Bulletin 827 (technical)
Bionomics and Physiology of
Culex nigripalpus (Diptera:Culicidae)
of Florida: An Important
i..A.. Uni J. K.of Fayar
i.F.A.S.-Univ. of Flon d-
Florida Agricultural Experiment Stations
Institute of Food and Agricultural Sciences
University of Florida, Gainesville
F. A. Wood, Dean for Research
ON THE COVER: Female Culex nigripalpus feeding on blood from thumb of the author.
Bionomics and Physiology of Culex nigripalpus
(Diptera: Culicidae) of Florida: An Important
Vector of Diseases
J. K. Nayar
Dr. Nayar is Adjunct Associate Professor, Department of Entomology and Ne-
matology; Affiliate Associate Professor, Department of Preventive Medicine,
College of Veterinary Medicine; and Entomologist, Florida Medical Entomol-
ogy Laboratory, Institute of Food and Agricultural Sciences, University of Flor-
ida, Vero Beach.
Culex nigripalpus is an important vector of diseases of man (St. Louis en-
cephalitis SLE), horses (Eastern equine encephalitis EEE), dogs (dog
heartworm Dirofilaria immitis), wild turkeys (wild turkey malaria Plas-
modium hermani and Avian poxvirus of wild turkeys) and is a major pest of
livestock in rural south-central Florida. It is essentially a tropical species, and
is present in the southern United States, Mexico, Central America, the northern
part of the South American continent, and in the Caribbean Islands. However,
it is abundant only in south-central Florida and in some of the Caribbean Islands.
In the tropics, where temperature is not the limiting factor during the breeding
season, rainfall usually governs the abundance of this mosquito species. All as-
pects of the life cycle of Cx. nigripalpus are governed by rainfall. The number.
of broods and generations of this multivoltine species vary each year depending
on the frequency of rainfall and/or artificial flooding. As many as 15 broods can
be recorded, reflecting at least 8 to 10 generations a year. The main breeding
season of Cx. nigripalpus is from June to November, with smaller populations
occurring from December to May.
Eggs are laid in a wide range of aquatic habitats a day or two after rainfall or
flooding. Decaying vegetative matter in the standing water emits certain attrac-
tant odors, such as those from hay-infusion. These eggs hatch within 2 days dur-
ing the main breeding season in south-central Florida. Larval development
under daily light-dark cycles culminates in pupation peaks, which are approxi-
mately 24 hours apart. The duration of larval development varies from 5 to 8
days and is affected mainly by the amount of available food, the salinity of the
water, larval density, and temperature. All of these are influenced by the fre-
quency of rainfall. The pupal stage lasts for 3 to 4 days, depending on tempera-
ture. These pupation peaks occur during the daylight hours and are followed by
emergence peaks 2 to 3 days later during the early part of the night.
The energy reserves status of an adult at emergence is determined by richness
of larval diet in the larval habitat, which is dependent on the amount of rainfall.
Upon emergence, the adults move to woodland areas. At first their peaks of
flight activity occur at sunset and sunrise. Then flight activity increases
throughout the night as they become older. These adults have a good potential
for dispersal, but seldom move out of their habitat readily. They prefer densely
wooded areas of high humidity, and will disperse when the relative humidity is
very high (usually 95% to 100%), such as after a late afternoon or early evening
rainfall. At night, adults can disperse up to 2 km, but during the day they rest in
shady woodland areas where the relative humidity is usually high.
Both sexes of newly emerged Cx. nigripalpus feed extensively on nectar. Fe-
males blood-feed in a distinct peak 2 to 3 days after emergence with a second
peak occurring 5 to 6 days later. Insemination begins 2 days after emergence and
virtually all the females of a brood are inseminated by the 5th day. Since these
females blood-feed 2 to 3 days after emergence, it appears that insemination is
not a prerequisite for blood-feeding as is sugar-feeding. Cx. nigripalpus has an
annual shift in its blood-feeding pattern, feeding mainly on avian hosts in the
winter and spring, and then shifting to an equal or greater proportion of feeding
on mammals in the summer and fall. This shift in feeding is probably influenced
by the afternoon rain showers during the summer and fall that cause Cx. nigri-
palpus to leave its wooded habitat and invade the adjacent open areas where
mammalian hosts are more likely to be found. Multiple blood-feeding is rare
during the summer and early fall, but is commonly observed in the winter and
early spring. Parous females appear 6 to 7 days after emergence.
Under certain conditions, Cx. nigripalpus has a tendency for autogenous egg
development. But in such females, fully developed eggs are seen only rarely.
Instead, the partially developed follicles are either resorbed or they degenerate.
Females that have ovaries with resorbed follicles are found year-round, but are
more common in the winter and spring. This is influenced by the frequency of
rainfall since females actively feed on blood sources at regular intervals during
the rainy season. This results in most of the blood-seeking females being either
nulliparous or parous, with only a very small percentage having resorbed ovarian
The daily survival rate of females varies from 65% in the dry season to 80%
or above in the rainy season. A higher daily survival rate and a greater proportion
of the females blood-feeding and ovipositing regularly results in a several-fold
increase in the population. In general, longer survival of females leads to a larger
number of infective mosquitoes in the population, since most viruses and para-
sites require 12 to 15 days after ingestion to reach the infective stage. All epi-
demics are associated with the presence of large populations of adult mosquitoes.
The chances of an epidemic are greatly increased when viruses and parasites are
in circulation and frequent rainfall increases the mosquito population. A com-
bination of environmental factors, especially frequent rain showers that increase
the population of Cx. nigripalpus along with the circulation of the viruses or
parasites, would make it a very good vector. This might explain the sporadic
occurrence of SLE and EEE viruses in Florida, both of which are associated
with Cx. nigripalpus.
Introduction ............. ..............................
Nomenclature ................. ............................ 2
Geographic Distribution .. ................... ............... 3
Distinguishing Characteristics ............................... 5
Diagnostic Characteristics of Different Stages ofCulex nigripalpus .... 5
Egg .................................................. 5
Larva and Pupa ....................................... 5
Adult ........................................ 6
Materials and Methods .................. ................... .. 8
Strains ofCulex nigripalpus Used ....................... . 8
Egg Collection and Egg Hatching ...................... .... 8
Larval Rearing .......................................... 9
Larval Marking with Radionuclide H3 32P 04 ............. ...... 9
Sampling Method for Larvae and Pupae .......................... 9
Sampling Methods for Adults ................................ 9
Experiments ............................................. 11
Broods and Generations ................ ................... 12
Egg .......... ........................................ 14
Focal Distribution ............................. ............. 14
Seasonal Occurrence and Associated Species ..................... 14
Embryogenesis and Egg Hatching ........................... .. 14
Larva and Pupa ............................................ 16
Focal Distribution .............. ............................. 16
Seasonal Occurrence .................................... 16
Associated Species ................ ....................... 16
Larval and Pupal Development ............................. .19
Larval Aggregations ................ ..................... 22
Adult .............. ............ ............... 23
Effects of Larval Nurture on Adult Characteristics at
Emergence and Survival Without Further Nourishment .............. 23
Focal Distribution .................... ............... 26
Seasonal Occurrence ................ ................. . 26
Associated Species ................. .. ................. 28
Copulation and Insemination ............................. . 29
Flight Activity ........................................ 30
Dispersal .................... ...... ........... 33
Feeding and Metabolism ............................... . 34
Nectar- and Sugar-feeding (carbohydrate-feeding) ............... 34
Metabolism of Carbohydrates ............................. 35
Blood-feeding and Metabolism of Proteins ..................... 35
Ovarian Development ........... ......... ... ............ 41
Oviposition ............ .... ......................... 47
Longevity .............................................. 50
Correlation of the Daily Survival Rates with Population on
Density and Vector Potentials .......................... .. 52
Vector Relationships ...................... ............. 56
V iruses .......... ............. ....... ............ 56
Malarial Parasites ................. .................. 58
Filarial Parasites ................ ...... .... ......... ... 58
Avian Poxvirus ........... .......... ................... 58
Antagonists and Potential Biological Control Agents ................ 60
Larvae .................. .......... .................. 60
Adult........... .......... .......................... 60
Resistance to Insecticides ................... ......... . 60
Control ofCx. nigripalpus Populations ................... . ...... 61
Control of Larvae ........... .......... .... ............ 61
Control of Adult ........... .......... . ... ............. 62
References Cited ............ ............................ 63
Culex nigripalpus Theobald belongs to the subgenus (Culex) of the genus
Culex, the tribe Culicini of the subfamily Culicinae in the family Culicidae, and
the dipteran suborder Nematocera. Formerly believed to be of little medical or
veterinary importance, it was proven to be the primary vector of St. Louis en-
cephalitis (SLE) during the 1961 and 1962 epidemic in the Tampa Bay area
(Chamberlain et al. 1964, Dow et al. 1964, Lewis et al. 1964 and Sudia &
Chamberlain 1964). This species is still considered the primary vector of SLE
virus in Florida and has also been implicated in the transmission of veterinary
and wildlife diseases dog heartworm, Dirofilaria immitis (Nayar & Sauer-
man 1975d), turkey malaria, Plasmodium hermani (Forrester et al. 1980, Young
et al. 1977), and Avian poxvirus (Akey et al. 1981). During the past 2 decades,
extensive laboratory and field studies on the bionomics and physiology of Cx.
nigripalpus have been conducted at the Florida Medical Entomology Laboratory
(FMEL), Vero Beach, Florida, including an in-depth physio-ecological study
conducted from 1975-1979 at the Tiger Hammock study area near FMEL (Nayar
et al. 1980). In an early report, Provost (1963) suggested that the biology of Cx.
nigripalpus should not be expected to differ markedly from that of any other
closely related Culex species. However, the more we have studied this species,
both in the laboratory and in the field, the more we have realized its differences
from the other mosquitoes belonging to the genus Culex. Provost (1969), in an
article entitled "The Natural History of Culex nigripalpus, stated that an un-
usual biological trait of the female is that an increase in the relative humidity
produced an increase in her general flight behavior and all associated physiolog-
ical activities. Since Cx. nigripalpus has been successfully colonized in the lab-
oratory (Haeger & O'Meara 1970), it has become the subject of much more
research, and it now seems that there is a need to review previous research on
this species. Therefore, this report is based on data and observations accumu-
lated by my group and other staff members of FMEL over the last two decades
of work with this species and other species of Florida mosquitoes, as well as
related work reported by others elsewhere. It is the purpose of this report to pro-
vide an organized source of information on this mosquito for use by both Public
Health Medical Entomologists and Mosquito Control personnel.
Bionomial taxon and synonyms: Culex nigripalpus Theobald 1901. (See
Knight & Stone 1977).
Culex nigripalpus Theobald 1901: (6, 9). Type-loc: St. Lucia Island, [Lesser
Antilles] (?NE). Dodge 1945: (L). Lane 1953: (6, 9, p, L). Carpen-
ter and LaCasse 1955: (6, 9, L). Bram 1967: (6, L; syn.). Belkin,
Heinemann, and Page 1970: (6, 9, L).
palus Theobald 1903: (6, 9). Type-loc: Barbados, [Lesser Antilles] (BM).
Belkin 1968: (lectotype desig.).
similis Theobald 1903: (9). Type-loc: Red Hills, Kingston, [Surrey], Ja-
biocellatus Theobald 1903: (9;flavipes var.). Type-loc: Trinidad (BM).
Stone 1956 (1957): (tax.). Stone, Knight, and Starcke 1959: (syn.).
Belkin 1968: (tax.).
microsquamosus Theobald 1905: (6, 9, L). Type-loc: Rio Corbre Canal
Dam, near Spanish Town, [Middlesex], Jamaica (BM). Belkin 1968:
,i.-., i;, ia.., Dyar and Knab 1906: (L). Type-loc: Zent, Costa Rica (NE).
carmodyae Dyar and Knab 1906: (L). Type-loc: [San Francisco Mines],
Santo Domingo, [Dominican Republic] (USNM).
factor Dyar and Knab 1906: (L). Type-loc: Tehuantepec, [Oaxaca], Mexico
,.:. ...t.... Dyar and Knab 1906: (L). Type-loc: Santo Domingo, [Domini-
can Republic] (USNM).
microannulata Theobald 1907: (d; Trichopronomyia). Type-loc: Stanley
Town, New Amsterdam, [Berbice], British Guiana (BM).
proximus Dyar and Knab 1909: (6). Type-loc: [Taboga Island], Panama
caraibeus Howard, Dyar, and Knab 1912 (1913), fig. 333: (9, L). Type-
loc: Barbados, [Lesser Antilles] (USNM).
prasinopleurus Martini 1914: (6, 9). Type-loc: Santiago de Cuba, [Ori-
ente], Cuba (BM).
azuayus Levi-Castillo 1954: (6, 9, p, L). Type-loc: Zhurucuchu, Azuay,
Ecuador (?USNM). Bram 1967: (syn.; info. on ?type).
Unless indicated by area or name, the binomial taxon, Cx. nigripalpus, is im-
plied in this report.
Cx. nigripalpus is essentially a tropical species occurring from the southern
United States southward to Brazil (Fig. 1). In the southern United States, it has
been recorded in: Alabama, Florida, Georgia (Carpenter & Chamberlain 1946,
King et al. 1944), Louisiana (King et al. 1944), Mississippi (Young & Chris-
topher 1944, Peterson & Smith 1945, Carpenter & Chamberlain 1946), North
Carolina (Carpenter et al. 1945), South Carolina (Carpenter & Chamberlain
1946, Weathersbee & Arnold 1947), Tennessee (Middlekauff & Carpenter
1944, Shlaifer & Harding 1946), and Texas (McGregor & Eads 1943, Rueger
& Druce 1950). Despite its rather wide distribution, Cx. nigripalpus is abundant
only in south and central Florida including the Keys. Throughout the remaining
southeast, it is usually of scattered or rare occurrence (King et al. 1960). It has
been recorded in all Central American countries and in the northern parts of
South America; e.g., Mexico (Palacios 1952), Ecuador, Panama, Colombia,
Venezuela, Guianas, Brazil, and Paraguay (Knight & Stone 1977). In the Car-
ibbean Islands, it has been recorded in the Greater Antilles, the Bahama Islands
(Porter 1967), the Lesser Antilles (Belkin et al. 1968), and Trinidad (Stone 1956
(1957)), but has not been reported in the Virgin Islands (Porter 1967, Belkin et
Since most of the research on Cx. nigripalpus has been conducted in Florida
at FMEL and at the West Florida Arthropod Research Laboratory, Panama
City, any discussion of biology and physiology will relate to the Florida strain
with an occasional mention of other strains.
Figure 1. Geographical Distribution of Culex nigripalpus Theobald. Dots represent
countries or states where Cx. nigripalpus has been recorded. In Florida, four dots are
used because of its abundance in south and central Florida.
Mosquitoes are usually collected as adults or as larvae, and it is therefore nec-
essary to be able to identify both adults and larvae to determine their relative
abundance in any particular habitat. In south and central Florida, Cx. nigripal-
pus occurs along with three other members of its subgenus Culex (Culex),
namely Culex salinarius Coq., Culex quinquefasciatus Say, and Culex restuans
Theobald. In the Keys it occurs with Culex bahamensis D. & K. and in north
Florida it occurs with Culex tarsalis Coq. and Culex territans Wlk (King et al.
1960). In the Bahamas, Cx. nigripalpus occurs with the closely related but less
known Culex (Culex) scimitar (Branch & Seabrook 1959). The adults of these
species can be identified macroscopically by their general coloration; Cx. nigri-
palpus is pale, without coloring, Cx. salinarius is golden red, Cx. quinquefas-
ciatus is straw yellow on the scutum, Cx. restuans is reddish, and Cx. bahamensis
has white banded tarsi (Provost 1969).
Larvae and adults of these can be further distinguished microscopically by
the diagnostic characteristics (cf. Dodge 1963, King et al. 1960).
DIAGNOSTIC CHARACTERISTICS OF DIFFERENT STAGES OF
Cx. nigripalpus eggs are laid in a wide range of aquatic habitats. The females
lay their eggs in rafts on the surface of water. Each egg raft contains ca. 200
eggs glued together in a definite pattern. The egg raft is whitish-green in color
when freshly laid, but changes to dark black within 1 to 2 hours. Macroscopi-
cally, they are indistinguishable from the egg rafts of other Culex species.
