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Early Evening Questing and Oviposition Activity by the Culex
(Diptera: Culicidae) Vectors of West Nile Virus in Northeastern
Author(s): Michael R. Reddy, Timothy J. Lepore, Richard J. Pollack, Anthony E.
Kiszewski, Andrew Spielman, and Paul Reiter
Source: Journal of Medical Entomology, 44(2):211-214. 2007.
Published By: Entomological Society of America
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BEHAVIOR, CHEMICAL ECOLOGY
Early Evening Questing and Oviposition Activity by the
Culex (Diptera: Culicidae) Vectors of West Nile Virus in
Northeastern North America
MICHAEL R. REDDY, TIMOTHY J. LEPORE, RICHARD J. POLLACK, ANTHONY E. KISZEWSKI,
ANDREW SPIELMAN,1AND PAUL REITER2,3
Department of Immunology and Infectious Diseases, Harvard School of Public Health, Boston, MA 02115
J. Med. Entomol. 44(2): 211Ð214 (2007)
To determine whether the Culex (Diptera: Culicidae) mosquitoes that transmit West
Nile virus (family Flaviviridae, genus Flavivirus, WNV) in the northeastern United States seek hosts
they deposited eggs. They traversed oviposition sites most frequently ?2 h after astronomical sunset,
?2 h after sunset during midsummer, they are more opportunistic during mid- to late fall. Because
applied at that time.
host seeking, oviposition, diel periodicity, Culex, West Nile virus
mainly during ßight (Mount 1998), interventions di-
rected against pathogen-transmitting mosquitoes re-
quire precise information regarding the diel period-
generally are applied widely in the face of the recent
emergence of West Nile virus (family Flaviviridae,
genus Flavivirus, WNV) in northeastern North Amer-
ica. Diverse behavioral functions might stimulate the
ßight responses of mosquitoes, including blood feed-
ing, nectar feeding, oviposition, and mating. In addi-
tion, any mosquito might undertake occasional ßights
to avoid predators, leave the breeding site, seek a
resting site, and some migrate. The issue of diel peri-
odicity of ßight activity has become crucial to the
conduct of these interventions due to the growing
health burden imposed by this epizootic.
The various members of the Culex pipiens L. com-
plex as well as Culex restuans Theobald mosquitoes
North America (Turell et al. 2001, Andreadis et al.
2004). Females of both species oviposit most actively
soon after dusk (MacDonald et al. 1981). Mating
feeding, too, is most intense just before midnight
(Anderson and Jaenson 1987). Although Cx. pipiens is
said to quest for hosts in a biphasic, crepuscular pat-
tern of activity in southern England (Service 1971),
these insects are said to quest throughout the night in
deposition and of host seeking by these mosquitoes in
the northeastern United States remains unknown.
It may be that adult Culex spp. in the northeastern
United States seek hosts at a different time of night
than when they oviposit. Accordingly, we compared
the periodicity of these activities in a New England
relating these behaviors to the onset of astral sunset.
Materials and Methods
Study Sites. Study sites were located in the gardens
of residences in Cambridge and Somerville in Massa-
chusetts, located ?2 km apart. Both sites included
well-manicured lawns, mature deciduous trees, and
ornamental shrubs characteristic of residential prop-
erties throughout these neighborhoods.
oviposition occurred by inspecting, every 2 h, be-
tween sunset and dawn, the surface of the water con-
tained in 51- by 38-cm black plastic pans (Vollrath
52636, The Vollrath Co., Sheboygan, WI) partially
of these oviposition pans, singularly spaced at ?15-m
intervals in sheltered sites near homes. Ovitraps were
set out during late afternoon and emptied the next
morning. In addition, egg rafts were harvested and
1Corresponding author, e-mail: email@example.com.
Control and Prevention, Fort Collins, CO 80521.
3Current address: Insectes et Maladies Infectieuses, Institut Pas-
teur, 75724 Paris, France.
0022-2585/07/0211Ð0214$04.00/0 ? 2007 Entomological Society of America
counted every 2 h during four nonconsecutive nights
each summer. We calculated a mean ? SE of egg raft
deposition per each 2-h interval through the night.
