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Northern Territory Department of Health Library Services Historical Collection
DL HIST
595. 772
WHE
1988MOSQUITO BREEDING AND BARDENS LAKES
FOR MORE INFORMATION CONTACT:
DEPARTMENT OF HEALTH AND COMMUNITY SERVICES MEDICAL ENTOMOLOGY BRANCH
GPO BOX 1701
DARWIN NT 5794
Telephone 20 8333 Peter WhelanSenior Medical Entomologist
HISTORICAL COLLECTION
00~ ~7
MOSQUITO BREEDING AND BARDENS LAKES
P.
I.WHELAN 1988
PETER WHELAN
Senior Medical Entomologist
MEDICAL ENTOMOLOGY BRANCH
DEPARTMENT OF HEALTH AND COMMUNITY SERVICES
P. 0. BOX 1701, DARWIN 5794 TELEPHONE ( 089) 20 8333
46284
MOSQUITO BREEDING AND BARDENS LAKES
1. 0 INTRODUCTION
2. 0
There has been a proposal by the owners of Bardens Lakes to purchase an adjoining section ( Section 1551, Hundred of Cavenagh) to complement their existing land holding in the area. The main entomological concern with the purchase of Section 1551 is to ensure that the construction and expanded operation of the proposed development does not increase the potential mosquito problems for either residents in the general area or to potential users of the development.
The application to purchase this land appears to be a retrospective arrangement, as the earthworks for the water retention development have already been at least partly developed on Section 1551. The application to purchase this area should be examined as part of the overall development, as this land is an integral component for the efficient management of the rest of the development.
The proposal should be examined in the light of the existing mosquito problems in the area, the effect of past and present development on mosquito breeding, the present environmental management of the area, and the possible construction and future management considerations that should be adhered to, in order to reduca present and potential mosquito problems.
EXISTING MOSQUITO PROBLEMS
The previous mosquito problem can be guaged by examining the results of part of the Medical Entomology Branch rural mosquito monitoring program. Adult mosquito numbers were monitored monthly at two sites near Lake Deane ( Map 1) from May 1986 to April 1987. The results of this monitoring are shown on Table 1. Overall, the monitoring results did not show a very large mosquito problem. The species with the highest numbers collected were Mansonia uniformis, Cul ex annulirostris, Anopheles bancroftii and Coquillettidia
xanthogaster. Both Ma. uniformis and ~
xanthogaster are not known to carry diseases, although they can be very significant pests. Ma. uniformis this general area. Both of these species are closely associated with the semi-aquatic reed Eleocharis sphacelata, which occurs naturally in Lake Deane.
An. is a potential vector of malaria nd is associated with shaded paperbark swamps and areas of Eleocharis reeds. Although An. farauti numbers were relatively low, the presence of this species is regarded with importance, as i t is the major vector of malaria in Australia. The overall numbers of malaria vectors in this area have not been large and these numbers would not be regarded with particular concern for a rural residential area with blocks over 2. 2 hectares.
Cx. annulirostris is a vector of various arbovirus diseases including Australian encephalitis and epidemic polyarthritis. This species breeds in open sunlit, grassy and reedy margins of freshwater swamps and lagoons. This species has been breeding in both Lake Deane and Bardens Lakes. Again, the overall numbers of this species have been relatively low and these numbers would not be of particular concern in a rural residential setting.
The current mosquito situation is indicated by the results of the recent adult mosquito trapping in Table 3. This indicates that there may be relatively higher An. banroftii and uniformis numbers at present when compared with two years ago. This could be within the expected fluctuations of mosquito numbers from year to year, but as the favoured breeding sites of both of these species are being extended, this may well be a real increase.
With the proposed tourist development in this area including caravan parks and possible camping grounds, the presence of the above mosquito species and the potential for increased mosquito numbers with development does pose a potential health problem and suggests that the existing and potential mosquito breeding areas should be reduced.
3. 0 THE EFFECT OF THE CURRENT DEVELOPMENT
One effect of the present development has been to increase the available area of mosquito breeding by increasing the amou t of the Eleocharis reeds in the horseshoe shaped Bardens Lake. The area of Eleocharis reeds appears to be expanding. The larval survey of the 21 /7/' 88 ( Table 2) indicates that both Lake Deane and Bardens Lake are breeding mosquitoes.
The area of Eleocharis reeds in Bardens Lake must now be regarded as an additional source of
~ xanthogaster and Ma. ~niformis and as these reeds develop further, the pest problems from these species will undoubtedly increase.
An. bancroftii appears to be restricted to the tree shaded Eleocharis reed and grass areas. While no An. bancroftii were collected in the main Bardens Lake, paperbarks are actively colonizing the margin of this area, and as these develop in conjunction with Eleocharis reeds, the lake area is likely to become a more significant source of An~ bancoftii.
Cx. annulirostris is primarily breeding in Lake Deane but not in Bardens Lake (Table 2). The reed areas in Bardens Lake were not a source of Cx.
annulirostris at present and this appears to be related to the upright state of the reeds. In Lake Deane, the reeds show some tendency to lodge, as the water level has receded, making access for fish predation on mosquito larvae more difficult. In Bardens Lake, with a constant water level being artificially maintained, the reeds are s t i l l actively growing and have shown no tendency to lodge. These Eleocharis reeds in Bardens Lake could become a source of Cx. annulirostris later in the year i f these reeds do in fact die or tend to lodge over.
The other major effect of the development has been the creation of a large shallow flooded swamp area in the water collection and retention area. As seen from the larval survey (Table 2), this area is a very significant source of Cx. annulirostris. This area is now being colonized by paperbark seedlings, and much of the original grasses and herbs have been drowned by inundation. This area has a large capacity to increase numbers of vector and pest mosquitoes.