Larva and Pupa
Since the egg rafts of different Culex species are indistinguishable from one
another, an early identification of the larvae has been sought. The following
characteristics distinguish the first instar larvae of the Culex species (Dodge
1966, modified by Haeger and Evans, Personal communication).
1. Dorsum of fourth abdominal segment devoid of pigment, first segment
easily distinguished from the others when larvae are 3 to 4 hours old, older
larvae show an intersegmental band of dark pigment on segments three
and five; siphon ratio 4:1.; lateral hair on anal segment single and 3 terminal
spines of antennae subequal to shaft ................... nigripalpus
Dorsum of all segments evenly pigmented in larvae less than one day
old ................................... ........... ... .2
2. Egg breaker surrounded in front by a clear, unpigmented area in chitin of
head capsule; antennal tuft usually bifid and head hairs B, C, & D ar-
ranged in a transverse line .............................. restuans
Egg breaker without a clear area ............... ............... 3
3. Siphon ratio 4:1 or more, its primary ring of sclerotization covering close
to 1/2 of length; larvae 24 hours old or older have 3/4 of siphon sclerotized;
3 terminal spines of antennae longer than shaft; lateral hair of anal segment
usually bifid ........... .. ... .. ............... salinarius
Siphon ratio 3:1; 3 terminal spines of antennae much shorter than shaft;
lateral hair of anal segment and antennal hair tuft bifid; primary ring of
sclerotization covering /3 or less of length .......... quinquefasciatus
The fourth instar Cx. nigripalpus larval can be identified by certain physical
features (cf. Carpenter and LaCasse 1955).
Antenna shorter than the head, constricted beyond antennal tuft, with the
part before constriction pale and spiculate and the part beyond constriction
darker and with few spicules. Antennal tuft large, multiple, barbed, inserted at
outer third of shaft and reaching well beyond tip. Head hairs: postclypeal 4
short, single; upper frontal 5 and lower frontal 6 usually 3-branched, barbed,
extending beyond preclypeus; preantennal 7 long, multiple, barbed. Prothora-
cic hairs: 1 to 3 long, single; 4 long usually double; 5 to 6 long, single: 7 long,
2- to 4-branched. Thorax densely clothed with fine spicules. Lateral abdominal
hair 6 usually 3- or 4-branched on segments 1 and II, usually double on Ill
through VI. Comb of eighth segment with many scales in a patch; individual
scale rounded apically and fringed with subequal spinules. Siphonal index 6.0
to 7.0; pecten of about 9 to 15 teeth on basal fourth of siphon; individual tooth
with 2 to 6 coarse teeth on one side; usually four paired siphonal tufts inserted
beyond pecten; proximal tuft usually double, occasionally single, as long or
longer than the basal diameter of the siphon; second and third tufts usually dou-
ble or triple and inserted somewhat laterally; distal tuft small, single to triple.
Anal segment completely ringed by the saddle, with coarse spicules present on
dorsoapical surface; lateral hair usually single, sometimes double, usually a
little shorter than the saddle; dorsal brush bilaterally consisting of a long lower
caudal hair and an upper caudal tuft of three hairs, (one long and two short);
ventral brush well developed, confined to the barred area; gills 1 to 3 times as
long as the saddle, bluntly pointed.
The Cx. nigripalpus pupa can be recognized as follows: (cf. Belkin et al.
1970). Chaetotaxy apparently variable. Trumpet not flared, pinna small. Me-
dian caudal part of abdominal sternite II with short, sharp spicules. Abdominal
hair 5-IV usually with 4 branches.
The adult Cx. nigripalpus has certain distinguishing features (cf. Belkin et al.
1970, Carpenter & LaCasse 1955).
Female: Medium-sized species. Head: Proboscis dark-scaled, usually paler
underneath on basal half; palpi short, dark. Occiput with pale golden-brown
scales that are narrow and curved, and dark erect forked scales dorsally, with a
patch of broad dingy-white scales laterally. Thorax: Integument of scutum
brown; scutum clothed with fine dark bronze-brown scales. Scutellum with
brown setae and fine dark bronze-brown scales on the lobes. Pleura with few or
no scales, rarely more than 5 or 6 scales in any single group. Abdomen: Tergites
clothed with dark-brown to black scales with bronze to metallic blue-green re-
flection; narrow white basal bands occasionally present on some segments;
basolateral white-scaled patches present. Venter pale-scaled. Legs: Legs dark-
scaled with bronze to metallic blue-green reflection; posterior surface of femora
and tibiae pale. Wing: Length 3.0 to 3.5 mm. Scales narrow, dark.
Male: Coloration similar to that of the female. Labium with false joint before
the middle, surrounded by numerous long setae. Palpus exceeding proboscis
from about the basal third of segment 4; ventral surface with small patches of
whitish scales at base and apex of segment 4 and a few on base of segment 5;
bristles more numerous and longer on segment 3, extending from about middle
of segment. Abdominal tergites with very indistinct dingy pale transverse bands
sometimes developed, segments 5, 6 with more distinct basolateral dingy white
Male Genitalia: Terminalia, eighth tergite bearing many short stout setae.
Lobes of ninth, tergite broadly rounded, separated by a deep emargination about
the width of one lobe, each lobe bearing many slender setae. Tenth sternite
densely crowned with short spines, the apical ones pointed, the outer ones blunt;
basal arm long, stout, strongly curved, sclerotized. Phallosome consists of two
large sclerotized plates. Each plate bears a long pointed basal dorsal arm, not
bent at a right angle; a long pointed basal process nearly as large as the dorsal
arm; and a stout curved ventral arm, finely denticulate on outer surface beyond
middle; between the dorsal and ventral arms arise 4 strong teeth. Claspette ab-
sent. Basistyle about 212 times as long as the mid-width, clothed with many
setae, much longer on outer aspect. Subapical lobe undivided, bearing three long
strong rods hooked at tips, a large broad leaflike filament, and a strong seta.
Dististyle about half as long as the basistyle, bearing one or two small setae on
inner surface before apex; claw short, blunt.
MATERIALS AND METHODS
STRAINS OF CX. NIGRIPALPUS USED
Experiments conducted in the field were performed either with different
stages of wild population near Vero Beach, Florida, or with F1 progeny from
field-collected blood-fed females. Different stages needed were reared in the
laboratory under conditions prescribed by the experiments.
Experiments conducted in the laboratory were performed either with F, prog-
eny from field-collected blood-fed females or from the colonized strain estab-
lished by J. S. Haeger of FMEL, Vero Beach. During the colonization of this
species he was able to obtain a significant level of insemination by introducing
colonized Ae. taeniorhynchus adults into the same cage. After several genera-
tions this extra interspecific stimulation was no longer necessary (Haeger, un-
published). Haeger and O'Meara (1970) later incorporated wild genes in this
colony by introducing Fi Cx. nigripalpus males into cages with colonized Cx.
nigripalpus females at irregular intervals and obtained an insemination rate of
approximately 75%, but when F, females were similarly introduced to either Fi
or colonized males, the rate of insemination was less than 2%.
EGG COLLECTION AND EGG HATCHING
In the field, eggs of Cx. nigripalpus can be collected in artificial pools or
containers (Smith & Jones 1972, Lowe et al. 1974). Smith & Jones constructed
artificial pools by stapling black plastic material to a wooden frame (30 inches
long, 18 inches wide, and 3 inches deep or 75 by 45 by 7.5 cm.). These ar-
tificial pools were then imbedded so that the top of the frame was level with the
ground in shady areas near a large collection of water. Of the egg rafts that were
laid, only 2.5% failed to hatch, and 89% of those that did hatch were Cx. nigri-
palpus and the remainder were other Culex species. Those pools, which con-
tained either plain water or infusion, yielded similar numbers of egg rafts. When
the pools contained crushed 40% hog supplement at a rate of approximately 8 g/
400 ml of water, 3 times as many eggs were laid. Other investigators have used
tubs of approximately the same size and obtained similar results (Lowe et al.
In experiments conducted at Tiger Hammock during 1978 and 1979, egg rafts
were collected in black tubs (60 cm x 45 cm x 30 cm) containing hay infusion
fortified with brewer's yeast.
In the laboratory, simultaneous egg hatch was achieved by allowing blood-
fed females of wild or colonized strains to oviposit in a dish containing hay in-
fusion placed into their cages for 1 hour just before light-off. The egg rafts were
removed from the oviposition dish, placed on filter paper squares on top of moist
cellucotton in small plastic boxes, and maintained at predetermined tempera-
tures (Nayar 1968a). By varying the temperature, embryogenesis was prolonged
and hatching delayed, and the use of moist filter paper further forestalled hatch-
ing for 1 to 2 hours after the embryonic development was completed. When the
rafts were then placed into water, a simultaneous hatching occurred. Thus hatch-
ing could be obtained that was both simultaneous and at a predetermined time
and newly hatched larvae could be reared to the adult stage and used for both
laboratory and field experiments (Nayar et al. 1979, 1980).
In the field, most mosquito larvae feed on microscopic fauna, such as bacte-
ria, yeast, algae, and protozoa that are associated with decaying plant materials,
e.g., hay and leaf litter. In the laboratory, larvae of different Florida mosquito
species can be reared on either a ration of dry brewer's yeast supplemented with
liver powder, or a mixture (1:1) of dry brewer's yeast and lactalbumin (Nayar
1967, 1968a, Nayar & Sauerman 1970a, Nayar et al. 1979, 1980).
LARVAL MARKING WITH RADIONUCLIDE H332 04
Radionuclides have been used to mark mosquitoes during the larval stage to
study the dispersal and energetic of adult mosquitoes (Service 1976). Dow
(1971) marked Cx. nigripalpus by feeding the larvae during the entire 4th instar
with H332P04 in HCI at concentrations of either 0.5 of 0.3 /Ci/mL of tagging
medium. The newly emerged adults exhibited diverse radioactive counts, and
the females had damaged ovaries. Nayar et al. (1979) improved this technique
by allowing synchronously reared larvae, at 10 to 12 hours after reaching 4th
instar, to feed for 16 hours on the radionuclide H332PO4 at concentrations of
either 0.5, 0.25 or 0.125 /Ci/mL of tagging medium. After the 16-hour period,
the radioactive counts per larva averaged 28,062 counts per minute (cpm),
12,213 cpm, and 6,802 cpm for the three respective concentrations of 32. Dur-
ing later mark-release-recapture experiments, early 4th instar larvae were marked
with H332PO4 for 18 to 24 hours at a concentration of 0.4 tCi/ml. The following
radioactive counts were recorded: 15,795 + 1268 cpm per larva, 12,045 1132
cpm per pupa, 9,820 543 cpm per male and 11,645 725 cpm per female.
The subsequent 50% survival time on distilled water was 52 to 64 hours for
males and 56 to 68 hours for females and on 10% sucrose, this period was 32 to
46 days for males and 57 to 75 days for females. These survival times were iden-
tical to those of unmarked adults reared in a similar manner.
SAMPLING METHOD FOR LARVAE AND PUPAE
In the field, larvae and pupae were collected with a standard dipper.
SAMPLING METHODS FOR ADULTS
All sampling methods for adult mosquitoes are subject to bias. Bidlingmayer
(1967) and Service (1976) have summarized different sampling methods used
The sampling techniques may be divided into two major classes: those that
assess resting adults during their inactive periods, and those that require the
mosquitoes to be in voluntary flight to effect its capture.
Resting adults are rarely affected by meteorological conditions, whereas
those in flight are affected not only by the current meteorological conditions but
also by their physiological need for sugar, blood, and oviposition sites. There-
fore, such influences can distort population estimates. The trapping methods
used to collect mosquitoes in flight may be either non-attractant or attractant.
Non-attractant traps presumably do not divert the mosquito from its normal
flight path prior to capture. However, due to either shape or motion, these traps
may unintentionally affect the mosquitoes' flight. Attractant traps rely upon a
positive response from the mosquito to an attractant source, although the per-
centage of the flying population that responds is unknown.
Different sampling methods have been successfully used to collect resting and
flying Cx. nigripalpus.
Vehicle mounted suction traps and portable battery-powered aspirators col-
lected adults in different physiological stages from the leaf litter (Bidlingmayer
& Hem 1973, Nayar 1978), including blood-engorged females (Bidlingmayer
& Edman 1967).
a) The non-attractant traps used for sampling Cx. nigripalpus were suction
traps and truck traps (Bidlingmayer 1967). A comparison of the two types
of traps shows that collections varied depending on meteorological condi-
tions, such as temperature, humidity, wind velocity, and moonlight phases
(cf. Flight Activity).
b) The attractant traps used for sampling Cx. nigripalpus employed either a
vertebrate host, or CO2 (dry-ice), or both a vertebrate host and C02, or
light as attractants. This study used three common attractant traps:
i) New Jersey traps, which are routinely used to collect night flying mos-
quitoes including Cx. nigripalpus (Bidlingmayer 1971, 1974).
ii) CDC light traps with or without dry-ice have been used extensively to
collect Cx. nigripalpus in Florida (Boike 1963, Dow 1971, Provost
iii) Lard-can bait traps with or without additional attractants such as
dry-ice, a bird, a bird and dry-ice, and other traps such as Lumsden,
Trinidad, and Magoon, which are successfully used for collecting
blood-seeking Cx. nigripalpus (Aitken et al. 1968, Nayar et al.
1980, Provost 1955, Vickery et al. 1966).
c) Human bait was also used for collecting mosquitoes (Provost 1955).
In collecting Cx. nigripalpus, a comparison was made between lard-can traps
with CO2 as an attractant, and CDC light traps with and without CO2 and light
as attractants. The results showed that the CDC trap without any attractant con-
sistently caught the least number of mosquitoes, and the COz and lard-can traps
caught fewer than the CDC traps with attractants. A collection ratio based on the
CDC trap without attractants (Table 1) showed that the superiority of CDC traps
with attractants over CO2 and lard-can traps remained approximately 2:1 from
October to May, but suddenly became 6:1 in June. However, due to uncertainty
in the operation of CDC light traps, lard-can traps with attractants such as CO2
and host (chick) are usually preferred for collecting blood-seeking females
(Nayar et al. 1980).
Therefore, depending on the aim of the study, there are several traps that may
be used to collect Cx. nigripalpus in Florida.
Table 1. Ratio of Culex nigripalpus collections to CDC trap with no attractant, i.e. with
fan only. A, = lard-can trap with one CO, block, A2 = same with two CO2 blocks, B =
CDC trap without attractant, C = CDC trap with light, D = CDC trap with light and one
Cycle Dates (1975-1976) A1 A2 B C D
8 10/05-11/02 3.77 6.21 1.00 5.97 10.60
9 11/03-12/02 2.97 3.38 1.00 4.72 5.50
11 01/01-01/30 4.29 4.14 1.00 5.71 7.00
12 01/31-02/29 9.33 9.67 1.00 12.33 12.67
1 03/01-03/29 2.80 3.70 1.00 5.20 10.10
2 03/30-04/28 3.05 3.10 1.00 6.20 8.25
3 04/29-05/28 2.40 2.87 1.00 5.48 4.91
4 05/29-06/26 1.03 .99 1.00 6.07 5.09
9tni. '"inter) 3.79 4.13 1.00 6.15 8.18
8 and 3 (fan ring) 2.81 3.77 1.00 5.65 6.44
4 (summer) 1.03 .99 1.00 6.03 5.09
Total 4 3.52 4.01 1.00 5.90 7.20
In the laboratory, all experiments were conducted under standard conditions,
which are defined in the text along with each experiment. However, in the field,
a major study was conducted from 1975 to 1979, reported herein, in the Tiger
Hammock study area to establish the time and sequence of different physio-
logical and ecological parameters. Details of the study area and experimental
design have been published elsewhere (Nayar et al. 1980). Some of the published
and most of the unpublished data are reported in this report, and will be referred
to as 1976 or 1978 release of marked Cx. nigripalpus in the Tiger Hammock
BROODS AND GENERATIONS
Ecological surveys in south and central Florida show that Cx. nigripalpus is
a multivoltine species, and it is possible to record 8 to 10 generations in a year
with as many as 15 broods. The number of broods and generations vary each
year according to the rainfall and/or artificial flooding of the breeding areas.
During the main breeding season (June through November 1978), a survey of
the eggs, larvae, pupae, and newly emerging adults that were present in a citrus
grove near Tiger Hammock study area revealed 9 distinct broods. Each occurred
after a rainfall heavy enough (25 to 125 mm) to cause water to stand in the swales
or one time after artificial irrigation by the owner of the citrus grove (Table 2).
