The Cayman Islands are famous for their mosquitoes
which in the past reached levels that were unequalled anywhere else in the world.
In South Sound mangrove swamp in 1971 as many as 600 bites per minute on one
arm have been recorded. The record trap catch for one night in the Cayman Islands
is 793,103 from a single New Jersey light trap in Bodden Town in 1974. Out of
approximately thirty species occurring in Grand Cayman, six can probably be
regarded as constituting the main pest species. Mosquitoes, such as Psorophora
ciliata, Aedes sollicitans, Wyeomyia vanduzeei, are occasionally reported as
nuisance but these tend to be sporadic and short lived problems and are usually
highly localised in nature.The most Important Species of mosquito found in the
Cayman Islands are :
Aedes taeniorhynchus
Aedes taeniorhynchus is the most abundant and important
pest mosquito in Grand Cayman. MRCU was formed specifically to control this
species. Accordingly, the majority of the unit's efforts, both in research and
operations, has been to this end. Most of our aerial and ground spraying techniques
and applications, as well as physical/permanent control, have focused on Aedes
taeniorhynchus. More Information about mosquitoes of the Cayman Islands.
The black salt-marsh mosquitoes, Aedes taeniorhynchus,
is by far the most serious pest species. The MRCU was formed specifically to
bring about its control. It is also the main pest species in Little Cayman and
one of the main pest species in Cayman Brac.
Breeding occurs mostly in black mangrove, but also
in mixed mangrove habitats and buttonwood swamp. It has a wide tolerance of
salinity and can be found in both seawater flooded areas and freshwater habitats,
and has even been found in artificial containers. However, the brackish water
that lies for several weeks at a time in black mangrove habitats during the
rainy season is favoured, especially in and around mangrove pools and areas
with Batis maritima ground cover. Alternate drying and flooding conditions are
necessary. Eggs are laid in damp mud as water levels retreat and a dry period
can result in huge "banks" of eggs building up. When tide or rain flooding next
occurs these hatch to give enormous numbers of larvae over huge areas. The synchronous
emergence of such broods is the basic cause of Cayman's mosquito problem.
Aedes taeniorhynchus is a strong flier so large
numbers emerging in one part of the island quickly spread to all areas. In Cayman
Brac it mainly breeds at the west end of the island where the only mangrove
swamps occur, but biting is experienced everywhere. The synchronous nature of
broods and the ability to move rapidly into new areas explains a fact which
perplexed many early observers : that larvae are difficult to find when adults
are extremely abundant.
Although Aedes taeniorhynchus broods can occur at
any time of the year, following rain or tide flooding, the largest numbers coincide
with the rainy season with peaks in May-June and August-September. During periods
of high numbers biting can be experienced at any time of the day or night but
major activity peaks occur at dawn or dusk. The dusk peak begins 15 minutes
after sunset and peaks 15 to 30 minutes later. By an hour after sunset most
activity has ceased. The dawn peak is similar, ending 15 minutes before sunrise.
Numbers in light traps have diminished greatly since
control measures began in 1966. The average nightly catch for all traps combined
was 446 in 1972 rising to 2862 in 1974 and dropping to 136 in 1987, which was
regarded as a bad year. When the first New Jersey light trap was set up at the
airport in grand Cayman in 1966 the average catch in May was 20,000. Catches
in 1986 and 1987 for that one trap were down to 5.4 and 11.2.
This mosquito is one of the most common species in
the Cayman Islands and can be a serious biting nuisance.It has remarkably catholic
tastes in breeding sites being found everywhere except the more saline parts of
the mangrove swamp. It is even found in artificial containers and crab holes.
In the Brac it breeds in the Bluff as well as the
coastal platform.Its numbers rise rapidly in June, with the beginning of the
rains, and decline very slowly, reaching low levels only in March. Male Cx.
nigripalpus have been observed swarming at high level and huge numbers occasionally
impact on spray planes flying low over the mangrove areas at dusk.
This is the second largest mosquito found in the
Cayman Islands after Ps. Ciliata. In Grand Cayman it is by far the most common
Psorophora species but in the other islands it is outnumbered by Psorophora
pygmaea. It is a floodwater mosquito breeding in a wide range of habitats, including
the less saline mangrove and buttonwood swamps, but most commonly in fresh water
pasture and limestone pavement pools. Here it is commonest where there is emergent
vegetation. In Cayman Brac it is found along the length of the north coast in
grass and rock pools, favouring open, sunlit areas.