To record the oviposition activity of mosquitoes
remotely, a low-light (0.08-lux) monochrome charge-
coupled device (CCD) camera (BWC-724I with an
8-mm lens, Crest Electronics, Greensboro, NC) was
entire pan. An infrared illuminator (Scopus IRI-2000,
Rock House Products, Middletown, NY) was posi-
tioned nearby such that the water surface was illumi-
emitted by these infrared (IR)-light emitting diodes
(LEDs) is beyond the range visible by birds (Varela
et al. 1993) and mosquitoes (Allan 1994).
Video imagery was captured using a VHS extended
time recorder with date and time data on each frame,
and these were subsequently reviewed on a high-
were then reviewed and relevant events tabulated.
Because sunrise and sunset times vary daily, we con-
verted the local time associated with each recorded
as an interval after astral sunset. Although the species
of mosquito could not be assessed by these recorded
images, mosquitoes could reliably be distinguished
from other insects, and all such mosquitoes were con-
sidered to be Culex spp.
Beginning at sunset, the Þrst 5 min of each quarter
hour was designated as a representative sample, and
this constituted an observation period. Mosquitoes
entering the visual Þeld of the camera during that
period were counted. To derive an estimate repre-
senting the total number of mosquitoes approaching
the oviposition site, we combined the counts during
the four 5-min observation periods of each hour, and
number of mosquitoes that visited the oviposition site
during the night.
Host-Seeking Activity. To record the host-seeking
or “questing” behavior of mosquitoes remotely, we
used lard-can traps (Lepore et al. 2004), baited either
with a pigeon or a European starling. An internal
screen prevented mosquitoes from contacting the
bird. Traps were deployed near the tops of trees, 10Ð
12 m above the ground). A 0 lux waterproof IR-sen-
Amityville, NY) incorporating an array of forward
facing IR-LEDs was afÞxed to the end of a 1-m bar
attached to one end of a lard can trap. The light and
camera were positioned to illuminate and view the
exterior of the screen funnel at one end of the trap.
Electrical cables for powering the camera and for
transmitting images were secured onto the rope used
to suspend the trap. A VHS video recorder and mon-
itor were sited under protective cover in a nearby
residence. Videotapes were subsequently reviewed
visually at normal speed, and on a frame-by-frame
basis of particular scenes of interest, and each event
was correlated to the time-clock that was integrated
determined, mosquitoes could reliably be distin-
guished from other insects.
To assess the species composition of mosquitoes
that entered the trap, we recorded questing behavior
traps. Mosquitoes were harvested from these cham-
where they were identiÞed according to morpholog-
ical characters and their abundance tabulated. We
calculated the mean number of mosquitoes (? stan-
dard error) entering the trap during each two hour
interval across the period of observation. In the Þrst
the weather was cold and unsettled. In the second
year, observations were conducted in early August
during a period of warm weather.
First, we recorded when mosquitoes ßew in the
vicinity of pans of fermented hay infusion. Activity
was recorded remotely on videotape by means of an
infrared illuminated imaging camera for ten noncon-
secutive nights. Although we observed a total of 3,096
mosquitoes ßying above the oviposition pan, some
proportion of these likely were repeated over-ßights
by the same mosquitoes. Maximum activity occurred
during the second hour after sunset (Fig. 1). Mosqui-
toes traverse potential oviposition sites most fre-
quently ?2 h after sunset.
We then determined when the characteristic egg
rafts of Culex mosquitoes were deposited on hay in-
fusion contained in plastic oviposition pans. During
deposited. More than half of the egg rafts deposited
during the entire night were laid during the Þrst 2 h
after astral sunset, and about two-thirds of the rafts
were recorded during the Þrst 4 h (Fig. 2). The eggs
of Culex mosquitoes generally are laid at ?2 h after
We determined when Culex mosquitoes approach
avian hosts during the early fall (year 1) when local
Error bars represent the standard error of the mean.
Mean number of mosquitoes traversing the sur-
212JOURNAL OF MEDICAL ENTOMOLOGY
Vol. 44, no. 2
when these mosquitoes are most abundant (year 2)
(Spielman 2001). Mosquitoes were detected ßying
near the trap openings 783 times, and the traps sam-
pled 268 times. Although mosquitoes were active
throughout the night during the Þrst year of observa-
tion (Fig. 3a), nearly two-thirds of all observed mos-
sunset (Fig. 3b) during the following year. Noctur-
nally active mosquitoes seem to approach avian hosts
?2 h after sunset during midsummer but more op-
portunistically during mid- to late fall.
