INTRODUCTION
Bates (1949) was the first mosquito biologist to classify mosquito
life cycles into logical groupings. He did so by dividing the
life cycle types into 4 temperate and 4 tropical categories. His
system separated species that bred continuously from those that
had life cycle types with overwintering mechanisms. Bates' type
species for temperate areas included: Aedes cinereus, Aedes
caspius, Anopheles claviger and Culex pipiens.
Bates chose not to use type species for the tropical life cycles
and designated his groupings by genus or breeding habitat. Unfortunately,
the classification system developed by Bates has limited value
for mosquito control workers in New England. Two of his type species
occur only in Europe and one of his major life cycle types is
not exhibited by any North American mosquito. As a result, the
classifications of Bates have value as a model but the system
cannot be applied to many mosquitoes found in the USA
Pratt (1959) proposed an alternate system that relied on 3 biological characteristics: 1) overwintering stage, 2) place where the eggs are laid, and, 3) number of generations each year. Pratt's system resulted in eleven life cycle types, including seven exhibited by mosquitoes in New England. Pratt's system is considerably more useful but is not without problems. His simplified classification covers so broad a geographic area that a single species can exhibit as many as 3 different life cycle types over its natural range. The system is also so generalized that salt marsh, fresh floodwater and container breeding pests are all grouped together in a single life cycle type.
In the system we propose, we limit life cycle classification to
northeastern species to minimize the wide variation one sees transcending
tropical, temperate and arctic conditions. We present information
here only for New England but hope to eventually expand classifications
for the entire northeastern region. We use a common representative
species to designate each life cycle type to avoid the confusion
of alternating between habitat, generic and species categorization.
Lastly, we have added larval habitat to the classification system
to emphasize its usefulness for control purposes. As a result,
our system is based upon 4 characteristics including: 1) where
the egg is laid, 2) typical larval habitat, 3) number of generations
per year and 4) stage of the life cycle that overwinters. Using
this designation we recognize 13 life history types for mosquito
species found in New England states.
PROPOSED LIFE CYCLE CLASSIFICATIONS
Aedes communis Type
This is the univoltine northern Aedes life cycle typical
for snow pool mosquito species of the northern United States and
Canada. Typical larval habitats are characterized by lack of aquatic
vegetation and thick lining with leaves. In many cases tannins
from decomposing leaves turn the habitat water dark brown. When
the trees that shade this habitat leaf out, the pools dry down
completely and become mere depressions in the forest floor. Eight
New England species fit into this category including: Ae. communis,
Ae. diantaeus, Ae. excrucians, Ae. implicatus, Ae. intrudens,
Ae. provocans, Ae. punctor and Ae. stimulans.
Aedes abserratus Type
This is a variation of the univoltine northern Aedes life
cycle that includes Culiseta species with winter hardy
eggs and univoltine Aedes found in permanent rather than
transient water habitats. Most of the mosquitoes in this group
can be associated with some form of aquatic vegetation and selective
dipping produces greater numbers of larvae than dips that are
performed on a random basis. Rarely are any of the Aedes included
in this group found in woodland pool situations. Six New England
species fit into this category including: Ae. abserratus, Ae.
aurifer, Ae. decticus, Ae. fitchii, Cs. morsitans and Cs.
minnesotae.
Aedes canadensis Type
This represents a second variation of the univoltine northern
Aedes that includes species generally listed as having
a single generation but frequently reappear more than once during
a single breeding season. Larvae that hatch from eggs in late
spring probably represent a portion of the overwintering population
that requires more than one flooding for complete egg hatch. The
rather large broods that sometimes appear in fall raise questions
regarding the accuracy of these species being classified as univoltine.
Three New England species fit into this life cycle type including:
Ae. canadensis, Ae. cinereus and Ae. sticticus.
Aedes vexans Type
This life cycle type includes the multivoltine Aedes /
Psorophora commonly referred to as floodwater mosquitoes.
The entire group undergoes accelerated larval development and
can pupate within 4-5 days of egg hatch if water temperatures
are favorable. We have excluded mosquito species with salt tolerance
for control purposes. Five species are found in New England including:
Ae. vexans, Ae. flavescens, Ae. trivittatus, Ps. ciliata and
Ps. ferox.
Aedes sollicitans Type
This life cycle type represents a variation on the multivoltine
Aedes / Psorophora life cycle used by mosquitoes with desiccation
resistant eggs. The species included in this grouping have considerable
salt tolerance allowing them to use the vast expanses of salt
marsh wetlands along the coast that are unsuitable for most multivoltine
floodwater species. Three salt marsh Aedes use this life
cycle in coastal New England including: Ae. sollicitans, Ae.
cantator and Ae. taeniorhynchus.
Aedes triseriatus Type
This life cycle type represents another variation on the multivoltine
Aedes / Psorophora life cycle. The species
included in this category deposit their eggs just above the water
line in containers that will eventually be filled with rainwater.
Like most multivoltine species with desiccation resistant eggs,
rainfall patterns determine the number of generations each year.
Repeated rainfall results in frequent changes in water levels
and larval populations of mixed instars. Two Aedes exhibit
this life cycle type in New England, Ae. triseriatus and Ae.
henderson. If Ae. albopictus every reaches New England,
it would exhibit an Ae. triseriatus type of a life
cycle.
Anopheles quadrimaculatus Type
This life cycle type has been described as the basic multivoltine
Culex / Anopheles life cycle represented by Culex
pipiens in the classification systems of both Bates and Pratt.