It can develop from egg to adult very quickly, four
days in warm summer weather, and is a fierce biter. Enormous numbers can occur
after extensive rain flooding and can cause a serious biting nuisance to people
and cattle, the former being much alarmed by its large size. However, peaks
of abundance are of shorter duration than those of Ae. taeniorhynchus.
Light traps in Grand Cayman show that its numbers
increase very rapidly right at the beginning of the rains and remain high, except
for a slight reduction in mid-rains, until October when they drop sharply to
very low dry season levels. It was much more common in light trap catches before
1980 than since.
An. Grabhami is the most common Anopheline species
in the Cayman Islands. In Grand Cayman it breeds in fresh water pools in grass
and rock areas, preferably with surface vegetation, but not in the more saline
button wood swamp. It is common for longer in the year than An. Albimanus, only
reaching low densities at the end of the dry season. After the beginning of the
rains it increases faster than An. Albimanus and remains at high levels from July
to December.
Anopheles albimanus is widespread in all three islands
breeding in fresh and brackish water.The greatest numbers in Grand Cayman have
been found in buttonwood pools with a salinity of about 18 %.It also occurs in
fresh water grass pools, Typha swamps and shallow wells. It shows a strong preference
for un-shaded localities and is not found in deep wells. In Grand Cayman it is
present in low numbers through the dry season and does not increase until July,
the start of the rainy season. Numbers usually peak in August and decline slowly
through the latter, wetter part of the rains. From 1973 to 1979 numbers in trap
catches declined rapidly and have remained at a low plateau ever since. The mosquito
readily bites man and animals and is an efficient vector of malaria.
Culex quinquifasciatus, the southern house mosquito,
breeds in and around dwellings in Grand Cayman. In Grand Cayman it is frequently
found in drums used to collect rainwater from roofs and in pots containing plant
cuttings. It favours contaminated water and will breed in large numbers in septic
tanks and in shallow wells used by cattle.
This mosquito is found in Grand Cayman and Cayman
Brac but it is not common and is rarely found in light traps. It is, however,
known to bite occasionally. It is the largest mosquito found in the Cayman Islands
and its larvae occur in fresh water with thick emergent vegetation, especially
flooded pasture and the edges of reed and sedge swamps. It is a floodwater species
and eggs are laid on damp mud.
The larvae that hatch when flooding occurs are predaceous
on other mosquito larvae and have been found feeding on Cx. nigripalpus and
Psorophora columbiae in Grand Cayman. In Cayman Brac it has been found in an
open grassy pool on the northern coastal platform in association with Ae. tortilis,
Psorophora pygmaea and Ps. Columbiae larvae. The small numbers in light traps
indicate a steady build up in numbers from the mid dry season to the end of
the rains.
Ae. sollicitans is one of the less common mosquitoes
in Cayman but it is a fierce biter when encountered. It appears in light traps
regularly in small numbers in Grand Cayman, even less frequently in Cayman Brac
and has not been recorded from Little Cayman. It is basically a salt march, floodwater
mosquito like Aedes taeniorhynchus but it avoids wooded sites and , therefore,
is not found breeding in mangrove swamps. In Grand Cayman it breeds in open grass
and reed areas which are usually fairly fresh. In Cayman Brac it has been found
in an unshaded muddy depression on the south west coast where the salinity was
near that of sea water. In Grand Cayman it appears to be present through most
of the year at similar levels except for a pronounced peak in May. This indicates
a response to the first rains of the rainy season but no such response to later
rain. Traps show it to have been much more common before 1980 than after.
W. vanduzeei occurs in all three islands and can be
very common but it does not enter light traps readily. In Cayman Brac it can be
one of the main pest species at times, biting during the day. Fay traps produced
very large numbers of this species and very little else (except for Ae. mediovittatus
and some Cx. bahamensis). It breeds in water that collects in the axils of the
leaves of bromeliad plants. These grow in the dry woodland areas of Grand Cayman
and Little Cayman and on top of the bluff in Cayman Brac. In Grand Cayman it has
only been found in the bromeliad Tillandsia balbisiana and appears not to utilise
other bromeliads or other plants such as sisal or banana which also collect water
in their leaf axils. The small number of W. vanduzeei taken in light traps in
Grand Cayman show a very distinct peak from April to June and by July numbers
have usually dropped to low levels. It would appear from this that the first rains
of the year produce a burst of breeding activity but further rain has little effect.