We then determined when mosquitoes attain close
the collecting chambers of bird-baited lard-can traps.
In total, 93 mosquitoes were collected. The collection
two-thirds of these mosquitoes entered the collecting
chambers during the Þrst 2 h after astral sunset, and
nine-tenths during the Þrst 4 h (Fig. 4). These Culex
mosquitoes generally attain close contact with avian
hosts during the Þrst few hours after sunset.
epizootic of WNV has stimulated many North Amer-
ican communities to apply adulticidal aerosols in at-
tempts to lessen the force of transmission and risk of
infection. Because mosquitoes would be most suscep-
tible to adulticidal aerosols while in ßight, “. . . it is
imperative that spraying take place when Culex mos-
quitoesÑthe primary target mosquitoesÑare most
active. This means spraying between dusk and dawn”
(New York State Department of Health 2001). Effec-
tive adulticidal interventions against WNV require in
depth knowledge concerning when and why they ßy
Cx. pipiens and Cx. restuans mosquitoes are said to
activity that precedes mating occurs solely during
early evening (Nielsen and Haeger 1960). Although
they seek nectar most avidly at midnight, some of
these mosquitoes engage in this activity throughout
most actively during the Þrst 3 h after sunset (Mac-
Donald et al. 1981). Nocturnally active Culex mosqui-
toes quest throughout night, after a monophasic, cre-
quitoes, expressed as the proportion of the total egg rafts
deposited on the surface of hay infusion during four con-
Nocturnal oviposition activity of Culex spp. mos-
proaching a bird, expressed as the percentage of the total
number of mosquitoes traversing the entrance of a bird-
baited trap during four nights of observation during (a)
September of year 1 and (b) August of year 2. Error bars
represent the standard error of the mean.
Nocturnal questing activity of mosquitoes ap-
mosquitoes collected from bird-baited traps captured on
1 and 2. Error bars represent the standard error of the mean.
Proportion of the total number of Culex spp.
March 2007REDDY ET AL.: EARLY EVENING Culex ACTIVITY
(Service 1971). They seem to ascend slowly into the Download full-text
of each night (Dabrowska-Prot 1966). These Culex
mosquitoes seem to quest for hosts and oviposit con-
temporaneously, and mainly during the crepuscular
hours, including several hours after sunset as well as a
more transient episode early in the morning.
mainly support the zoonotic cycle of West Nile virus
in New England, approach prospective oviposition
sites at about the same time that females actually
repeatedly before ovipositing, as suggested by the
large disparity between the number of traversing
ßights that we observed and the number of egg rafts
discovered on the surface of the water. Our observa-
quitoes oviposit early in the night (MacDonald et al.
Mosquitoes seemingly approach the entrance of a
bird-baited trap more often than would be suggested
by examining the actual number trapped. The ap-
ßy in the vicinity of a trap in a tentative manner,
abundance of mosquitoes observed in ßight as op-
quitoes seem to enter bird-baited traps at about the
same time that they ßy in the vicinity of these traps
(Service 1971), and our videotaped observations as
well as our hourly assessments of actual overnight
collections conÞrm that Culex mosquitoes quest for
avian hosts most actively soon after astral sunset.
depend on precise understanding of the diel period-
icity of the mosquitoes that transmit the West Nile
virus. Peak host-seeking and oviposition-related ßight
activities of these Culex mosquitoes correspond
closely, at ?2 h after sunset. Applications of adulti-
cides against the Culex mosquitoes that serve as the
main vectors of West Nile virus in the northeastern
United States thus would seem most efÞcacious if
delivered during these peak ßight periods.
The authors gratefully acknowledge: Nicole Arrigo,
Patrick Barton, Adam Bemis, Andrew Broadbent, Peter
Dodd, Henry Goldfarb, Annette Lee, Sejal Shah and Nicole
This research was supported by grants RO1AI 52284 and
RO1AI 44064 provided by the National Institute of Allergy
and Infectious Disease of the National Institutes of Health
Disease Control and Prevention.
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Received 26 March 2006; accepted 31 October 2006.
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