The mean air temperature from June to early October was between 24.70C and
27.80C. The first pupae appeared 6 to 9 days after the rainfall or flooding and
pupation lasted for the next 4 to 8 days. This variation in the onset of pupal ec-
dysis and the duration of adult emergence could not be related to the amount of
rainfall (Table 2). The total duration of development time from flooding to adult
emergence varied from 12 to 15 days during June to the middle of October (Table
2), but during the latter part of October and the first part of November, the tem-
perature was lower and development was delayed.
Table 2. Number and durations of broods of Culex nigripalpus recorded in a citrus grove near Vero Beach, Florida in 1978.
Mean Duration of Brood
Rainfall Temperature Days Development
No. of Amount of Air When First Adult Emergence First Flooding to
Brood Date (mm) oC Pupae Observed Date Duration Emergence
1 6/21-22 47 27.8 8 6/30-7/3 4 12
2 7/2-4 121 25.9 9 7/12-15 4 13
3 7/20-21 60 25.9 8 8/2-7 6 14
4 7/31-8/2 125 27.1 7 8/14-21 8 15
5 9/3 25 25.5 7 9/15-19 5 12
6 9/18-25 Artificial 25.5 7 9/27-10/1 5 12
7 9/30-10/3 53 24.7 7 10/10-14 5 12
8 10/11 25 24.7 6 10/22-28 7 13
9 10/19 41 22.9 15 11/3-20 20 35
Cx. nigripalpus breeds in more or less permanent collections of water where
its eggs are laid. A more detailed account of its focal distribution is given under
'Larva and Pupa.'
SEASONAL OCCURRENCE AND ASSOCIATED SPECIES
Cx. nigripalpus eggs occur along with eggs of other Culex species. During a
1971-1972 study at Seashore Key (north Florida) egg rafts of Culex species were
collected and individually reared to 4th instar for species identification. This
resulted in the following proportions: 24.2% Cx. nigripalpus, 67.4% Cx. quin-
quefasciatus, 7.3% Cx. restuans, and 1.0% Cx. salinarius (Lowe et al. 1974).
Cx. quinquefasciatus was present throughout the entire collection period, but
was more abundant during the warmer months of May to August. Cx. restuans
was found only during the winter and spring months except for a single collection
in July, and Cx. salinarius was collected only in August 1971. Cx. nigripalpus
was collected mainly from September to December and only sporadically from
April through August (Lowe et al. 1974).
In experiments conducted at Tiger Hammock, egg rafts were collected from
August to November 1978, and May to June 1979, and were individually hatched
and identified during the first instar larva stage (cf. Identification of first instar
larvae). All of the egg rafts collected from August to November 1978 were Cx.
nigripalpus, but in May 1979, when fewer egg rafts were laid, 56.2% were Cx.
nigripalpus, 37.2% were Cx. quinquefasciatus, and 2.3% and 4.3% were Cx.
salinarius and Cx. restuans, respectively (Table 3). By the end of May to early
June, the population of Cx. nigripalpus had greatly increased and the total num-
ber of eggs collected was almost equivalent to those collected in summer (Table
3). At this time, 89% of the collected egg rafts were Cx. nigripalpus, 10.7%
were Cx. quinquefasciatus, 0.2% were Cx. salinarius, and no egg rafts of Cx.
restuans were present.
EMBRYOGENESIS AND EGG HATCHING
The eggs of Cx. nigripalpus do not undergo either quiesence or diapause, but
hatch immediately upon the completion of embryogenesis. Eggs laid in the lab-
oratory hatch after 18 to 20 hours at 350C, 20 to 21 hours at 320C, 27 to 28 hours
at 270C, 40 to 41 hours at 220C, 55 to 56 hours at 200C, and 122 to 126 hours
at 150C (Nayar 1968a). They do not hatch at 100C. This indicates that the du-
ration of embryogenesis is temperature dependent since the Q10 value is 2.0 for
a 100C range of temperature from 22 to 320C, and is much higher than 2.0 for
any 100C range lower than 220C. Therefore, in south and central Florida, eggs
laid in the field during the peak of the breeding season when the mean temper-
ature is between 24.70C and 27.80C would be expected to hatch within 2 days
Table 3. Egg rafts of Culex species collected in 18 black troughs in the Tiger Hammock
study area, Vero Beach.
Percentage of Different Species
Total No. Cx. Cx. quinque-
Date Collected Collected nigripalpus fasciatus Cx. salinarius Cx. restuans
9/1/78 3825 100 0 0 0
9/20-30/78 4378 100 0 0 0
10/10-20/78 4426 100 0 0 0
11/1-11/78 2401 100 0 0 0
5/1-12/79 254 56.2 37.2 2.3 4.3
5/29-6/9/79 3311 89.0 10.7 0.2 0
LARVA AND PUPA
Cx. nigripalpus larvae and pupae are found in ditches, grassy pools, catch
basins, tubs, crabholes near the bases of trees, permanent pools in swamps,
holes in the coral along the coastal littoral, pond ground pools, and beached
boats (Belkin et al. 1970, Horsfall 1955, Carpenter & LaCasse 1955, Provost
1969). Although not considered common in artificial containers, usually fre-
quented by Aedes aegypti, a 1963 container survey in Dade County found the
larvae of Cx. nigripalpus in 37 different classifications of containers, the most
common being buckets, tires, fish ponds, containers with cuttings, and pools
(Heidt 1964). In spite of the fact that Cx. nigripalpus larvae and pupae have been
found in highly polluted water in Puerto Rico (Root 1922); they generally are
uncommon in such habitats primarily favored by Cx. quinquefasciatus (Provost
1969). In addition to the habitats already stated, in Florida, Cx. nigripalpus lar-
vae also occur in waste waters dispersed by spray irrigation, from sewage or
citrus-processing plants, grove swales during periods of irrigation or rainfall,
and most recently are recorded from the phosphate pits in the Polk County area
among the matted plants, in fresh water saw-grass marshes and in bay-head for-
ests (Haeger 1979).
Larvae and pupae of Cx. nigripalpus are found throughout the year in the ex-
treme southern United States (Carpenter & LaCasse 1955). From June to No-
vember 1978 in and around the Tiger Hammock area, a biweekly survey of 16
sites (e.g., irrigation ditches, citrus grove swales, reservoir ponds, roadside
ditches, and pasture ditches) revealed the presence of larvae and pupae in 188 of
the total 738 collections. Very few larvae of any instar were present in the June
collection, but as the rainy season began and the citrus groves were irrigated,
the number of larvae increased, with a maximum of 47.01% collected in July
1978 (Table 4). The main breeding season in the Vero Beach area is from July to
November (Table 4), but larvae and pupae are also collected in smaller numbers
throughout the rest of the year. Table 5 shows that a maximum of 95.06 larvae
and pupae per dip were collected in July 1978 and a minimum of 2.3 larvae and
pupae per dip were collected in June 1978.
In the 1963 container survey in Dade County, the larvae and pupae of Cx.
nigripalpus were found breeding along with the larvae ofAe. aegypti, Cx. quin-
quefasciatus, Culex (Mel.) pilosus and Anopheles crucians (Heidt 1964).
Table 4. Number of Culex nigripalpus larvae and pupae collected in a biweekly sampling from 16 sites of different types in and around the Tiger
Hammock study area in 1978.
Month of Rainfall Sites Examined Larvae in Different Instars Total Larvae % Total
Collection mm Negative Positive % Positive 1 2 3 4 P Collected Collected
June 87 92 16 14.8 1 164 92 119 38 414 0.57
July 262 83 36 30.3 5424 9511 9677 8985 625 34,222 47.01
August 75 118 31 20.8 982 1532 2671 2780 451 8,416 11.56
September 107 70 26 27.1 1558 3390 5523 1774 101 12,346 16.96
October 150 126 45 26.3 2614 3472 1612 1368 771 9,837 13.51
November 89 61 34 35.8 71 919 872 3886 1816 7,564 10.39
Total 550 188 25.4 10,650 18,988 20,447 18,912 3802 72,799 100.00
Table 5. Average number of larvae and pupae of Culex nigripalpus collected per larval
dip per site during each month from June to November 1978.*
Average No. Collected Per Dip
Month of Each Instar All Instars
Collection 1 2 3 4 P Per Site/Day
June <1 1 <1 <1 <1 2.30
July 15.1 26.9 25.0 1.7 1.7 95.06
August 3.2 4.9 8.6 9.0 1.4 27.15
September 6.0 13.0 21.2 6.8 <1 47.48
October 5.8 7.7 3.6 3.0 1.7 21.86
November <1 3.7 2.6 11.4 5.3 22.25
*Ten standard dips were made biweekly at each of the 16 sites.
During 1978, in a field survey of the larvae and pupae present in the waste
waters dispersed by spray irrigation from sewage and/or food-processing plants,
Cx. nigripalpus larvae were found breeding with Cx. quinquefasciatus, Cx. sal-
inarius, Aedes vexans, and Psorophora columbiae.
In a biweekly survey of 16 sites (cf. Seasonal Occurrence), Cx. nigripalpus
larvae and pupae were found in association with different species during differ-
ent months of the year (Table 6). The other species most commonly found from
July to November were: Cx. quinquefasciatus, Ae. vexans, An. quadrimacula-
tus, Ps. columbiae, Ps. howardii, and Uranotaenia lowii (Table 6).
Haeger (1979) reported Cx. nigripalpus breeding along with other species in
a variety of habitats. Some examples are: in natural lakes, man-made ditches,
canals, and borrow pits, where water plants have become matted due to several
years of repeated freezing or herbiciding. Cx. nigripalpus larvae were found
breeding along with An. quadrimaculatus, An. crucians, Cx. salinarius, Cx.
(Mel.) opisthopus, Cx. (Mel.) pilosus, Mansonia spp. and Cq. perturbans. In
Table 6. Numbers of larvae and pupae of Culex nigripalpus and other mosquito species
breeding in 16 sites of different types in and around the Tiger Hammock area in 1978.
Species Jun. Jul. Aug. Sept. Oct. Nov.
Culex nigripalpus 395 365 3,555 14,789 2,866 127
Culex salinarius 45 1 44 0 0 0
Culex quinquefasciatus 43 0 570 50 94 0
Aedes vexans 3 106 391 0 0 14
Aedes sollicitans 2 0 0 0 0 0
Aedes atlanticus 54 62 0 0 36 0
quadrimaculatus 1 0 0 15 31 15
Anopheles crucians 0 9 0 9 9 0
Psorophora columbiae 782 107 478 248 13 12
Psorophoraferox 0 39 11 0 0 14
Psorophora ciliata 0 8 4 0 0 0
Psorophora howardii 0 5 107 28 18 4
Uranotaenia lowii 2 0 0 6 26 42
saw-grass marshes, Cx. nigripalpus larvae have been found breeding along with
An. quadrimaculatus, An. crucians, Cx. (Mel.) erraticus, Cx. (Mel.) pilosus,
Cx. salinarius, and Ps. columbiae. In bay-head forests, swamp forests, and cy-
press swamps, where the floor is irregular and often forms small pools after a
heavy rain, Cx. nigripalpus larvae have been found breeding along with Culiseta
melanura, Cx. (Mel.) peccator, Ae. atlanticus, Cx. (Mel.) opisthopus, Ae. in-
firmatus, Ps.ferox, Ps. howardii, Ur. sapphirina, An. crucians and An. quad-
LARVAL AND PUPAL DEVELOPMENT
In the laboratory when larvae of Cx. nigripalpus are reared on standard food
under a light-dark cycle of 12L: 12D, the pupal ecdysis exhibits a diurnal rhythm
(Fig. 2a-f), where the peaks of pupation occur approximately 24 hours apart and
are independent of the temperature. Studies on the growth and development of
Cx. nigripalpus larvae showed that ontogenetic timing, the rate of growth, en-
dogenous diurnal rhythm, and the synchronization of both pupation and emerg-
ence peaks were affected by nutrition, larval density, salinity of the rearing
medium, and temperature, as in other mosquito species (Nayar 1967, 1968a,
Nayar & Sauerman 1970a).
The following developmental patterns were observed when Cx. nigripalpus
larvae were reared on 2 different diets (basic ration BR, and 2XBR) at three
constant temperatures (220C, 270C, and 320C, Fig. 2). The onset of pupal ec-
dysis was found to be temperature dependent, 167, 102, and 78 hours after
hatching at 220C, 270C, and 320C, respectively (Fig. 2a, c, e) and had a Qo1
value of more than 2.0 for the 100C range from 220C to 320C (Nayar 1968a).
The number of pupation peaks that occurred was dependent on both the temper-
ature and the quantity of food. On a basic ration there were between three and
five peaks at each temperature (Fig. 2a, c, e), but when the ration was doubled,
a single major pupation peak occurred at 220C (Fig. 2b), and two major peaks
occurred at 270C and 320C (Fig. 2d, f). At higher temperatures these peaks had
smaller standard deviations (-1.52 hour and -2.59 hours at 320C and 270C,
respectively) than at the lower temperature (5.32 hours at 220C), where a
breakdown in the rhythm became evident (Fig. 2a). The duration of pupal ec-
dysis was mainly affected by the amount of available food. With a smaller quan-
tity of food, pupation lasted between 75 and 90 hours (Fig. 2a, c, e) whereas on
more food, pupation lasted only 46 to 52 hours (Fig. 2b, d, f) between 220C and
320C, thus indicating that the duration of pupal ecdysis was synchronized by the
larger quantity of food (Nayar 1968a). The diurnal peaks of pupation occurred
at different hours during the LD cycle depending on the temperature. At 22C
the pupation peaks were near the light-to-dark transition, 16h00m and 19h00m,
at 270C they were near the dark-to-light transition, between 05h00m and 10h00m,
and at 320C they culminated at midday between 10h00m and 13h00m.
72 t 96
t ONSET OF PUPIL ECDYSIS
BR BASIC RATION
2BR 2xBASIC RATION
/A.-- \ ^ ___^----- -_-
S144 168 192
HOURS AFTER HATCH
-- EMERGENCE PEAKS
'--'DURATION OF PUPIL ECDYSIS
Figure 2. Patterns of pupation and emergence in Culex nigripalpus as affected by two
levels of food at three constant temperatures. Ordinate: 2-hour running averages of num-
ber of pupae per hour. Modified from Nayar (1968a) and Nayar et al. (1978).
t I I
Larval development was also affected by the salinity of the rearing medium
and the larval density, when combined with variations in the available food. At
a lower salinity, the amount of food and larval density did not affect the onset of
pupal ecdysis (101 to 103 hours) but had a marked effect on its duration (47
hours on high food and 90 hours on low food and 75 larvae per pan compared to
75 hours on high food and 113 hours on low food and 200 larvae per pan, Table
7). A higher salinity of the rearing medium, with a basic ration, delayed the
onset of pupal ecdysis by 24 hours, but shortened its duration. The addition of
more food prevented such a delay in onset when the larval density was low but
had no effect at higher densities (Table 7).
The length of the pupal stage was only affected by temperature with a duration
of 57.0 hours at 220C, and 24.5 hours at 270C, and 27.5 hours and 320C, and a
Qio value of 2.1 for the 10C range from 220C to 320C (Nayar 1968a).
The emergence rhythm was dependent on the pupation rhythm and the inter-
val separating them was affected by the temperature but not the light regimes
(Nayar 1968a). Deviations from the larval LD 12:12 light cycles, imposed dur-
ing the pupal stage shifted both the onset and the mean time of the emergence
peak 1 to 3 hours (Nayar et al. 1978). This slight shift was not due to entrain-
ment, but was a result of the light cycle affecting the duration of pupal develop-
ment, and hence, the timing of the emergence peaks.
Unlike Ae. taeniorhynchus (Nayar 1967, Provost 1960), the male to female
ratio was invariably 50:50 during each peak, showing that the onset of both pupal
ecdysis and adult emergence was similar for both sexes (Nayar 1968a).
Since Cx. nigripalpus larvae pupated from early morning to late in the eve-
ning, depending on the temperature, and accounting for differences in the length
of the pupal stage at various temperatures, adult emergence generally occurred
during the second or third night after pupation (Fig. 2).
