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Pest Status
Background
Host Specificity
Chapter 7
Culiseta inornata (Williston), a Mosquito
(Diptera: Culicidae)
J .A. SHEMANCHUK, H.C. WHISLER and S.L. ZEBOLD
Culiseta inomata (Williston) is a very common mosquito species in the Prairie
Provinces of Canada. It is found not only in the open country but also in the aspen
grove regions. Females of this species pass the winter as fertilized females in rodent
burrows, caves, root cellars, rock piles, and abandoned buildings. Generally, the
species does not appear in large numbers until late July. There are several generations
in a season. C. inomata is a common pest of livestock and occasionally of man, and is
a vector of western equine encephalomyelitis and California encephalitis (Morgante &
Shemanchuk 1967, Shemanchuk & Morgante 1967).
Concern about the environmental contamination with pesticides has led many
countries to examine the use of biological agents for the control of mosquitoes. An
aquatic fungus, Coelomomyces psorophorae Couch (Chytridiomycetes: Blastocladiales),
parasitic on larvae of C. inomata was discovered in the irrigated areas of southern
Alberta in 1956 and has been found there every year since. Larval mortalities up to
80% have occurred in some of the breeding ponds. This fungus was also found
infecting C. inornata larvae in the irrigated areas of Saskatchewan, which indicates a
wide geographic distribution.
The persistence of this fungus in the irrigated areas of southern Alberta and
Saskatchewan and the ability to colonize the host species of mosquitoes in the
laboratory prompted us to study further this particular host-parasite combination.
Laboratory experiments resulted in the discovery of a previously unknown obligate
alternate host the copepod, Cyclops vernalis, in the life-cycle of Coelomomyces
psorophorae (Whisler et al. 1974, 1975). In the life-cycle, the zygote seeks out and
attaches to the intersegmental membranes of the mosquito larva and penetrates the
cuticle leading to the development of hyphal bodies, mycelium, and ultimately thickwalled
resistant sporangia in the coelom of the larva, which kill the larva. Under
appropriate conditions, these sporangia release zoospores of opposite mating type
which infect the alternate host, Cyclops vernalis. Each zoospore develops into a
thallus and eventually gametangia. Gametes of opposite mating type fuse either in or
outside of the copepod to form the mosquito-infecting zygote.
The discovery of the life-cycle led to the development of a successful laboratory
procedure for the propagation of the fungus and to a better understanding of its basic
biology.
With the reliable technique which was developed for consistent and significant
infections in C. inornata in the laboratory, host specificity studies of Coelomomyces
psorophorae were possible. The possibility of infecting Aedes vexans (Meigen)
received particular attention because larvae of this mosquito with light infections of
Coelomomyces psorophorae have occasionally been found in field sites where heavily
infected larvae of C. inornata were also found. Larvae of A. vexans, A. dorsalis
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