4. 0 PRESENT ENVIRONMENTAL MANAGEMENT
The current practice of keeping Bardens Lake topped up is developing the paperbark species around the margin and enabling the Eleocharis reeds to expand. This will increase the area of potential mosquito breeding. There is no evidence of conscious management to reduce the extent of the Eleocharis reeds or the paperbark. In fact, these species would probably be encouraged as they would reduce wave action erosion on the constructed banks. The water collection area has been bunded without levelling or clearing of existing vegetation.
This lack of levelling has enabled the area to become a series of shallowly flooded pools, and as the bunded vegetation dies, the additional nutrient load is creating very favourable mosquito breeding sites.
In order to reduce areas of Eleocharis reeds, the margins of any of the lake area that has to be maintained at a constant water level should have a slope
of 1: 1 horizontal to vertical, with the minimum slope being 2: 1 horizontal to vertical, and be at least 2. Sm deep. This form of treatment should also be considered for Lake Deane and may involve considerable reconstruction on the more shallow margins. In the water collection area, the entire area should be cleared of standing vegetation, and excavated to ensure that the maximum area of the water storage is 2. 5m deep at the end of the wet season. The water collection area should also be levelled to eliminate any residual ponding as the water is drawn off, and should have an internal drainage system, so that all the water can be drained off.
6. 0 SUGGESTED MANAGEMENT PRACTICE 6. 1 Eleocharis control
The Eleocharis reeds minimized by excavation weedicide program. If a carried out, a suitable must be applied as two apart, and maintained appears. In this way a Eleocharis margin can offer wave protection, breeding areas, as well mosquito breeding area.
in Bardens Lakes can be as above, or by a regular weedicide program is to be weedicide such as Roundup i n i t i a l sprays, one month whenever renewed growth thin (no more than 1m wide)
be maintained, which would aesthetic appeal and fish as reducing the available
6. 2 Water level management
7. 0
Water level manipulation in the water collection and storage area could be one method to reduce mosquito breeding. If the water collection and storage area is reconstructed as above, i t should be relatively easy to manage the water level to either flood or strand marginal areas during the wet season, to prevent mosquito breeding. In the dry season, a continual reduction of the water level in this area, as Bardens Lake is topped up, will be beneficial in allowing the marginal areas to dry out rapidly.
ADULT MOSQUITO CONTROL
Regular adult mosquito control in the long term is not an option. Occasional adult mosquito control can be carried out, but should only be done in conjunction with the Medical Entomology Branch, so that the recommended equipment, insecticides and insecticide rates are used.
Personal protection from mosquitoes must continue to be the main protection for visitors to this development.
8. 0 CONCLUSION
There is no entomological objection to the purchase of Section 1551 1 as long as the development adheres to construction and management guidelines to reduce the available area of mosquito breeding. As this land is already partially developed as a water retention area and is currently a mosquito breeding area1 the only options available appear to be to allow the purchase of this land and i t s subsequent development, as an integral part of the overall development. If the land is not purchased, i t is recommended that the Department of Lands and Housing ensures that whoever has control over this land, completely rehabilitates the land to its original landform and reduced mosquito breeding capacity.
Peter Whelan
Senior Medical Entomologist 28th July 1988
of
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SITE
MOSQUITO LARVAL SURVEY LAKE DEANE AREA MEDICAL ENTOMOLOGY BRANCH
NUMBERS OF VARIOUS GROHTH STAGES IN SAMPLE VOLUMES
VEGETATION SPECIES NUMBERS OF
LARVAE IN FIVE STANDARD DIPS
NUMBERS OF LARVAE PER
BUCKET
1st-2nd: 3rd-4th 1st-2nd: 3rd-4th
Lake Deane Site A
Lake Deane Site B
Bardens Lake Site C
Bardens Lake Site D
Bardens Lake Site E
Paperbark shaded, Eleocharis reeds.
Paperbark shaded, couch grass
An. bancroftii Hi. metallica
ex.
annulirostrisAn. bancroftii Hi. metal l i ca Cx. annulirostris
Sunlit grass margin Ad. catasticta with algae.
Sunlit
Eleocharis reeds
Semi-shade grass margin
An. meraukensis
An. meraukensis
2
Hater collection Shallow,flooded
ex.
annulirostris 6area.
Site F
area with remnant pools and dead vegetation.
An. annulipes 7
2
2 5
4 2 2
9
2
4
TABLE 3.
TRAP LOCATION
Rest side of Lake Deane
TRAP NO.
Peninsula on Bardens Lake 2
Rest end of Rater collection 3 area
Creek north of Bardens Lake 4
TOTALS
PERCENTAGE OF TOTAL
VECTOR MOSQUITO SURVEY - BERRY SPRINGS: BARDENS LAKE & LAKE DEANE 21ST JULY 1988 Numbers of female mosquitoes collected in carbon dioxide baited traps.
An. annulipes Ae. notoscriptus ex. species nr vishnui
An. bancroftii ex. species 32
Ha. uniformis Hi. elegans An. hilli
An.meraukensis
Cx. annulirostris
ex. hilli ex. species 155 Mi. metallica
Cx. quinquefasciatus Cq. xanthogaster Ur. nivipes
7 358 3 45 1 5 4 7 21 3
2 23 37 16 148
3 23 44 46 97
14 2 48 2 85 41
12 418 3 3 174 16 6 2 154 499 2
0. 93 32. 3 0. 08 0. 23 0. 23 13. 4 1. 24 0. 08 0. 46 0. 08 0. 15 11. 9 38. 6 0. 08 0. 08 0. 15
TOTALS
653
229
218
194
1 294
100
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2. OVERALL DEVElOPMENT PLAN 1 • 5 000
BARDENS LAKES TOURIST PARK
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