Table 7. Onset of pupation, duration of pupal ecdysis in Culex nigripalpus as affected by
varying quantities of food, density of larvae, and salinity of medium at 270C under LD
(Quantity of food, salinity, Onset of Pupation Duration of
& density of larvae/pan)* (hours after hatch) Pupal Ecdysis**
BR, 5% SW, 75 102 90
BR, 5% SW, 200 102 113
BR, 20% SW, 75 124 70
2 BR, 5% SW, 75 101 47
2 BR, 20% SW, 75 102 28
2 BR, 5% SW, 200 102 75
BR, 20%, 200 124 75
2 BR, 20%, 200 123 70
SOURCE: Nayar, 1968a (modified).
* BR-low diet and 2BR-high diet, SW-sea water.
** The duration of the first 5% of pupal ecdysis.
Under certain field conditions, especially during the last instar, larvae of dif-
ferent mosquito species form aggregations of hundreds of individuals (Nielsen
& Nielsen 1953, Horsfall et al. 1973). No detailed studies have been made on
these aggregations in the field. When Cx. nigripalpus larvae are reared in the
laboratory under ideal conditions, they are usually immobile, and photonega-
tive. But, when they are temporarily crowded for 8 hours during the light phase
of an LD 12:12 cycle and normally fed, they tend to form one or two large com-
pact clusters comprising 80% to 90% of the larvae in the pan at times (Nayar &
Sauerman 1970b). The larvae hang almost perpendicular to the water surface
and the rosette-shaped clusters are immobile and relatively insensitive to shad-
ows. On the other hand, when temporarily crowded larvae are starved, their
threshold for the photonegative response is lowered and they form several dense,
small clusters in the shaded corners of the pan. These dense clusters are much
more active than those of well-fed larvae, have a slight vertical movement, and
are very sensitive to any movement from above or around them, thus exhibiting
an intense alarm reaction.
The onset of pupal ecdysis is delayed by 24 hours and its duration is length-
ened when temporarily crowded larvae are starved, suggesting that this is a con-
dition of stress that delays larval development. However, pupal ecdysis remains
synchronized, whether the temporarily crowded larvae are fed or starved.
EFFECTS OF LARVAL NURTURE ON ADULT CHARACTERISTICS AT
EMERGENCE AND SURVIVAL WITHOUT FURTHER NOURISHMENT
Maximum expression of size, body weight, and energy reserves (Table 8) is
present in the adults at emergence when Cx. nigripalpus larvae are reared in the
laboratory under ideal environmental conditions, with maximum quantity of
food, and with either a low larval density or temporary crowding in fresh water
at a constant temperature of 270C (Nayar 1968b, Nayar & Sauerman 1970b). A
combination of low food quantity, constant high larval density, and high salinity
produces adults with a minimum of size and weight (Table 8). Both size and
weight also vary inversely with the temperature, since larger and heavier adults
are produced at lower temperatures than at higher temperatures (Nayar 1968b).
At emergence, differences in size and weight between male and female Cx. ni-
gripalpus are slight (Table 8) when compared to other mosquito species, where
these differences are pronounced (cf. Nayar 1969, Nayar & Sauerman 1970b).
Starving 4th instar larvae for varying time periods prior to pupation produced
adults of similar size but varying body weights and energy reserves (Table 9),
thus indicating that both are continually accumulated during the last stages of
4th instar and up to pupation. Therefore in nature, if the amount of food available
to the 4th instar was diluted by rainfall or depleted by overcrowded conditions,
the resulting adults would be lower in weight and have less energy reserves. This
became apparent when the survival of the emerging adults was observed. When
larvae were not starved during the 4th instar, the 50% survival time of the newly
emerged adults was normal (91 to 93 hours, Table 9), but as the starvation period
of the larvae was increased, their 50% survival time as adults was greatly re-
duced (66 to 72 hours, Table 9). Survival time was dependent on the amount of
lipid present in the adult at emergence, but was not affected by the amount of
glycogen. Approximately 35% of the dry body weight at emergence was lost by
the time 50% survival time was reached, whereas the lipid and glycogen re-
serves were reduced by 79% to 95% of their original amount. Therefore, with
advancing age, unfed adults became exhausted and died due to a depletion of
A direct correlation between the depletion of lipid and 50% survival time was
established when Cx. nigripalpus larvae were reared on specific diets and then
analyzed as adults at either 12- or 24-hour intervals for any depletion of their
energy reserves (Nayar & Pierce 1977). The glycogen and triglyceride reserves
declined exponentially, i.e., the depletion (utilization) rate of both glycogen and
triglyceride was proportional to the amount of each that was present at emerg-
ence. The rate of utilization was between 50% and 74% for each 24 hour period,
but utilization decreased as the mosquito aged. A 90% depletion of the triglyc-
Table 8. The adult characteristics of Culex nigripalpus reared under maximum and minimum tolerable levels of salinity, food, and density at 270C
under LD 12:12, and under aggregated conditions.
Morphological Energy Reserves
Rearing Conditions (basic Characteristics Weight Percent of drdybd weight
ration, % salinity, and Wing length x wing breadth Dry body weight
larval density) Sex (mm) ,(Ig/adult) lipid glycogen
BR, 20% SW, 200* 2.62 x 0.73 395 8.1 9.5 4.14 6.4 0.35
9 2.86 x 0.83 416 15.8 9.1 2.09 5.5 0.43
2 BR, TW, 75 S 2.90 x 0.79 698 14.4 21.3 3.68 8.7 0.91
9 3.08 x 0.88 743 59.3 17.8 3.97 7.5 1.45
Aggregated conditions** 2.86 x 0.78 468 15.2 12.0 1.21 7.5 1.31
9 3.15 x 0.96 509 16.1 9.4 1.62 5.3 0.43
SOURCE: Nayar, 1968b, Nayar & Sauerman 1970b (modified).
* BR = basic ration; SW = sea water; and TW = tap water.
** Two thousand larvae from five pans were temporarily crowded in one pan of 500 mL of fresh water for 8 hours daily during the light period of the LD 12:12 cycle, and
simultaneously fed brewer's yeast (Nayar & Sauerman 1970b).
Table 9. The adult characteristics of Culex nigripalpus reared on two basic rations in 5% sea water with 200 larvae per pan at 270C under LD 12:12,
and starved for varying times during the instar larval stage.
Starved Morphological Characteristics
Before Onset Wing Weight Energy Reserves 50% Survival
of Pupation length Wing breadth Meandry weight Percentage of dry body weight Times
(hours) Group Sex (mm) (mm) (p.g/adult) lipid glycogen (hours)
20 A 2.86 0.08 0.77 0.05 387 7.98 13.5 2.14 5.3 0.82 66
12 B 2.90 0.08 0.81 0.05 508 20.12 19.3 1.86 5.1 0.93 83
0 C 2.94 0.06 0.83 0.03 662 11.32 21.1 2.61 8.0-t 1.20 91
A y 2.93 0.09 0.86 0.04 396 8.01 14.2 2.23 5.4 0.73 72
B 3.19 0.12 0.95 0.03 533 14.13 17.2 2.01 4.3 0.98 88
C 3.18 0.13 0.93 0.05 709 9.23 16.6 0.86 5.3 0.97 93
SOURCE: Nayar 1968b
eride reserves coincided with a 50% survival time. The 50% survival times var-
ied from 58 to 78 hours for males and 62 to 82 hours for females. The maximum
difference between 50% survival time and 0% survival was 8 to 10 hours only,
indicating that adults rapidly die off once they start to die (Nayar & Pierce 1977).
In order to compare the size, reserves, and 50% survival times of laboratory
reared mosquitoes with those in the field, pupae were collected from citrus grove
swales near Vero Beach at 3-week intervals during 1975 to 1976. The emerging
adults had an average dry body weight of 450 I/g, of which 5% to 8% was lipid,
and 5% to 6% was glycogen. Their mortality was recorded at 4 hour intervals,
resulting in an average 50% survival time of 54.5 -12.5 hours for males and
63.6 13.4 hours for females. However, pupae collected during February pro-
duced adults that survived almost twice as long, with a 50% survival time of 115
-30 hours for males and 130 -25 hours for females. An improvement in these
results might occur if the natural foods are concentrated either through evapo-
ration or enrichment of the breeding waters. But when larvae develop in food
deficient waters, weaker adults are produced.
The adult Cx. nigripalpus emerges during the early part of night and imme-
diately settles in the grass and shrubs that are marginal to the larval site. By
daylight these adults move to areas of dense vegetation, such as oak or cypress
hammocks with dense canopy. Provost (1969) observed that during the daytime
Cx. nigripalpus rests in dense vegetation, close to the ground, or even within
the leaf litter or ground detritus. The hotter and drier the day, the deeper the
adults will penetrate such concealment, while on humid, overcast and cooler
days, they are less compelled to hide themselves. The nighttime climate in Flor-
ida permits them to rest anywhere from the tree tops to the ground. Cx. nigri-
palpus is generally regarded as an outdoor species, but when populations are
large, they occasionally enter houses (Carpenter & LaCasse 1955). These ob-
servations were experimentally confirmed recently when newly emerged 32p_
marked, unfed adults were released at Tiger Hammock (Nayar et al. 1980). In
the area surrounding the release site (circle with 0.4 km radius), more resting
adults, both marked and unmarked, were collected during the day from the
young and mature oak hammocks than from the Brazilian pepper-shrubs and
pine-palmettoes (Table 10). In the adjoining area (a circle with a radius of 0.4 to
1.2 km), the concentration of adults in the oak hammocks was even more pro-
nounced. On the other hand, blood-seeking females were more evenly distrib-
uted throughout all vegetational areas, with the exception of open pastures
The records of both New Jersey light traps and chick-baited cans, collected
over a period of 11 years at the FMEL show that the breeding of Cx. nigripalpus
is at its lowest level from January through March, followed by a slow population
Table 10. Number and percentage of unmarked and 32P-marked adult Culex nigripalpus taken by aspirator collections (resting adults both sexes)
and in bait-can traps (blood-seeking females) from different habitats. Adults were released and recaptured during a 12-day period.*
Aspirator (resting adults both sexes) Bait-Can Traps (blood-seeking 9)
3 and 9/site/collection 9/site/trap
No. of Collections or Unmarked Marked Unmarked Marked
Habitat Traps No. (%) No. (%) No. (%) No. (%)
Within 0.4 km
Brazilian pepper-shrub 2 680 ( 5.9) 3.0 ( 2.2) 1813 ( 22.8) 3.5 ( 15.7)
Pine-palmetto 2 539 ( 4.7) 6.5 ( 4.8) 1782 ( 22.4) 4.5 ( 20.2)
Young oak hammock 5 3624 ( 31.6) 59.4 ( 43.6) 2614 ( 32.9) 7.6 ( 34.1)
Mature oak hammock 3 6636 ( 57.8) 67.3 ( 49.4) 1735 ( 21.8) 6.7 ( 30.0)
Totals 12 11,479 (100.0) 136.2 (100.0) 7944 ( 99.9) 22.3 (100.0)
Within 0.4 to 1.2 km
Pasture 2 154 ( 1.5) 0.0 ( 0.0) 965 ( 13.7) 0.0 ( 0.0)
Pine-palmetto 2 455 ( 4.5) 0.5 ( 9.5) 1549 ( 21.9) 3.0 ( 31.6)
Citrus grove 4 490 ( 4.8) 0.75 ( 14.3) 2199 ( 31.1) 3.25 ( 34.2)
Mature oak hammock 4 9057 ( 89.2) 4.0 ( 76.2) 2355 ( 33.3) 3.25 ( 34.2)
Totals 12 10,156 (100.0) 5.25 (100.0) 7068 (100.0) 9.5 (100.0)
SOURCE: Nayar et al. 1980
*50,000 adults released
buildup from April through June; concurrent with a sharp increase in rainfall,
the peak population occurs from July through October, with a gradual decline
during November and December (Provost 1969). These data were confirmed
when CDC light traps with dry-ice, rotated nightly among five sites, were used
to collect mosquitoes from October 1975 through May 1976 at Tiger Hammock.
The Cx. nigripalpus population dropped precipitously from October to Novem-
ber and was lowest during January and February (Table 11). By May the popu-
lation had built back up to the October level and in June was at its usual summer
peak (Table 11).
Table II. Trapping of Cu
light trap with one CO2 b
lex nigripalpus in Tiger Hammock, 1975-1976 using CDC
No. of No. of Adults
1. The collection data were first transferred into
9 thru 2 Mw 64
8 and 3 Mw 17
4 M, 12
Mw (Williams Mean), i.e. the antilog of (x + 1) -
the logarithum ofx + 1.
Cx. nigripalpus occurs along with various other species in the same ecologi-
cal areas. In south-central Florida, it has been collected along with all of the 50
or more species present at different times throughout the year. Collection meth-
ods, however, determine their correlation to a particular species.
In experiments at the Fisheating Creek Wildlife Management Area in south-
central Florida, Glades County, Cx. nigripalpus was collected at weekly and bi-
weekly intervals from April through October 1977. In the bait can collections,
it was associated with 14 species, and in CDC light traps, it was associated
with 15 species. Six of the species were not common in either of the collections
(Table 12). In north-central Florida, at the Lochloosa Wildlife Management
Area, Alachua County, Cx. nigripalpus was associated with 10 different species
when collected with bait-can traps and CDC light traps, but seven of the species
were not common to both types of traps.
Table 12. Average number of adults per trap of mosquito species collected at 2-week
intervals at the Fisheating Creek Wildlife Management area, Glades County, Florida
and Lochloosa Wildlife Management Area, Alachua County, Florida from April to
Fisheating Creek Area
COPULATION AND INSEMINATION
Field observations on the sexual behavior of newly emerged mosquitoes show
that pairing of sexes takes place soon after emergence. It is implied that this is
when insemination occurs, even though it was never proven that these observed
females were uninseminated prior to or after the pairing. It is therefore a com-
mon assumption that the females of some species become inseminated a few
hours after emergence or just as soon as they can fly. In the laboratory, however,
the caged females of some of the same species do not become inseminated until
they are several days old (Lea & Evans 1972). In a Cx. nigripalpus colony, none
of the females were inseminated 3 days after emergence, but all were insemi-
nated by day 9 (Nayar & Pierce 1980).
Swarms of male Cx. nigripalpus have been observed above solid markers such
as trees during the evening twilight starting at 15 to 20 minutes after sunset
(Provost 1969). In the laboratory, male Cx. nigripalpus began to swarm at dawn
with an average light intensity of 0.72 to 2 log lux and stopped swarming at 1.32
log lux. At sunset, the swarms started at 0.63 to 1.13 log lux and stopped at
0.08 to 3 log lux (Nielsen & Nielsen 1962). Whether females were actually in-
seminated during these swarms has not been conclusively established.
In order to determine the age at which Cx. nigripalpus females are insemi-
nated in nature, Lea & Edman (1972) released on a wooded island in a fresh
water swamp, marked males and females that were both 22 hours old. They had
also released males 5 to 7 days earlier at the same site. The females were recap-
tured during the night of the release and for two subsequent nights with CO2-
baited light traps and during the mornings from the ground litter with a tractor-
mounted power aspirator. Upon dissection, it was discovered that few of these
females were inseminated until the third night. A positive correlation was also
found between the rate of insemination and the distance from the release point
at which the recoveries were made.
Lea and Edman's (1972) conclusions were confirmed during a 1976 study at
Tiger Hammock, when inseminated 3P-marked females were first recaptured
at 54 hours of age, after being released between the ages of 6 to 30 hours. All of
the recaptured females were inseminated by 90 hours. This study also showed
that those females which dispersed further from the release area during the first
two days were inseminated earlier than the ones that remained nearby.
An investigation of the flight activity patterns of virgin and mated Cx. nigri-
palpus adults having different nutritional experiences was conducted in the
acoustic bio-room under a LD 12:12 light cycle (both square- and sine-waves) at
270C, where flight sound could be recorded continuously for at least two weeks
(Nayar & Sauerman 1969). In square- and sine-wave light cycles, both virgin
males and females maintained on 10% sucrose had a bimodal flight pattern
(Nayar & Sauerman 1974a). The activity started about 24 hours after emergence
and thereafter a peak of about 30 minutes of flight activity occurred at light-off
and a peak of 20 minutes occurred at light-on (Fig. 3). There was no discernible
activity during the remaining dark phase of the LD cycle. But as these adults
matured, began to feed on sugar and blood, and developed eggs, their activity
increased throughout the night (Fig. 3). A similar flight pattern was observed in
the field when adults were collected with suction traps for different durations
during the night (Bidlingmayer 1974). Further laboratory experiments showed
that starvation caused an initial increase in activity for 24 hours, but the activity
decreased on subsequent days. Flight activity of mated females, however, was
extended throughout the dark phase both on 10% sucrose and sucrose plus a
S2 I I I 6 7 I I I I 9I
12 24 36 48 60 72 84 9
HOURS AFTER EMERGENCE
Figure 3. Flight activity pattern in Culex nigripalpus male and female after emergence in LD 12:12 on 10% sucrose at 270C.
blood meal. When females were fed blood and sucrose, more flight activity oc-
curred after oviposition than before. These observed changes in activity are con-
sistent with changes in behavior in the field from mating to host-seeking and
host-seeking to oviposition.
