A CLASSIFICATION SYSTEM FOR NORTHEASTERN MOSQUITO LIFE CYCLES
WAYNE J. CRANS1 and JAMES R. McNELLY2
1Mosquito Research & Control
Department of Entomology, Cook College
PO Box 231, Cook College New Brunswick, NJ 08903
2Cape May County Mosquito Commission
PO Box 66 Cape May Courthouse, NJ 08210
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
This is New Jersey Agricultural Experiment Station Publication No. E-40400-04-97 supported by state funds and funds from the New Jersey State Mosquito Control Commission.
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.
please e-mail comments and/or questions on this article to Wayne J. Crans