, P,0.001).
There was no signiÞcant difference in nymphal abun-
dance between the 2 control sites (Q51.923, P.
0.05) or between the 2 burn sites (Q51.387, P.0.05)
by the pairwise multiple comparison procedure
(Dunn method). There was a signiÞcant difference in
larval abundance between the East Haddam control
site and the other 3 treatment sites (H520.154, df 5
3, P,0.001), but there was no signiÞcant difference
in larval abundance between the unburned and
burned tracts within Cockaponset State Forest (Q5
1.626, P.0.05). The majority of the larvae (98.6% of
248) at the 1st burn site were recovered from a single
plot located near the hemlocks, and a stream, where
the impact of the burn was slight, and '10 m from a
rabbit warren. The failure to recover any larvae at the
moderate to severely burned site and more heavily
burned portions of the 1st burn site suggests that many
engorged females may not have survived. However,
Fig. 1. Mean number of I. scapularis collected per 100 m
2
from April through November 1992 at the control and burn
sites in Cockaponset State Forest. (A) Nymphs. (B) Larvae.
(C) Adults. Arrow 1 indicates burn 1 (15 April, Truck Road
site); and arrow 2 indicates burn 2 (21 May, Spruce Ledge
site). Data for the East Haddam control site is not shown.
July 1998 STAFFORD:FOREST BURNS TO CONTROL TICKS 511

few larvae were recovered at the unburned woodlands
adjacent to the 1st burn site. By fall, there was no
signiÞcant difference (H53.151, df 53, P50.369) in
the number of I. scapularis adults recovered between
the burned and unburned areas, although fewer adults
were recovered from the more intensely burned site.
An insufÞcient number of ticks were tested to provide
a good measure of risk for transmission of B. burgdor-
feri, but the infection rate was low (7.1% of 14) in the
nymphs tested from the woodlands adjacent to the
burn site and the East Haddam control site (5.3% of
19). No spirochetes were detected in the 4 nymphs
tested from the 1st, moderately burned site.
These results are similar to earlier studies on burn-
ing of ground vegetation on ticks. The abundance of
larval and nymphal A. americanum was reduced by
burning the vegetation, but the adults were not af-
fected (Hoch et al. 1972). The reduction in nymphal
abundance in this study was greater than that ob-
served (49% fewer I. scapularis nymphs) by Mather et
al.(1993), but the single spring burn on Shelter Island,
although igniting surface leaf litter, did not penetrate
the soil humus layer. No information on larval and
adult I. scapularis later in the season was provided.
Vegetative burns on Great Island, MA, reduced adult
I. scapularis by 70 and 80% following the burns, but the
effect of the burn on adult tick abundance could no
longer be detected 1.5 yr later (Wilson 1986). Reduc-
tions in tick abundance in this study appears to be
affected primarily by the intensity of the burn and
degree to which the litter was consumed. The hotter
Þre performed later in the spring (burn 2, May 21)
resulted in 97% fewer nymphs. Only 1 nymph was
recovered at the severely burned site and few ticks
were recovered from any of the plots located in areas
where the Þre was more intense. Many of the ticks
recovered within the 1st (Truck Road) burn site were
from the plots located near wetland and more dense
vegetation where the burn intensity was less.
Recent efforts to control I. scapularis have focused
upon methods that could reduce the local abundance
of the tick at individual homes. Environmental mod-
iÞcation or vegetative management as a method to
reduce the abundance of I. scapularis, including con-
trolled burns, has received little attention, although a
number of studies have examined the impact of de-
stroying the vegetation on other tick species (Wilson
and Deblinger 1993). Area-applied acaricides have
been found to be effective in controlling I. scapularis
(Schulze et al. 1987, Schulze et al. 1991, Stafford 1991a,
Solberg et al. 1992, Curran et al. 1993). The use of a
wood chip border at the lawn edge and removal of leaf
litter has been found to reduce tick populations
around the home (K.C.S., unpublished data), and the
removal of leaf litter in wooded residential plots was
shown to reduce signiÞcantly the abundance of I.
scapularis nymphs (Schulze et al. 1995). Mowing the
vegetation reduced adult I. scapularis by as much as
70% (Wilson 1986).
However, controlled burns are limited to a large
scale, and burning may ultimately increase tick abun-
dance. As reviewed by Wilson and Deblinger (1993),
several studies have shown that burning woodlands
can improve deer browse and the density of Peromy-
scus spp., a major host for immature I. scapularis. This
may increase tick densities. Indeed, researchers con-
ducting vegetative studies at the 2 burn sites in this
study in the years following the burn noted that black-
legged ticks appeared to be more abundant than pre-
viously, but these tick densities were not quantiÞed.
The destruction of vegetation by controlled burns,
unless done annually, does not appear to be effective
in reducing the abundance of I. scapularis. Alternative
technologies aimed at reducing adult tick populations
on their primary host, the white-tailed deer, may have
more potential for the control of I. scapularis over
large areas and could be applied in residential settings
where burns cannot be performed.
Acknowledgments
We thank Tia Blevins and Collen Moser (Connecticut
Agricultural Experiment Station) for technical assistance.
Permission to conduct the studies at the state forest was
provided by the Connecticut Department of Environmental
Protection.
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Received for publication 13 August 1997; accepted 26 De-
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July 1998 STAFFORD:FOREST BURNS TO CONTROL TICKS 513