Cx. nigripalpus females were maintained ad lib. on 10% sucrose and a sample
of 20 was flown once a week for 10 weeks on a flight mill. They flew tethered at
a speed of approximately 1000 meters per hour for a period of 4.5 hours (Nayar
& Sauerman 1973a). After 10 weeks, the surviving females were unable to fly
due to their old age. Control females held at rest on 10% sucrose, required two
weeks of ad lib. feeding before their reserves stabilized to a maximum of 2.5 to
4.0 cal (10.5 to 16.8 joules) per female for triglyceride and 0.5 to 0.75 cal (2.1
to 3.1 joules) per female for glycogen. The free sugar levels, however, remained
very low. After tethered flight, no consistent or significant difference in the de-
pletion of triglyceride or free sugar was apparent between those females that
flew and those at rest (Fig. 4). Only glycogen showed a consistent and signifi-
cant depletion during tethered flight (Fig. 4). The flown females, which weighed
an average of 2.6 to 3.4 mg, expended between 0.06 and 0.07 cal (0.25 and 0.29
joules) per 1000 meters, and at the end of the 4.5-hour period, had expended 15
to 28 cal (63 to 117 joules) per hour per gram of energy (Nayar & Sauerman
6.0-3.0 ------ SUGARS (FREE)
--- UPID (TRIGLYCERIDE)
0 1 2 3 4 5 6 7 8
AGE AFTER EMERGENCE (WEEKS)
Figure 4. Fluctuations in levels of energy reserves in flown and non-flown Culex
nigripalpus females maintained on sucrose solution. From Nayar & Sauerman (1973a).
It is not possible to observe directly the flight patterns of mosquitoes in the
field, but they can be indirectly studied through the use of non-attractant col-
lecting techniques when both environmental conditions and the physiological
state of the mosquito can be evaluated (Bidlingmayer 1971, 1974). Bidling-
mayer used suction traps within and adjacent to wooded swamps, and vehicle
aspirators within daytime resting habitats to collect mosquitoes. From his data
he found that certain species including Cx. nigripalpus remained in the woodland
throughout the day, and even though a few mosquitoes did move into the fields,
they were usually found near shrubs. Only a small number ventured outside their
natural habitat at night, except after an early evening rain, when the relative hu-
midity was above 90% (Bidlingmayer 1974, Dow & Gerish 1970, Provost
1973). Therefore, even though Cx. nigripalpus has a great dispersal potential
(both the energy reserves and flight capacity), it rarely moves far from the
shaded woodlands and swamps unless the humidity is high.
Results from suction and truck trap collections showed that the light intensity
of the moon, wind velocity, and temperature also affected the flight activity of
Cx. nigripalpus (Bidlingmayer 1967, 1971, 1974). During the full moon phase,
1.7 times more Cx. nigripalpus adults were collected than during the new moon,
indicating that flight activity was greater with the higher intensity of moonlight
(Bidlingmayer 1967, Provost 1959).
Mosquito collections were reduced by about 60% with wind velocities of 10
to 20 mph (4.5 to 8.9 meters per second). Winds above this prevented any flight
activity. Only a few more adults were collected when early morning tempera-
tures were 220C than were collected at temperatures of 190C to 21 C. The col-
lections were significantly smaller when morning temperatures dropped to
between 180C and 160C, and even fewer were collected below 160C.
Approximately seven times more blooded Cx. nigripalpus females were col-
lected in suction traps at dawn than at dusk (Edman & Bidlingmayer 1969, Bid-
lingmayer 1975). Even though gravid females flew at both evening and morning
twilight, more were usually collected in the morning than in the evening. The
number of parous females that were flying gradually increased during the night,
being lowest in the evening and greatest at dawn. On moonlit nights, more
blooded and gravid females of Cx. nigripalpus were trapped in the fields than in
the hammocks (Bidlingmayer 1971, 1974).
All species of mosquitoes disperse varying distances after emergence from
their larval habitats. The extent to which a brood may disseminate is influenced
by the weather conditions following emergence, the direction and speed of the
air mass bearing them during flight, and the availability of nutritional sources,
resting places, and oviposition sites.
The dispersal of Cx. nigripalpus marked with radiophosphorous and fluores-
cent dyes was studied in a uniform citrus-growing area west of Vero Beach (Dow
1971). Recoveries were made with CDC light traps arranged in a geometrical
pattern extending 5 km from the point of release. A total of 0.13% of the released
radioactive females were recaptured within a 1.6 km radius. A few individuals
were collected up to 5 km from the release point.
Cx. nigripalpus is not thought of as a migratory species, but increased flight
activity 2 or 3 days after emergence is observed when larvae are reared under
temporarily crowded conditions (Nayar & Sauerman 1973b). This could result
in extensive dispersal of the population. The tethered flight performance of Cx.
nigripalpus on a flight mill also indicates that they possess a good potential for
dispersal (Nayar & Sauerman 1973a).
In another dispersal study at Tiger Hammock during 1976, 2P-marked Cx.
nigripalpus were collected from the oak hammocks with dry-ice chick-baited
lard-can traps and portable power aspirators (Nayar et al. 1980). An average of
1.87% (0.81% to 3.01% in four experiments) of the released adults were recap-
tured within a radius of 1.2 km and of the total adults recaptured, 82.4% were
collected within 0.4 km radius, with only 17.6% collected from the surrounding
area within 0.4 to 1.2 km radius (Nayar et al. 1980).
FEEDING AND METABOLISM
Adult mosquitoes feed primarily on carbohydrates usually from plant nec-
tars. However, most female mosquitoes also require protein sources such as ver-
tebrate blood (Clements 1963). Carbohydrates serve as the adult's main source
of nutrition and energy, whereas proteins are utilized by the female for the de-
velopment of eggs.
Nectar- and Sugar-Feeding (Carbohydrate-Feeding)
In the field, Haeger (1955) observed the males and the females of Cx. nigri-
palpus feeding on the honey dew of green aphids covering the leaves of Bidens
sp. Later he observed Cx. nigripalpus feeding on the nectar sources (extra-flo-
ral) as well as floral nectar of Cassia brachiata (Partridge pea), Crotalaria mu-
cronata crotalariaa), Serena repens (Saw palmetto), Urena lobata (Caesar weed),
Ilex cassine (Ilex), Lantana camera (Lantana), Citrus sinensis (Orange), and
the honey dew from Coccus viridis on Baccharis halimifolia (Groundsel bush)
Bidlingmayer and Hem (1973) examined field-caught Cx. nigripalpus fe-
males collected by suction trap and portable power aspirators for the presence of
fructose by the cold anthrone method. More of the specimens collected near a
salt marsh had fed on nectar than those collected within a maple swamp area.
Nectar feeding was then monitored for a year in the area adjacent to the salt
marsh, and was found to vary seasonally, being greatest from April through Oc-
tober (12% to 24%).
In 1976, marked Cx. nigripalpus were released at Tiger Hammock and both
resting adults and blood-seeking females were collected for a period of 11 days
and tested for the presence of fructose and glucose using a paper chromato-
graphic technique (Nayar 1978). Overall they contained more glucose (67% to
77% for males and 57.8% to 100% for females) than fructose (15.6% to 40.3%
for males and 3.0% to 69.1% for females) (Nayar 1978). Fewer blood-seeking
females (4.6% to 12.6%) showed the presence of fructose than resting females
(35.7% to 69.1%). Similarly fewer blood-fed females contained fructose,
whether resting (3.0% to 26.0%) or blood-seeking (0.0% to 15.4%). During the
11 days of collection, there were fluctuations in the relative quantities of glucose
and fructose (Nayar 1978). The presence of these free sugars in the recaptured
adults showed that they had imbibed a nectar meal, since unfed mosquitoes do
not contain free sugars, and did not show fructose or glucose in the chromato-
graphic tests. Twelve hours after their release, 100% of the resting females con-
tained both glucose and fructose (1.9 units of glucose and 1.1 units of fructose),
as compared to having neither at emergence. As the resting females aged, the
number which contained fructose declined. Whereas most of the recaptured
resting females (98.4%) contained glucose, and only 65.3% contained fructose
Metabolism of Carbohydrates
Mosquitoes emerge with varying levels of energy reserve (stored glycogen
and triglyceride), the amount of which is species specific (Nayar 1968b, Nayar
& Sauerman 1970b). If newly emerged mosquitoes are starved for 2 to 4 days,
their energy reserves become depleted and they begin to die. However, if mos-
quitoes are fed a meal of simple sugar prior to death, it is rapidly absorbed
(Nayar & Sauerman 1975a, Van Handel 1965, Van Handel & Lum 1961).When
sugar is imbibed, its rate of utilization follows an exponential curve of decline in
which the rate of its disappearance is proportional to the amount of sugar pres-
ent. This sugar is converted into energy reserves (glycogen and triglyceride),
and as the amount of free sugar declines, the female becomes increasingly de-
pendent on these reserves. When Cx. nigripalpus females (average weight 2.0
mg per female) were fed a meal of 1.6 ItL of 50% glucose, they utilized the
glucose at a rate of 34 hours per cal (4.19 joules) per mg (Nayar & Sauerman
1975a). An analysis of these females at 24-hour intervals, indicated that the ex-
ponential rate of decline was followed up to 72 hours, but afterwards a slower
rate of decline occurred. As the free sugar was absorbed, glycogen and triglyc-
eride were synthesized and accumulated as energy reserves (Fig. 5). Both gly-
cogen and triglyceride began accumulating within 24 hours after the sugar meal.
Glycogen reserves reached a maximum by 24 hours and thereafter gradually de-
clined, however, triglyceride required 144 hours to peak before decreasing.
Therefore, glycogen accumulation is not dependent upon the amount of free
sugar present, although the accumulation of triglyceride is so dependent.
When newly emerged Cx. nigripalpus females were given either a 5%, 10%,
or 25% solution of sucrose in a potometer, made from a glass tube 20 cm long
and 0.4 cm o.d., bent into a J-shape, one arm 4 cm long and the other arm 13
cm long (Nayar & Sauerman 1974b), the volume imbibed per female per day,
was greatest with 5% sucrose solution and least with 25% sucrose solution dur-
ing the first 5 days (Nayar & Pierce 1980). However, when these volumes were
converted to calories per female per day, the caloric intake was much higher on
*--* SUGARS (FREE)
E- LIPID (TRIGLYCERIDE)
0 I IIl ldI
HOURS AFTER A GLUCOSE MEAL
\ r \
0 48 96 144 192 240
HOURS AFTER A GLUCOSE MEAL
Figure 5. Synthesis of glycogen and triglyceride by Culex nigripalpus females after a
50% glucose meal. Modified from Nayar & Sauerman (1975a).
25% sucrose than on the other two concentrations. Both the volume and the ca-
loric intake of the sucrose increased rapidly after the first day, reached a maxi-
mum during the next 5 days, and then declined sharply, stabilizing at a
maintenance level with minor fluctuations during the subsequent 4 weeks (Nayar
& Pierce 1980). Similar patterns of sugar intake have also been observed in the
blowfly Phormia regina (Gelperin & Dethier 1967), Ae. taeniorhynchus (Nayar
& Sauerman 1974b), and F1 Cx. nigripalpus females maintained on a 10% ra-
dioactive sucrose solution ad lib. (Nayar et al. 1979). The total amount of sugar
present in those females that fed on the 5% or 10% solution was very low (0.08
to 0.33 cal (0.34 to 1.38 joules) per female) and rarely changed, suggesting a
rapid metabolism on free sugar following ingestion. However, at higher concen-
tration (25% sucrose) a large amount (0.63 to 0.89 cal (2.64 to 3.73 joules) per
female) of free sugar was always present in the females. After the first week,
glycogen accumulation stabilized and gradually decreased in females fed on
either 5% or 10% sucrose, but remained high in females fed on the 25% sucrose.
Triglyceride reserves were consistently lower in those females which fed on 5%
sucrose when compared with the other two concentrations, especially 25% su-
crose, where the amount of triglyceride reserves remained very high. As ob-
served in both Ae. taeniorhynchus and Cx. nigripalpus (Nayar & Pierce 1977,
Nayar & Sauerman 1974b), a maximum accumulation in the energy reserves
(glycogen and triglycerides), at a specific concentration of sucrose, regulates
any further long-term intake of sucrose. At death, most of the females still had
a large amount of energy reserves, indicating that death in older mosquitoes is
not due to a lack of reserves but a cessation of body functions and a result of
Most carbohydrate metabolism studies were conducted on female mosqui-
toes, because it was previously shown that the males of bothAe. sollicitans and
Ae. taeniorhynchus did not synthesize triglyceride when maintained ad lib. on
a glucose solution (Van Handel & Lum 1961). This was interpreted to mean that
the males of all mosquito species did not synthesize triglyceride from carbohy-
drates (Van Handel & Lum 1961, Provost 1969). However, during our experi-
ments, where both male and female Cx. nigripalpus were maintained ad lib. on
a 10% sucrose solution, it was discovered that the males also synthesized gly-
cogen and triglyceride from the sucrose (Fig. 6). The amount synthesized by
these males was less than ihat found in females, but the pattern of their synthesis
and accumulation was identical to that reported for females (Fig. 6). Thus it
appears that in this aspect the males of Cx. nigripalpus differ from the males of
Ae. sollicitans and Ae. taeniorhynchus.
Blood-feeding and Metabolism of Proteins
Host range and feeding. In both the field and laboratory, the mosquitoes'
range of hosts varies with each species and the hosts' availability. The blood
oo00 / 1 0--0
0 2 4 6 8 10
Figure 6. Synthesis of glycogen and triglycerides in Culex nigripalpus adults maintained
on 10% sucrose after emergence.
sources of mosquitoes collected from the field can be identified in the laboratory
by a precipitin test (Edman 1974, Tempelis 1975). Cx. nigripalpus is an ex-
tremely opportunistic mosquito, feeding mainly on mammals such as cattle and
rabbits, and birds of the orders Ciconiiform (herons, egrets, and ibis), Passeri-
form (perching birds), and Galliform bobwhitee quail and turkeys) (Edman
1974). The feeding habits of Cx. nigripalpus were further examined at three dif-
ferent locations near Tampa. At Keystone, the first site, most of the feedings
were on Gallinaceous birds (76%), the majority of which were undoubtedly
chicken since sentinel flocks were located within the area, followed by Passeri-
form (10%), Charadriiform (shore birds, gulls, and terns) (8%), Strigiform
(owls) (3%), and Ciconiiform (2%). Among the mammals, ruminants (54%)
and rabbits (23%) were the most common source of blood, followed by equine
(9%), canine (6%), opossum (3%), feline (2%), and armadillo (1%). Fletcher,
the second site, was wetter. Therefore, wading (Ciconiiform, 43%) and shore
(Charadriiform, 36%) birds were the most common avian hosts identified, along
with rabbits (39%) and armadillos (32%) as the mammalian hosts. At Big Bend,
the third site, Galliform birds (76%), and rabbits (70%) were the dominant
blood sources. At all three sites, the feeding patterns reflected the faunal popu-
lation of the area. Man does not seem to be a favored host, at least in rural areas
where in biting collections, only 4 of the 1471 females that attempted to feed
were Cx. nigripalpus (Provost 1955), and only six Cx. nigripalpus had fed on
man out of 2002 engorged mosquitoes collected with a power aspirator (Provost
Under laboratory conditions, female Cx. nigripalpus fed readily on three spe-
cies of restrained birds (Chicken Gallus domesticus, Ground Dove Columi-
gallina passerina, Barn Owl Tyto alba), three species of restrained mammals
(Gerbil Meriones unguiculatus, Rabbit Oryctolagus cuniculus domesticus,
House Mouse Mus musculus) and human volunteers Homo sapiens, during
the light period, ingesting between 3.47 to 4.35 mg per female (Nayar & Sauer-
Cx. nigripalpus females were also collected in traps baited with seven differ-
ent species of animals, including two species of lizards and four species of ro-
dents. However, the bulk of the females were attracted to the chick-baited traps
(Aitken et al. 1968).
