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Northern Territory Department of Health Library Services Historical Collection
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MOSQUITO SITUATION IN JABIRU, 1983
1.0. Mosquito Species
The mosquito problems at Jabiru have been under
investigation by the Department of Health for a number of years, and altogether over thirty-eight different species of mosquitoes have been found. However, only a few mosquito species occur in sufficient numbers in the Jabiru residential area to cause pest problems and have a potential to spread mosquito borne diseases.
The mosquitoes of most concern are Culex annulirostris, the common banded mosquito, Aedes vigilax, the salt marsh mosquito, and some Anopheles species.
The salt marsh mosquito can be an occasional problem in November to February. It breeds i n the salt marshes of the South and East Alligator and migrates long
distances. Very little can be done to control i t at i t ' s breeding grounds because they are inaccessible and extensive.
The common banded mosquito and the Anopheles species can be a problem from January to October, but generally, the greatest problems occur from April to September
or after the 'wet'. These mosquitoes breed in shallow, reedy, _ grassy swamps and creek lines, and flooded areas, of which Baralil Creek is a very important site, due to its closeness to Jabiru and because the lower parts of Baralil Creek breed mosquitoes into the late dry season.
2.0 Mosquito Numbers
DL HIST 614.4323 WHE 1983
Investigations carried out around Jabiru i n September this year indicated very high numbers of mosquitoes, especially the common banded mosquito~ Using special traps, over 6,000 of these mosquitoes were caught in one trap Cv)· one night on the outskirts of Jabiru
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near Baralil Creek. This figure represented approximately 1,400 mosquitoes biting one person in an hour, just after , sunset. These figures are extremely high. The highest figures recorded in the Northern Territory by these traps have been 11,000 on tne Mary River flood plain, with the highest numbers ever recorded at the worst area near Darwin in the region of 2,000 per trap per night. High numbers, (up to 1,100) have been recorded even inside the residential area of Jabiru.
The highest numbers recorded from Jabiru have always been bordering Baralil Creek, and recent investigations showed very high numbers occured around the south and west sides of the town.
3.0 Diseases Carried
Both salt marsh mosquitoes and the common banded mosquito can carry epidemic polyarthritis, which can be a mild to severe arthritic disease. The common
banded mosquito can also carry Australian Encephalitis.
The very high numbers of the common banded mosquito means that there is a considerable potential for the people of Jabiru to contract a mosquito borne disease, and cases of Epidemic Polyarthritis have been reported from Jabiru for a number of years.
4.0 Option for Control
There are a number of options open to try to reduce the severe pest problems, as well as to decrease the risk of disease.
a) Engineering out the breeding grounds
This can not be done for the salt marsh mosquitoes that breed so far awayp The engineering possibilities around Jabiru would include making Baralil Creek more free-flowing, or digging large, deep, steep-sided billabongs in the bed of Baralil Creek, lower down
the creek, to drain the upper reaches early in the dry season.
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This option would be expensive, and may not be possible due to the proximity of the National Park.
b) To apply an insecticide to the breeding grounds The breeding groundsJjust after the wet season has finished,are very extensive, with Baralil Creek and Condorl Creek and other areas further away
contributing to the problem. The application of chemicals to the breeding grounds is a specialist job and would be very expensive, as i t would have to be done almost weekly for a large part of the year over a large area.
c) Mosquito fogging
This appears to be the only possibility for some relief from the large numbers of mosquitoes. If the edges of Baralil Creek and areas outside the townJ
where la~ge numbers of mosquitoes are found, are fogged, less mosquitoes would invade the town. The success of such fo9ging relies largely on wind direction and good access,right to the edge of the creeks,
other breeding areas, and places where the mosquitoes rest during the day. The chemical that is recommended for fo9ging adult mosquitoes is Maldison (Malathion).
This is a very safe insecticide, contrary to a lot of misplaced criticism of i t . It is recommended by World Health Organisation and the National Health &
Medical Research Council for adult mosquito control.
A visiting toxicology expert for the U.S. has said recently that after high use of Malathion in the
U.S., often in agricultural situations where much higher rates are applied, they could not criticise the use of Malathion for any adverse toxicological effect to
people and the environment.
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The Malathion fogging would be applied to the breeding and resti~g areas, not in the residential areas. Out of consideration for objections by some people to the use of any insecticide, fogging should be done only
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along the creek and the outskirts of the town, when the wind direction is not blowing the fog into the residential areas. A recommended fogging route has been drawn up.
The recommended amount of Malthion for foggi~gJworks out to less than lfl.oz. per acre. It is applied as an aerosol of very fine droplets and relies on killing mosquitoes as the mosquitoes fly through the fog and contact the very fine droplets of insecticide. The insecticide would have no residual effect. If the
toxicity of Malathion is compared to natural pyrethrin, i t is about twice as safe for skin contact as pyrethrin, and almost three times as safe orally as pyrethrin.
If Malathion is compared to Baygon, which is approved for application inside the household, Malathion is 15 times safer orally, although i t has almost the same skin contact toxicity.
Although there will still be mosquitoes biting people when fogging is carried out, there will be a temporary reduction for at least one day. More importantly, i t lowers the average age of the mosquitoes near the town. There would then be more of the newly emerged mosquitoes biting people than older mosquitoes. As i t
is the older mosquitoes that transmit diseases, the fogging would lower the chances of mosquito borne disease.
Fogging should only be done infrequently to lower the chances of the mosquitoes becoming resistant to the insecticide. For this reason, the fogging must be based closely on a mosquito monitoring program.
At the most, fogging should only be done 3 times per week when the mosquito numbers are very high, and once per week normally. If there/ are very few mosquitoes
no fogging should be done. People should rely on personal protection against mosquitoes in addition to fogging
operations.
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5.0 Recommendations
It is recommended that access tracks be constructed, and mosquito fogging be carried out, based closely
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on a mosquito monitoring program~
P. WHELAN,
SENIOR MEDICAL ENTOMOLOGIST