The eggs must remain moist to hatch and are, therefore, laid directly
on the surface of the water instead of an area that will flood
at a later date. Breeding is continuous, thus, all instars are
represented in typical breeding habitat. Culex pipiens,
the type species for this life cycle in previous classification
systems, is pollution tolerant and rarely found with most multivoltine
Culex and Anopheles. The breeding habitat for the
life cycle type we are presenting always includes clear water
that supports emergent vegetation. As a result, we have excluded
Cx. pipiens from this swamp life cycle and include only
species that are non-pollution tolerant. Five New England species
utilize this cycle including: An. quadrimaculatus, An. earlei,
An. punctipennis, Cx. territans and Uranotaenia sapphirina.
Culex salinarius Type
This life cycle type is the multivoltine Culex / Anopheles
counterpart of the life cycle utilized by floodwater Aedes
with salt tolerance. The eggs, however, are deposited directly
on standing water rather than moist mud exposed at low tide. Few
of the species in this group breed directly on tidal marshes.
Most reach greatest abundance in areas adjacent to salt marshes
where fresh water from the upland drains onto coastal habitats.
Each of the species in the group is capable of breeding in areas
where salt concentrations are minimal but all reach greatest concentrations
in coastal areas of the region. Two species utilize this life
cycle type in New England, Cx. salinarius and An. bradleyi.
Culex pipiens Type
This life cycle type represents a third variation on the multivoltine
Culex / Anopheles life cycle. Pollution tolerance
becomes a limiting factor and allows representatives to utilize
water that excludes most freshwater breeders. Species that belong
to this group tend to be urban, rather than rural, pests. Three
New England species exhibit this life cycle type including: Cx.
pipiens, Cx. restuans and Culiseta inornata.
Culiseta melanura Type
This life cycle type is a fourth variation of the multivoltine
Culex / Anopheles cycle with overwintering occurring
in the larval rather than the adult stage. Adult populations build
gradually during the course of the breeding season with a peak
in late summer to provide the larvae that ultimately overwinter.
Two species utilize this life cycle type in New England, Cs.
melanura and An. crucians.
Coquillettidia perturbans Type
The Coquillettidia perturbans life cycle type is monotypic
with only the type species representing the model. The larvae,
as well as the pupae, have respiratory apparatus capable of extracting
oxygen directly from submerged plant material. As a result, neither
larvae nor pupae come to the surface to breathe. Coquillettidia
are also unique because they take one full year to complete a
single generation. Overwintering is accomplished in the larval
stage in any instar trapped by the onset of winter.
Anopheles walkeri Type
The Anopheles walkeri life cycle type is also monotypic
represented solely by the type species in North America. Anopheles
walkeri has a unique winter hardy egg which distinguishes
it from other anophelines found in this habitat. Because it overwinters
in the egg stage, An. walkeri is the first anopheline to
appear as a larva in the spring. Anophelines that overwinter as
mated females are emerging from hibernation, finding a blood meal
host and producing eggs during the time frame that Anopheles
walkeri is completing larval development and preparing for
pupation.
Wyeomyia smithii Type
This life cycle type is typical for a number of container breeding
mosquitoes. The pitcher plant mosquito, Wyeomyia smithii
serves as the model but some treehole species behave similarly.
The Orthopodomyia may deposit their eggs in moist areas
of the habitat near the water line, rather than directly on the
water itself. This group overwinters in the larval stage, occasionally
frozen in the ice. Two species exhibit this type of life cycle
in New England, Wy. Smithii and Or. signifera. The predacious
species, Toxorhynchites rutilus septentrionalis also undergoes
this life cycle type but is only found in the extreme southern
portions of New England.
Aedes thibaulti, a species that is uncommon in the northeast,
does not logically fit into any of the categories we present.
Aedes thibaulti is a crypt breeder that is quite common
in the southern United States but shows isolated populations well
into Canada (Belton and French 1967). In New Jersey, the species
can occasionally be found with Cs. melanura in subterranean
crypts, in the darkest recesses of upturned trees and under
the base of hollowed out trees. Its breeding habitat has not been
located in New England. The species has a single generation each
year and overwinters either as an egg, if the larval habitat is
dry in fall, or as larvae that hatch in fall and spend the winter
in 2nd or 3rd instar (Lake 1967). Until
more is known of the exact life cycle type exhibited by this unique
single generation mosquito in New England, the species will remain
unclassified.
CONCLUSIONS
The life cycle classification system that we propose here should
be useful for mosquito biologists as well as mosquito control
agencies in the northeast. We have used common species representatives
to designate each of the life cycle types. A cursory knowledge
of basic mosquito biology should be all that is needed to recognize
differences between life cycle types. The system is being proposed
only for the northeastern United States at this time but can easily
be modified for other geographic areas of the country. Most of
the basic life cycle types will remain the same. We recommend
substituting locally common species as type representatives for
each of the cycles for use in other areas.
ACKNOWLEDGEMENTS
This is New Jersey Agricultural Experiment Station Publication
No. E-40101-03-97 supported by state funds and funds from the
New Jersey State Mosquito Control Commission.
REFERENCES CITED
Bates, M. 1949. The natural history of mosquitoes. The Macmillan Company, New York, NY. 379 pp.
Belton, P. and D.E. French. 1967. A specimen of Aedes thibaulti collected near Belleville, Ontario. Canad. Entomol. 99:1336.
Lake, R.W. 1967. Notes on the biology and distribution of some Delaware mosquitoes. Mosquito News 27(3):324-331.
Pratt, H.D. 1959. A new classification of the life histories of North American mosquitoes. Proc. NJ Mosquito Exterm. Assoc. 46:148-152.