Metabolism and blood-feeding patterns after emergence in the labora-
tory. Blood is essential for the preservation of most mosquito species be-
cause, with the exception of a few autogenous species that develop eggs from
their reserves, the protein in blood is essential for egg development. When either
blood or sugar is fed to starved Ae. sollicitans females, glycogen and triglycer-
ides are synthesized and stored as reserves (Van Handel 1965). Regardless of
whether sugar or a calorically equivalent amount of blood is imbibed, the rate of
triglyceride synthesis is the same, but glycogen is synthesized ten times faster
from the sugar than from blood. The energy reserves that are accumulated from
the blood are used for survival and flight (Clements 1955, Nayar & Van Handel
In spite of repeated attempts, both virgin and mated 1- to 4-day-old female
Cx. nigripalpus refused to feed ad lib. on a chick when they had been previously
starved (Nayar & Sauerman 1975b). A comparison of their blood-feeding habits
when maintained on either distilled water or 10% sucrose proved that without a
prior meal of sucrose, very few females of this species would take blood from a
tethered vertebrate host (Nayar & Pierce 1980). In this regard, Cx. nigripalpus
differs from most other Florida mosquitoes, whose females blood-feed readily
on vertebrate hosts without prior sugar-feeding. However, when virgin or mated
female Cx. nigripalpus were maintained on 10% sucrose and then offered a
blood source, more than half of them blood-fed by day 3 post-emergence, and
the remainder blood-fed on subsequent days (Nayar & Pierce 1980).
In a mark-release experiment at Tiger Hammock during 1976, when a syn-
chronized brood of newly emerged, unfed Cx. nigripalpus was released into the
field and the marked blood-seeking females were recaptured daily in bait-can
traps, only 2.2% of the marked blood-seeking females were recaptured 24 hours
after their release, when they were a maximum of 30 hours old. However, 49.3%
and 24.9% were recaptured at 54 hours and 78 hours, respectively, making a
total of 76.4% recaptured in the first peak (Fig. 7). Less than 1.0% were re-
covered during the next 48 hours, but 174 hours and 198 hours after release an-
other 19.2% were recaptured, constituting a second peak of blood-seeking
4 I I
COL-chick-baited lard-can trap.
I 2 3 4 5 6 7 8 9 10 II 12
DAYS AFTER RELEASE
Figure 7. Blood-seeking pattern of marked Culex nigripalpus females collected with
C02-chick-baited lard-can trap.
Blood-feeding patterns in nature. Cx. nigripalpus has an annual shift in
its blood-feeding pattern, feeding mainly on avian hosts in the winter and spring,
and then switching to an equal or greater feeding on mammals in the summer
and fall (Edman & Taylor 1968). This feeding pattern is similar to that observed
with Culex tarsalis in California (Hayes et al. 1973, Reeves et al. 1963, Tem-
pelis et al. 1965, Tempelis et al. 1967). The time and magnitude of this shift
varies from year to year in accordance with the onset and duration of the rainy
season (Edman 1974).
In the summer and early fall when afternoon showers occur frequently, more
Cx. nigripalpus move from wooded areas to the adjacent open habitats (Bidling-
mayer 1971), where mammalian hosts are more likely to be found. The cause of
this seasonal change in their flight behavior is difficult to assess, but the effect
of temperature and light is inadequate to explain the phenomenon since humidity
seems to greatly influence the activity of Cx. nigripalpus.
Reeves (1971) introduced another consideration which involves the relation-
ship of mosquito density to host selection. In analyzing the shift from birds to
mammals by both Cx. tarsalis and Cx. nigripalpus, he hypothesized that since
larger mosquito populations coincide with the shift from birds to mammals, this
may have an effect on mosquito feeding behavior, which can be interpreted in
two ways: (1) the increased population results in mosquitoes interfering with
each other during feeding or (2) the increased biting activity on the preferred
host makes this host intolerant to the mosquito. The latter has been shown to play
an important role in the feeding behavior of Cx. nigripalpus (Edman et al.
Mosquito behavior and host receptivity also influence blood-feeding patterns.
Certain mosquito species do not leave protected areas, such as woodlands, seek-
ing blood, while others move readily into open areas (Bidlingmayer 1971). This
can determine to some extent the host selected for feeding. When species are
reluctant to leave these protected areas, they are restricted to feeding on the
fauna that share the same woodland habitat. However, mosquitoes that fly into
open areas increase their host choice to include pastured animals.
Until recently, a host's anti-mosquito behavior had not been ascertained to be
a key determinant in host preference, but it is now recognized that certain hosts
do demonstrate heightened activity when attacked by mosquitoes. Rodents are
abundant in nature and in many cases the most common animal group present.
However, they seldom serve as mosquito hosts in any significant degree. The
infrequency of feeding on rodents may be due to their inaccessibility during
times of peak mosquito biting activity and their defensive behavior (Edman &
Kale 1971). Seven Ciconiiform bird species were studied and variations in their
receptivity to the biting ofCx. nigripalpus were observed (Edman & Kale 1971,
Webber & Edman 1972). Fifteen different anti-mosquito movements were rec-
ognized, such as foot-slapping and foot-pecking, and on those birds that were
most active, the mosquitoes were least successful in obtaining a blood meal.
When the birds were confined in cages, few Cx. nigripalpus were engorged and
high mortality occurred, since some birds were visually observed killing and
occasionally eating the mosquitoes. Five of the bird species which actively seek
out their food in nature, had a highly developed anti-mosquito behavior, whereas
the two species that showed little anti-mosquito behavior also wait for their food
in a motionless stance. Neither the color, size, nor weight of the host affected
the feeding success of Cx. nigripalpus (Edman & Kale 1971). Olfactory attrac-
tiveness did not seem to be a factor either, since they fed equally well on different
species of restrained birds. Age may play a role, however, since nestling birds
can readily serve as hosts (Kale et al. 1972).
Multiple blood-feeding. Multiple blood-feeding, defined as blood meal
comprised of two or more feedings, the last being taken before the protein from
the first meal is completely digested, has been reported in several species of
mosquitoes in the field (Boreham & Garrett-Jones 1973, Tempelis 1975). How-
ever, multiple blood-feeding in the field has not been reported in Cx. nigripal-
pus. In the laboratory, Edman et al. (1975) observed that virgin F1 Cx.
nigripalpus females that were less than half full of blood, usually attempted to
refeed when a second blood meal was offered to them within 6 hours, but few
refed 12 or 24 hours later. When they were offered the second blood meal after
6 hours, the proportion that refed declined as the volume of the interrupted meal
increased. This decline appeared to be related to the initiation of egg develop-
ment. Those females that were three-quarters full of blood usually failed to re-
In a mark-release-recapture experiment at Tiger Hammock during 1976, 32P-
marked, sugar-fed Cx. nigripalpus females confined to holding cages were al-
lowed to feed ad lib. on restrained chicks prior to being released into the field.
Approximately 60% fed, but most of them only partially. During the first week
of collection, 6.2%, 15.4%, 23.9%, and 10.8% of females that were recovered
seeking blood at 78, 102, 126, and 150 hours, respectively, comprised a total of
56.3%. Between 20% and 40% of those recovered during these four days were
partially blood-fed and possibly were attempting to complete the blood meal.
This, therefore, might be considered as a case of multiple blood-feeding by Cx.
nigripalpus in the field before the completion of the first gonotrophic cycle even
though the first blood-feeding occurred before release. The second peak of
blood-seeking occurred between 222 and 270 hours (6 to 8 days into the exper-
iment) after release, with 18.7% of the total females recovered. In other mark-
release experiments, only 0.6% to 0.9% of the blood-seeking females had a
small amount of blood in their midguts at the time of their capture.
There is no indication that Cx. nigripalpus undergoes diapause during the
winter months in central and south Florida when the population is at its lowest
level, since blood-seeking females are easily collected in bait can traps during
this time. From October 1976 through August 1977, blood-seeking Cx. nigri-
palpus were collected in Tiger Hammock at 3- to 5-week intervals using modi-
ebped.i m A
Figure 8. Ovarian development in Culex nigripalpus. All photographs were taken from freshly dissected material using high-dry Nomarski interfer-
ence contract microscopy. (A) Nulliparous follicle with evenly beaded pedicel (e.b.ped.); (B) Resorbed follicle with enlarged beaded pedicel
(r.e.ped.); (C) Parous follicle with one dilatation (d); (D) Nulliparous follicle with dilatations containing relic of degenerated follicle (d.f.); (E) Parous
follicle with 3 dilatations (Laboratory reared). g germarium; f follicle; scale line represent 50/1. From Nayar & Knight (1981).
fied chick-bait-can traps without CO2. Females from each collection were
examined for ovarian development and parity, and those with ovaries in a quies-
cent early stage II were characterized into three categories (Fig. 8; Nayar &
Knight 1981). The first category were nulliparous females in which the follicles
were small ovoid spheres containing a few coarse yolk granules surrounding the
oocyte nucleus. The second category were parous females in which the follicular
sheath was either sac-like or shrunken to form a small distinct dilatation which
was separated from the next resting follicle by a connecting stalk. The third cat-
egory were females with resorbed follicles, in which the tunica of the follicle
showed signs of distention posterior to the resting follicle but with no connecting
stalk separating them, and sometimes several distentions being contiguous to
each other, and clearly distinguishable from two or more parous follicles. These
resorbed follicles did not resemble the degenerating follicles of Cx. nigripalpus,
which had distinct dilatation like the parous follicles and contained follicular rel-
ics. The latter resembled those observed in diapausing Culex species from tem-
perate climates (Eldridge et al. 1972, Oda & Kuhlow 1976).
The simultaneous occurrence of all three types of females in the population
indicated that both newly emerged and older females were present throughout
the year (Fig. 9, Nayar & Knight 1981). Few newly emerged nulliparous fe-
males were collected from October through March with the exception of the De-
cember collection. Females with parous follicles were also less abundant during
October and November, and again from February through June. Females with
resorbed follicles were generally present when the nulliparous females were
fewer in number. The presence of a large number of nulliparous females in the
collections indicated the emergence of a new brood, while more parous females
indicated the survival of an older brood seeking a second or even a third blood
Correlating the abundance of these three types of females with the mean tem-
perature and total rainfall during the 2 weeks prior to each collection revealed
that females with resorbed follicles were more abundant during the cool and/or
dry months of October to March and less abundant during the warm, wet months
of April through August (Fig. 9); with corresponding larger number of nullipa-
rous females collected in bait cans when rainfall increased during the previous
month (Nayar & Knight 1981).
In the laboratory, when newly-emerged Cx. nigripalpus were maintained on
10% sucrose at temperatures of 180C, 240C, and 300C, ovarian development
progressed at varying rates (Nayar & Knight 1981). Ovarian development was
retarded at 180C; the follicles in most of the females were still in stages No-N
120 hours after emergence and a few had started to degenerate. By 144 hours,
these ovarian follicles had progressed to stage II and between 16.7% and
36.7% of the females had some resorbed follicles. Follicular development pro-
ceeded at an accelerated rate when the females were maintained at 240C and
300C, reaching stage II at 72 hours and 24 hours, respectively. At the same time,
a large percentage of the ovaries contained resorbed follicles and by 168 hours
some follicles were showing signs of degeneration. After 96 hours at 300C
(860F), a small percentage (3.3% to 10%) of the females' ovaries had developed
to stages III to V, providing evidence for autogenous egg development in this
species at elevated temperatures. Autogenous egg development has also been
previously observed from both the Tampa Bay and the Vero Beach area (Provost
1969). However, this species' potential for autogeny is very low, therefore all
field populations should be considered functionally anautogenous.
ONDJ F M A M J J A S
Figure 9. A) Distribution of nulliparous, parous, and resorbed females of Culex nigri-
palpus from October 1976 through August 1977. Nulliparous *--*, parous o--o,
and E----D resorbed females. B) Mean temperature and rainfall during 1976-1977. From
Nayar & Knight (1981).
In another experiment, 4-day-old Cx. nigripalpus were fed either a full meal
to repletion or a partial meal (one-fourth) on a restrained chick and then dis-
sected at 12 hour intervals to observe their ovarian development (Nayar & Knight
1981). Most of the females had ovaries which contained significant numbers of
resorbed follicles prior to the blood meal, but within 12 hours some of these
females' ovaries (6.6% to 23.3%) had begun to develop. The number of females
in which follicular development had initiated reached a maximum between 36
and 48 hours, and was similar on either size meal of blood. However, after 48
hours, as the development progressed beyond stage III, the number of follicles
that continued to mature in each female decreased more rapidly in those which
were fed a partial blood meal than with a full blood meal. Similar results were
also observed inAe. aegypti (Lea et al. 1978) where above a certain minimum
amount, the size of the blood meal had little effect on the number of oocytes that
began to deposit yolk within a short time. However, in Cx. nigripalpus, as ma-
turation of follicles progress, the number that continued to develop was de-
pendent on the amount of blood ingested and the availability of sucrose after the
blood meal. By 96 hours, the females had stage V follicles, and with a full blood
meal, the number matured per female (210 8.3) was similar whether main-
tained on 10% sucrose or distilled water. However, after a partial meal, the num-
ber of follicles per female that matured on distilled water was almost half that
matured on 10% sucrose. Yet more of these partially fed females had developing
follicles on distilled water, while more of them had resorbed follicles on 10%
sucrose. The appearance of functional resorbed follicles in partially fed females
was also demonstrated inAe. aegypti (Lea et al. 1978).
The amount of time required for Cx. nigripalpus eggs to mature to stage V is
temperature dependent, taking 72 hours at 300C, 96 hours at 240C and 168
hours at 180C (Nayar & Knight 1981). When maintained at these temperatures,
more females had mature stage V follicles at the higher temperatures (240 and
300C) than at the lower (180C). Temperature also affected the resorption of fol-
licles depending on the amount of blood ingested. When trace amounts of blood
were ingested, more resorbed follicles occurred at 180C than at 300C. However,
resorbed follicles did not appear in fully fed females maintained at either 24C
or 300C, but at 180C, 3.3% had resorbed follicles. Similar results were obtained
with Culex tritaeniorhynchus summorosus (Mogi et al. 1972), Cx. pipiens (Bel-
lamy & Brackan 1971), and Cx. nigripalpus (Edman & Lynn 1975). Therefore,
gut stretching previously hypothesized by some investigators is not a prerequisite
for activating of the ovaries, but the ovarian response is closely related to the
quantity of blood ingested, because, as the intake of blood is increased, more
eggs are matured.
In a mark-release experiment at Tiger Hammock during 1976, a synchronized
brood of newly emerged, unfed, Cx. nigripalpus was released into the field and
32P-marked resting adults were recaptured with portable aspirators from 7h30m
to 9h30m daily. Marked females had stage I oocytes until 42 hours after emerg-
ence. By 66 hours their development had proceeded to stage II, and stage III,
Table 13. Number (percentage) of recaptured 32P-marked Culex nigripalpus females with different stages of oocytes at different hours after
Age in Hours Oocyte Stage Mean No. of Stage V Oocytes/Female
After Emergence I II III IV V 32P-marked Unmarked*
18 54(100.0) -
42 54(100.0) -
66 21(100.0) -
90 8( 88.9) 1(11.1) -
114 4(44.4) 3(33.3) 2( 22.2) 190 200
138 2( 40.0)** 2(40.0)** 1 ( 20.0) 165 267
162 3( 42.9)** 4( 57.1) 211 252
186 1( 25.0)** 3( 75.0) 182 213
210 1(100.0) 126 159
*Seven unmarked gravid females were dissected at each time interval.
**Presumed or designated as parous.
IV, and V oocytes appeared from 90 to 138 hours after emergence (Table 13).
Marked gravid females were first collected at 114 hours, and all females re-
covered after age 138 hours were either gravid with eggs or parous that had re-
cently oviposited. Marked gravid females matured an average of 187 25.6
stage V oocytes per female with the range from 85 to 328 oocytes per female.
Unmarked gravid females captured at the same time in this experiment averaged
235 20.1 stage V oocytes per female with the range from 64 to 375 stage V
oocytes per female.
Upon investigating the effect of different blood sources on the ovarian devel-
opment of Cx. nigripalpus, no clear-cut relationship was found between the vol-
ume, weight, and caloric value of the ingested blood, and the number of eggs
developed and laid (Table 14, Nayar & Sauerman 1975c, 1977, Nayar unpub-
lished). Avian blood did not promote the development of more oocytes or the
oviposition of more eggs than mammalian blood on a per tl or /g basis. How-
ever, differences were observed when chicken blood and human blood were
compared on a per mg basis, since Cx. nigripalpus developed 19% more oocytes
per mg of chicken blood than per mg of human blood. In general, human blood
was inferior to all the other avian and mammalian blood sources in the develop-
ment of oocytes. The near absence of DL-isoleucine in human blood, when com-
pared to its titer present in other bloods, may be responsible for the development
of fewer eggs on human blood.
Judging from the great variety of larval habitats recorded in the literature (cf.
Larval Focal Distribution), the female is relatively non-selective in her choice of
During a study at Tiger Hammock from August to November 1978 and May
through June 1979, eggs were collected in tubs containing a hay-infusion me-
dium fortified with brewer's yeast, which was replenished daily. In each of six
experiments, 18 tubs were monitored daily over an 11-day period for each ex-
periment for newly laid egggs. Each day, the egg rafts were collected between
7h00m and 10h00m, brought to the laboratory, set up in individual vials, and
identified as first instar larvae (cf. Identification of first instar larvae). A pattern
in the oviposition was observed in a typical experiment, indicating that more
eggs were laid in the tubs when the relative humidity was low (<80% RH) dur-
ing the early part of the night (between 20h00m and 24h00m) and fewer were laid
when the relative humidity was high (>90% RH) (Fig. 10). This phenomenon
was more apparent when rainfall occurred prior to sunset, since very few eggs
were laid that night or the following morning before collections were made (Fig.
10). High humidity increases the flight activity of adults (Dow & Gerrish 1970,
Provost 1973) and may have allowed the gravid females to disperse from the
wooded areas seeking more natural breeding waters rather than ovipositing in
the tubs. Such an increase in flight activity has been documented by the capture
of large numbers of Cx. nigripalpus during or immediately following a rainy
Table 14. Quantitative analysis of a blood meal ingested to repletion from seven vertebrate hosts by 11-day-old sugar-fed 9 9 of F1 Culex nigripalpus
Parameters Chicken Dove Owl Gerbil Rabbit Mouse Human
a 4.35 3.79 4.19 5.18 3.47 3.58 3.92
b 0.58 0.78 0.75 0.78 0.76 0.78 0.75
c 0.95 0.06 1.76 0.12 1.32 0.10 1.74 0.13 1.33 0.12 1.65 0.11 1.86 0.12
d 0.13 0.21 0.18 0.15 0.22 0.22 0.19
e 0.22 0.46 0.32 0.34 0.38 0.46 0.47
f 160 9 179 8 175 9 159 5 169 15 173 9 116 4
g 37 47 42 31 49 48 30
(a) Mean wet weight of blood ingested/ 9 (mean of 5 samples of 4 9 /sample)
(b) Mean dry weight of blood ingested/ 9 (gg/ 9) (mean of 5 samples of 4 9 9/sample)
(c) Calories (mean SE) of blood ingested, analyzed individually (10 9 9/treatment) by the biochromate oxidation method.
(d) Dry weight (mg) per mg of ingested blood.
(e) Calories per mg of ingested blood.
(f) The mean number (LSE) of matured oocytes (25 9 9/group)
(g) The number of oocytes matured per mg of wet weight of blood ingested
SOURCE: NAYAR & SAUERMAN, 1977 (modified)
T 1000- -100 -IOC
* o--o---.o.--.--o--- o
a / 1 lo
I I \
\ 00 I -90 -80
S00- 1 -90 -20
W., I I %I
" I II
200 -70 -40
o -i i- | -- --U U -- -- i -- -- -- U -40
0 2 4 6 8 10 12
DAYS OF COLLECTION
Figure 10. Oviposition pattern by Culex nigripalpus in the field in artificial oviposition
period (Bidlingmayer 1971, 1974, Boike 1963, Dow & Gerrish 1970, Provost
1969, 1973). On the other hand, the rainfall may have diluted the infusion me-
dium and thus reduced any odors emanating from the water. After a few days,
fermentation of the infusion would have again produced an odor strong enough
to stimulate gravid females to lay eggs. Observations in the field substantiate
that more egg rafts are laid a day or two after a rainfall of 25 to 30 mm.
In the laboratory, F1 (wild) Cx. nigripalpus females seldom became insemi-
nated, and even though they blood-fed and developed mature eggs, they did not
lay these eggs. Colonized females became inseminated, blood-fed, and devel-
oped eggs. However, only a small percentage oviposited immediately after the
eggs were mature (Nayar & Pierce 1980). By 2 weeks, approximately 18% of
the females had laid eggs and in the subsequent seven weeks, 40% of the females
oviposited. During this nine-week period and after repeated blood meals, 66.7%
of the females laid eggs but many never oviposited. Wild blood-fed females
were collected in a chick-baited lard can trap and allowed to oviposit under lab-
oratory conditions. After five days an average of 23% had oviposited, and during
the following four days, a total of 50% of the females oviposited (Nayar & Pierce
1980). This indicates that under laboratory conditions, Cx. nigripalpus does not
lay eggs as soon as it matures but rather waits for the proper stimulus.
Oviposition by both colonized and wild caught blood-fed females is not syn-
chronized, as a small number of females oviposit each day rather than in a peak.
Since few females oviposit in the laboratory, either the conditions for oviposition
are not satisfactory or other undetectable factors are present which inhibit their
oviposition (Nayar & Pierce 1980).
In a mark-release experiment at Tiger Hammock during 1976, when a syn-
chronized brood of newly emerged, unfed Cx. nigripalpus was released into the
field, and marked resting adults were recaptured with portable aspirators from
07h30m to 09h30m daily, marked females with stage II parous ovaries with one
distinct dilatation per ovariole appeared at 138 hours of age. This suggested that
these females had oviposited during the previous night. Many blood-fed, 138-
hour-old resting females with oocytes in stages III to V were collected. Some of
them were parous, as they had already started to mature the next oocyte. All
marked females recovered had parous stage II ovaries by 222 hours of age, or
about 72 to 96 hours after the appearance of the first parous females. Biparous
marked females were recaptured at 222 hours of age. Only one marked female
was recaptured 3 weeks after release with three dilatations.
Comparison of nulliparous and parous marked resting females showed that
3% of the total recaptured were parous. A similar comparison among the daily
collections of unmarked females captured showed 43.0% of the captured were
The 50% survival time of newely emerged, unfed Cx. nigripalpus (colonized
and Fi) varies from 2.5 to 3.5 days for males and 2.5 to 4.5 days for females
(Nayar 1968b, Nayar & Pierce 1977, 1980). Therefore, after emergence, adult
Cx. nigripalpus cannot survive either in the laboratory or in nature at summer
temperatures much beyond 3 or 4 days unless they replenish their energy re-
serves. When female Cx. nigripalpus ingested a meal of 50% sucrose they had
a 50% survival time of 8.5 days (204 hours) and a 100% mortality at 9.1 days
(216 hours) (Nayar & Sauerman 1975a).
The 50% survival times of mosquitoes on a meal of sugar vary with the con-
centration of sugar they imbibe. This was demonstrated with Ae. sollicitans by
Van Handel (1965) and has also been observed in Cx. nigripalpus. Both colo-
nized and Fi adults of Cx. nigripalpus maintained ad lib. on 5%, 10%, and 25%
sucrose solution at 270C and 75% RH, had 50% survival times of 37.5 to 38.3,
53.4 to 53.5, and 42.2 to 50.9 days, respectively for females, arid 19.8 to 23.5
days at all three concentrations for males (Nayar & Pierce 1980).
The 50% survival time for F1 Cx. nigripalpus females maintained ad lib. on
10% sucrose at 270C and 70% RH was 65 days (Nayar & Sauerman 1973a). The
prolonged survival of most mosquitoes maintained on sucrose indicates that the
sucrose alone satisfies the metabolic requirements for general maintenance and
flight (Galun & Fraenkel 1957, Nayar & Sauerman 1971, 1973a, 1975a), with-
o 150- -30W
-1976- -- 1977 ---
Figure 11. Survival of field collected Culex nigripalpus at outdoor temperatures.
out undue reduction of tissue nitrogen (Thayer & Terizan 1971, Van Handel
1965). After 2 weeks of ad lib. feeding on the sucrose solution, the energy re-
serves (glycogen, and triglyceride, or caloric reserves) of these females reached
a maximum level (Nayar & Sauerman 1973a, 1975a), and provided continuous
energy for their normal physiological functions, such as flight, respiration, mat-
ing, and general metabolism. Triglyceride, in addition to free sugar, served as
the substrate for general maintenance and respiration (Nayar & Van Handel
1971), and its availability in large quantities prolonged survival. Therefore, en-
ergy reserves declined as the females approached 50% survival time.
When wild blood-seeking females collected from the field from October 1976
to August 1977 were maintained on distilled water under outdoor conditions,
their 50% survival times varied inversely with temperature. A high 10-day mean
temperature after the collection produced a 50% survival time of 53 to 112
hours; however, when the temperature was lower, the 50% survival times were
129 to 256 hours (Fig. 11). These survival times were similar to those observed
in unfed females at the same temperatures.
It is impossible to calculate the 50% survival time of mosquitoes in the field,
so other methods must be used (Service 1976). During one study, marked Cx.
nigripalpus adults were recovered with CDC light traps, assuming that these
light traps caught active females without bias and that the mortality rate was
constant from day to day. The daily survival rate was calculated from the num-
ber of marked females recovered each day (Dow 1971), and was found to be 0.81
or 81% daily for 176 radioactive females recaptured.
In 1976 at Tiger Hammock, a series of four experiments was conducted re-
leasing 32P-marked Cx. nigripalpus into the field. Resting adults were recap-
tured with portable aspirators and blood-seeking females were recaptured with
lard can traps baited with chicks and COz. In all four releases, the number of
marked adults that were recaptured increased steadily during the first 3 days and
then gradually declined during each successive day. For combined data from
four experiments, for the first 4 days, 61.6% of all females were recovered,
28.9% were recovered during the second 4 days, and 9.5% during the final 3 to
4 days. The decline in recovery was not as sharp for the males, since 47.4% of
the males were recovered during the first 4 days, 38.5% during the second 4
days, and 14.4% during the final 3 days (Nayar et al. 1980). Fewer adults were
collected during the first 2 days after release than during the 3rd day, suggesting
that during these first 2 days most of the dispersal took place. Assuming that the
mortality rate did not change from day to day, the daily survival rate, calculated
from the number caught each subsequent day (Gillies 1961), was 66% in Au-
gust, 75% in early September, 79% in late September, and 76% in October(Fig.
12), giving an average daily survival rate of 72% for the females (Nayar et al.
1980). The daily survival rate of the males during these 3 months was 84%.
Similar results were obtained with Culex tarsalis where the daily survival rate
varied between 64 to 77% from June to September (Nelson et al. 1978).
CORRELATION OF THE DAILY SURVIVAL RATES WITH
POPULATION ON DENSITY AND VECTOR POTENTIALS
A higher daily survival rate for Cx. nigripalpus is associated with moderate
night temperatures and higher humidities due to rainfall that occur in south-
central Florida from August to October. Mathematical analysis of the relation-
ship between the daily survival rates, blood-feeding, and oviposition reveals
that below a 70% daily survival rate, less than 1% of the original brood lives
beyond 13 days and takes the epidemiologically important second or third blood-
meal (Fig. 13). However, 3% of the brood lives beyond 13 days at a 75% daily
survival rate, and 6% of the brood at an 80% daily survival rate (Fig. 13). An
increase in the daily survival rate, therefore, raises the reproductive potential
of the population, since the population is barely replaced in the next generation
when the daily survival rate is only 70%, but at a daily survival rate of 80%,
the population can easily be doubled (Table 15). Natural weather conditions
vary from day to day, and daily survival rates fluctuate accordingly, but if both
the weather conditions and available food sources are optimal, daily survival
rate remains high and the density of the population can greatly increase.
Considering vector potentials, survival for 13 to 19 days with two to three
blood-feedings is necessary for the transmission of a pathogen (Figure 14),
which requires 12 to 15 days of incubation in the mosquito (Sudia & Chamber-
lain 1964, Young et al. 1977, Nayar & Sauerman 1975d). On the average, the
Cx. nigripalpus female is generally a poor vector since it usually does not sur-
vive much beyond 13 days. However, when environmental conditions remain fa-
vorable, the females live longer and their numbers increase several-fold with a
better chance for multiple blood-feeding. Consequently, Cx. nigripalpus can be-
come an excellent vector.
0 2 4 6 8 10 12
MEAN DAILY SURVIVAL= 66%
c. LATE SEPTEMBER 1976
0 2 4 6 8 10 12
b.EARLY SEPTEMBER 1976
0 2 4 6 8 10 12
MEAN DAILY SURVIVAL=75%
d. OCTOBER 1976
2 4 6 8 10 12
2 4 6 8 10 12
MEAN DAILY SURVIVAL=79% MEAN DAILY SURVIVAL=76%
DAYS AFTER RELEASE
Figure 12. Daily mortality rate of "P-marked Culex nigripalpus females in experiments
conducted from August to October 1976 at Tiger Hammock.
4 6 8
60% 65% 70%
10 12 14
N & 2nd OVIPOSITION &
EAL 3rd BLOOD- MEAL
DAYS AFTER EMERGENCE
Figure 13. Relationship of daily survival rate with blood-feeding and oviposition in Cu-
lex nigripalpus. Mark-release experiments suggested that Cx. nigripalpus females ac-
quired their first blood meal on day 2 after emergence. This was followed by first
oviposition during days 6 to 8, and second blood meal immediately following oviposition.
Second oviposition and third blood meal occurred during days 11 to 13 after emergence.
Regression lines at different daily survival rates indicate percent surviving at any given
time after emergence.
Table 15. Maximum potentials for an increase of adult populations. Hypothetical
population to start with 20,000 adults (10,000 9 9).
Mean No. of Eggs
Laid at Different Assuming Maximum
Daily Survival Rates of 10% of First
(200 eggs/ 9 9) Instar Larvae Proportion of
at on day 7 Become Adults Original Population
90% 960,000 96,000 x4.800
80% 420,000 42,000 x 2.100
70% 164,800 16,480 x 0.824
60% 56,000 5,600 x 0.280
50% 15,600 1,560 x 0.078
on day 12
90% 560,000 56,000 x 2.800
80% 137,400 13,740 x 0.680
70% 27,600 2,760 x 0.140
60% 4,400 440 x 0.022
50% 400 40 x 0.002
Duration in Newly emerged mosquito
Post-emergence maturation &
2-3 nectar feeding
Mating insemination &
First blood-meal (Possible intake of the pathogen)
Maturation of ovaries,
5-7 Maturation of the pathogen &
Maturation of ovaries & pathogens &
1-2 Nectar-feeding &
Third blood-meal (Possible transmission of the pathogen)
Figure 14. Hypothetical mosquito vector strategy in tropical climates.
Cx. nigripalpus is a vector for several viral agents and malarial and filarial
parasites that attack both man and other vertebrates. The ability of Cx. nigripal-
pus to support a number of pathogens makes it of great epidemiological impor-
The most important virus associated with Cx. nigripalpus is St. Louis en-
cephalitis (SLE) (Table 16) (Chamberlain et al. 1964, Dow et al. 1964, Lewis
et al. 1964). Presumably, Cx. nigripalpus was responsible for the 1958 outbreak
in Miami and was later proved to be the vector in the Tampa Bay area epidemics
during 1959, 1961, and 1962 which resulted in 315 cases and 55 deaths, and
caused adverse economic effects in Florida (Bond 1969, Bond et al. 1965,
1966, Hammon et al. 1966, Lewis et al. 1966, Quick et al. 1965). Sudia and
Chamberlain (1964) experimentally confirmed through transmission studies
that Cx. nigripalpus is a vector of SLE virus in Florida. One confirmed and two
presumptive cases were discovered during 1969 in Polk County (Wellings et al.
1972), but in 1977, a SLE epidemic in rural south-central Florida resulted in
110 laboratory confirmed or presumptive cases with eight deaths (Yeller 1978).
In 1979 six confirmed and three presumptive cases with no deaths were re-
corded again from south-central Florida (Arbogram 1979).
SLE virus isolates have always been made from Cx. nigripalpus during the
years when human cases were reported (Table 17), but usually no isolates were
made during the intervening years (Taylor et al. 1969, Wellings et al. 1972).
SLE virus has also been isolated from Cx. nigripalpus in Trinidad (Aitken et al.
1964) and Jamaica (Belle et al. 1964). However, whether Cx. nigripalpus is the
only or the major vector of SLE virus in Florida has not been satisfactorily con-
firmed, since single isolates were also made from Culex (Melanoconion) sp. and
Anopheles crucians during the 1962 epidemic (Chamberlain et al. 1964, Dow et
Another important virus associated with Cx. nigripalpus is Eastern Equine
encephalitis (EEE), which comprised 27% of the total viral isolates from 1962
to 1970 in the Tampa Bay area (Table 16) from May to November (Taylor et al.
1969, Wellings et al. 1972). In Trinidad during 1959 and 1960, EEE virus was
isolated from Cx. nigripalpus females collected in chick-baited traps (Downs et
al. 1959). The virus was originally isolated from Cx. nigripalpus in May 1959,
when North American birds had begun their northward migration back from
Trinidad. In view of the evidence of the north-south transport of the EEE virus
by migratory birds (Lord & Calisher 1970, Calisher et al. 1971), and the rec-
ognized bird-mammal feeding habits ofCx. nigripalpus (Edman & Taylor 1968,
Table 16. Arboviruses associated with Culex nigripalpus.
Infection Transmission Geographical
lab wild lab wild Location No. Isolates 1960-1979 References
St. Louis encephalitis (SLE) + + + Florida 78 Chamberlain et al. 1964, Dow
Eastern equine encephalitis
Venezuela encephalitis (VEE)
California encephalitis (CE)
Poxivirus avium from wild
etal. 1964, Lewis etal. 1964,
Aitken et al. 1964
Belle et al. 1964
Scherer, W F (Personal
Wellings et al. 1972
Downs et al. 1959
Theiler & Downs 1973
Harwood & James 1979
Rodaniche & Galindo 1961
Aitken & Spence 1963
Theiler & Downs 1973
Taylor et al. 1969
Theiler & Downs 1973
Wellings et al. 1972
Wellings et al. 1972
Wellings et al. 1972
Akey, Nayar & Forrester
Table 17. Number of human cases and isolates of St. Louis Encephalitis from wild
collected mosquitoes from 1959 to 1979.
Year Cases No. of Isolations Citation
1959-1962 315 42 Bond et al. 1965, Chamberlain et
al. 1964, Dow et al. 1964,
1964 1 1 Taylor et al. 1969
1969 1 4 Wellings et al. 1972
1977 110 26 Arbogram** 1977
1979 9* 5 Arbogram** 1979
*Six confirmed human cases were identified, and as of the end of 1979 there were three additional
**DHRS publications for administrative use only.
Edman 1974), the possibility of this species becoming infected and transmitting
the disease to humans in Florida is very real (Wellings et al. 1972).
Other viruses (Table 16) which have been isolated from Cx. nigripalpus are
of minor importance with little significance at the present time.
During 1977 and 1978, more than 21,000 females of 15 mosquito species
were trapped alive in an area in south Florida where many wild turkeys (Melea-
gris gallopavo) harbor malarial infections. By inoculating mosquito slurries into
uninfected domestic poults, Plasmodium hermani was demonstrated to be pres-
ent in Cx. nigripalpus, with one isolate being made in 1977 and two in 1978
(Table 18) (Forrester et al. 1980). Cx. nigripalpus previously shown to be a com-
petent experimental vector (Table 18) (Young et al. 1977), is thus believed to be
the primary natural vector of wild turkey malaria in Florida.
In a Vero Beach residential area, Cx. nigripalpus was the major species (39%)
collected, with 0.5% harboring various stages of Dirofilaria immitis (a dog
heartworm) (Table 18). When Nayar & Sauerman (1975d) fed 400 F, Cx. nigri-
palpus females on an infected dog, 10% became infected with 2 to 6 larvae per
female, and an average of 3 infective larvae in the proboscis. This indicates that
Cx. nigripalpus is potentially a natural vector ofD. immitis in Florida. No at-
tempts were made to transmit these parasites.
Recently, avian poxvirus was isolated from sentinel birds and experimentally
transmitted by Cx. nigripalpus (Akey, Nayar and Forrester 1981).
Table 18. Parasites associated with Culex nigripalpus.
Infection Transmission Geographical
Parasites lab wild lab wild Location No. Isolates References
Plasmodium hermani + + + + Florida 3 Young et al. 1977
(Wild turkey malaria) Forrester et al. 1980
Dirofilaria immitis + + Florida 3 Nayar & Sauerman 1975d
(dog heartworm) Sauerman & Nayar (unpublished)
ANTAGONISTS AND POTENTIAL BIOLOGICAL CONTROL AGENTS
The larvae of Cx. nigripalpus are attacked by both internal and external par-
asites. The internal parasites being evaluated as potential biological control
agents for use in integrated mosquito control programs are: (a) a nematode par-
asite, Romanomermis culicivorax (Levy & Miller 1978), (b) a fungal pathogen,
Lagenidium giganteium L. (culicidum) (McCray et al. 1973, Umphlett 1973),
(c) a naturally occurring fungal pathogen, Helicosporidium near parasiticum
(Fukuda et al. 1976), and (d) a bacterium, Bacillus sphaericus (Hertlein 1978).
External parasites collected along with Cx. nigripalpus larvae include the
predaceous larvae of Psorophora ciliata and Ps. howardii. Mosquito-eating
fish, such as Gambusia and guppies, also occur in the permanent waters where
Cx. nigripalpus larvae are found.
There are numerous reports of predaceous vertebrates (particularly birds)
finding and feeding on localized concentrations of adult mosquitoes. In addition,
during a mark-release-recapture experiment, several species of dragonflies,
such as, Anaxjunius (Drury) were observed feeding on Ae. taeniorhynchus as
they were released from cages (Edman & Haeger 1974). Similar observations
of feeding by dragonflies were made when Cx. nigripalpus were released during
a study at Tiger Hammock during 1976 and 1978.
RESISTANCE TO INSECTICIDES
An insecticide resistance surveillance program against organophosphate in-
secticides in operation since 1964 at the West Florida Arthropod Research Lab-
oratory, Panama City, Florida, indicates that the larvae of Cx. nigripalpus in
the field do not show any difference in their susceptibility to malathion, naled,
and fenthion from that of the susceptible laboratory strain (Boike & Rathburn
1972, Boike et al. 1978, 1979, Rathburn & Boike 1967). Laboratory selection
of Cx. nigripalpus at the LD80 level for 31 generations resulted in no significant
increase in tolerance to Paris green when compared to an unselected laboratory
colony (Rathburn & Boike 1973).
However, several adult populations of Cx. nigripalpus have become less sus-
ceptible (3X to 5X) to malathion than the colonized laboratory strain (Boike
& Rathburn 1972, Boike et al. 1978, 1979, Rathburn & Boike 1967).
CONTROL OF Cx. NIGRIPALPUS POPULATIONS
Success in mosquito control depends on knowledge of the species of mosqui-
toes and their habits in the particular region where they are to be controlled. With
this knowledge, efforts to reduce mosquito production can be more successful.
Therefore, the information presented in this report should be of help to Mosquito
Control personnel to achieve better control of this species in Florida.
Rathburn (1979) has detailed controlling methods for both larvae and adults
of Cx. nigripalpus and other mosquito species. These methods and others rec-
ommended for control of Cx. nigripalpus (Control of St. Louis Encephalitis
1976) are summarized below:
CONTROL OF LARVAE
a) Non-Chemical Control. One of the main non-chemical control meth-
ods is the permanent elimination of breeding sites in an environmentally
acceptable manner. Filling open ditches, subsoil drainage, and pumping
and diking are all effective in controlling breeding sites. Keeping fresh
water ponds, sewage stabilization lagoons, and open storm sewers free of
vegetation reduces mosquito production. In grove swales and irrigated
lands, water should not be allowed to collect and stand for more than 3 to
4 days at a time, since the larval development of Cx. nigripalpus requires
6 to 9 days (see 'Larval Development'). Borrow pits should be constructed
with steep shorelines and kept free of vegetation. The proper grading of
fields eliminates standing water and the mosquitoes that breed there.
b) Biological Control. -Biological agents mentioned above (see Antago-
nists) should be used when available.
c) Chemical Control. In areas which cannot be drained or filled at an
acceptable cost and where impounding or biological control is not possi-
ble, larviciding is a reasonable alternative. Insecticides that are currently
registered for use as larvicides are as follows:
chlorpyrifos (Dursban)** ............. 0.0125 to 0.05 lb/acre, or
1.0% briquettes (10% encapsulated at 1.5 ppm)/20 ft2 in small pools
(McDonald & Dickens 1970).
fenthion (Baytex)** .................. ... 0.05 to 0.10 lb/acre.
malathion (Cythion)** ................... . 0.4 to 0.5 Ib/acre.
parathion, ethyl ............................ .0.1 lb/acre.
parathion, methyl ............................ 0.1 Ib/acre.
temephos (Abate)** ..................... 0.05 to 0.1 lb/acre.
*These groups of insecticides have not been recommended for use in Florida by the
FAMA or DHRS as larvicides since 1957.
**Trade names in parentheses.
ii) Chlorinated hydrocarbons:*
methoxychlor ................... .. 2.0 lb/acre, as a prehatch.
iii) Petroleum Oils:
Diesel fuel oil No. 2 ....................... 10 to 20 gal/acre.
without spreading agent
Diesel fuel oil No. 2 ......................... 1 to 5 gal/acre.
with spreading agent
Propriety mosquito control oils (as Flit MLO, ARCO larvicide, GB-
1313, and Florida mosquito larvicide) ........... 1 to 5 gal/acre.
iv) Insect Growth Regulators (IGRs):
methoprene (Altosid SR-10, SR-10 sand granular, or Altosid
briquettes) .......................... 0.020 to 0.027 lb AI/acre.
All insecticides must be applied in strict compliance with the label and local,
state, and federal regulations.
An integrated control strategy which includes all methods that both reduce
mosquito populations and exert a minimum of harmful effects on the environ-
ment should always be the preferred approach. This includes environmental
management, the judicious application of insecticides, and the use of predator
fish and insect growth regulators. Such a plan should retard resistance to insec-
CONTROL OF ADULT
The control of adult Cx. nigripalpus in Florida is primarily based on chemical
methods (Rathburn 1979). However, most chemical control methods provide
only temporary relief, since unaffected mosquitoes usually disperse into the
sprayed areas after the spraying has stopped. Space spraying operations are most
effective in the early evening and at night, or in the early morning when the air
is cool and the wind velocity is low. Methods of formulations and application
used for space spraying are found in detail in Rathburn (1979) and in Control of
St. Louis Encephalitis (1976): spray formulations are summarized in the follow-
Ground application. Ultra-low volume (ULV) application with ground
equipment is at present the most popular method used to control adult Cx. ni-
gripalpus in Florida and can be very effective if used properly (Rogers 1978).
ULV requires a very small amount of a highly concentrated insecticide, e.g.,
chlorpyrifos at 2.1 fluid ounces (6.2 mL) per minute at 10 mph (16 km/h); fen-
thion at 0.3 to 2.0 fluid ounces (0.9 to 5.9 mL) per minute at 5 to 20 mph (8 to
32 km/h); malathion at 1.0 to 4.3 fluid ounces (3.0 to 12.7 mL) per minute at 5
to 10 mph (8 to 16 km/h); naled at 0.6 to 18 fluid ounces (1.8 to 53 mL) at 5 to
15 mph (8 to 24 km/h); pyrethins at 2.0 to 2.25 fluid ounces (5.9 to 6.7 mL) per
minute at 5 mph (8 km/h); and resmethrin at 9.1 fluid ounces (27 mL) at 5 mph
(8 km/h) (Rathburn 1979). These six insecticides have EPA label approval for
application as ULV aerosols by ground equipment in Florida (Rathburn & Boike
Thermal aerosol, mist, and dust applications have been used successfully for
many years, and are as effective as ULV applications. Tests in Florida with
ground dispersed dusts (19% and 7.5%) of carbaryl produced a 99% reduction
in the number of adult salt marsh mosquitoes at a dosage of 0.2 and 0.3 pounds
(90 g and 136 g) per acre (0.4 ha).
Thermal aerosol formulations currently recommended for use are chlorpyri-
fos, fention, and malathion at 40 fluid ounces (120 mL) per minute at 5 mph (8
km/h) and naled at 80 to 120 fluid ounces (240 to 350 mL) per minute at 10 to
15 mph (16 to 24 km/h) (Rathburn 1979).
Mist formulations currently recommended for use are chlorpyrifos, fenthion,
malathion, and propoxur at 100 fluid ounces (300 mL) per minute at 4 mph (6.5
km/h) and naled at 0.1 pound (45 g) Al per acre (Rathburn 1979).
Aerial application. Airplanes have been used for many years to apply in-
secticide dusts, pellets, sprays, and aerosols. The aerial ULV technique applies
0.5 to 3.0 ounces (16 to 93 g) of highly concentrated insecticide per acre to con-
trol adult mosquitoes. Three insecticides are currently approved for ULV appli-
cation from airplanes: malathion at 3 fluid ounces (9 mL) per acre, naled at 0.5
to 1.0 fluid ounce (1.5 to 3.0 mL) per acre, and pyrethrins and pip. butoxide at
0.06 ounces to 0.1 pound (1.8 to 45 g) AI per acre (Rathburn 1979, Rathburn
& Boike 1972, 1975).
Insecticides recommended for low volume aerial application are chlorpyrifos
at 0.025 to 0.05 pound (11 to 23 g) AI per acre, fenthion at 0.05 to 0.10 pound
(11 to 45 g) AI per acre, malathion at 0.15 to 0.36 pound (68 to 160 g) AI per
acre, naled (Tech.) at 0.05 to 0.10 pound (11 to 45 g) AI per care, and propoxur
at 0.05 to 0.175 pound (11 to 79 g) AI per acre (Rathburn 1979).
Some insecticides are also recommended for use as aerial thermal aerosols.
They are fenthion at 0.03 pound (14 g) AI per acre, malathion at 0.20 pound (90
g) AI per acre, and naled at 0.087 pound (39 g) Al per acre (Rathburn 1979).
Other Methods. Residual treatments for mosquito control are used in lim-
ited outdoor areas, such as small city parks, playgrounds, and picnic areas.
Water suspensions or emulsions with a low percent of insecticide are used to
The use of any insecticide to control the adult reaches only a small portion of
the total mosquito population and thus provides only temporary relief. Therefore
it is more economical to concentrate most control effort toward the larval stage.
Rogers (1978) recently presented a historical overview of mosquito control in
Florida for both pest and vector species.
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I thank Dr. John A. Mulrennan, Jr. and Elisabeth C. Beck of the Office of Entomol-
ogy, DHRS, Jacksonville; Dr. Carlisle B. Rathburn, Jr. and Arthur H. Boike of West
Florida Arthropod Research Laboratory, Panama City; Drs. M. D. Young and D. J.
Forrester of the Department of Preventive Medicine, College of Veterinary Medicine,
University of Florida, Gainesville; and Drs. Samuel G. Breeland, and D. M. Sauerman,
Jr., Mrs. P. A. Pierce and Mrs. J. W Knight of the Florida Medical Entomology Labo-
ratory, Vero Beach, for reading the manuscript and making helpful suggestions. I also
thank J. S. Haeger, D. M. Sauerman, Jr., C. W Hansen, L. A. Webber, J. W Knight,
R. A. Crossman, Jr., P A. Pierce, E D. S. Evans and others of the FMEL for their as-
sistance during the conduct of this study, and especially to'Mrs. P. A. Pierce for editorial
assistance. Appreciation is also expressed to Linda Rezmer and Jean Griesser for typing
This investigation was supported in part by the National Institutes of Health Grant
This publication was promulgated at a cost of $5,343.73, or $1.78
per copy, to provide information about Culex nigripalpus, an im-
portant disease vector in Florida.
All programs and related activities sponsored or assisted by the Florida
Agricultural Experiment Stations are open to all persons regardless of race,
color, national origin, age, sex, or handicap.
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