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Northern Territory Department of Health Library Services Historical Collection
A PRELIMINARY SURVEY OF MOSQUITOES OF THE FINNISS RIVER DISTRICT
DL HIST
595.772 BOO 1987
MAY 1987
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NT DEPARTMENT OF HEAL TH~ AND COMMUNITY SERVICES
HISTORICAL COLLECTION
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A PRELIMINARY SURVEY OF MOSQUITOES OF THE FINNISS RIVER DISTRICT, MAY 1987
by D. Booth, P. Whelan and G. Dobson Medical Entomology Branch
Dept of Health & Community Services
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A PRELIMINARY SURVEY DISTRICT, MAY 1987.
1 . Introduction
OF MOSQUITOES OF THE FINNISS RIVER
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square kilometres into Fog Bay, and is located approximately 100 km SH of Darwin. The potential mosquito breeding habitats in the study area to the north of the lower reaches of the Finniss River C Maps 1 & 2) are comprised of low lying black soil plains, mangroves, paperbark , grassy and reedy swamps.
The district is only a few hours drive from Darwin and is of increasing recreational and commercial importance. The latter is from fishing, prawning and tourism industries conducted in the district. Also, as an undeveloped area close to Darwin i t is the subject of a number of sub-division proposals.
This survey was undertaken to determine what mosquito species were present in the area, what type of habitats were most productive for mosquito breeding, and whether mosquitoes presented a potential health hazard for current and proposed land use.
2. Method
EVS CO2 baited traps were set at various habitat and vegetation types so that a representative sample of the mosquito fauna of the district could be collected (Maps 1 and 2) and relative numbers assessed. The traps were run from late afternoon to the following morning on the 19th (Inland area) and 20th (Coastal area) of May, 1987. Larval collections were made where possible C Map 1 and 2). Slides were taken of a representative sample of mosquito breeding habitats (presented as photos in Appendix 1).
3. Results and Discussion 3. 1 General
Thirty mosquito species were collected from the EVS traps
C Table 1) and by larval sampling C Table 2). Cul ex annulirostris a major vector of Australian encephalitis and epidemic polyarthritis, was collected in large numbers throughout the district. Anopheles species, some of which are potential vectors of malaria, were also distributed throughout the study area and occurred in very high numbers.
Aedes vigilax a major vector of epidemic polyarthritis was present, but as with most other Aedes species, did not occur in large numbers at this time of year.
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3. 2 Inland Area
There are extensive low lying areas subject to inundation along the course of the Finniss River that provide major mosquito breeding areas as the floodwaters recede. This survey was timed to coincide with the period of expected peak mosquito breeding in these receding waters.
All trap sites captured greater than the 100 Cx annulirostris per trap night standard used as a guide to the maximum acceptable level of that species in a residential area. The lowest catch for Cx annulirostris was 412 /trap night, while the highest was 4032/trap night, indicating the relatively high and widespread occurrence of this species.
Other man-biting species were also abundant. Fortunately Anopheles farauti s. 1., the Australian malaria vector, was not collected near this section of the Finniss, and only trap 6 (beside Leviathan Creek) produced any. However An bancroftii was very abundant throughout the Finniss flood plain area, occurring at up to 500/trap ( near paperbark swamps at sites 1 and 8). Although An bancroftii is only regarded as a secondary malaria vector, i t s high density here means the area should be considered receptive to malaria. Low numbers ofAn meraukensis and An novaguinensis were captured at trap site 4 (grassy floodplain near the river) and site 7 (beside freshwater mangroves on the northern side of the road). The former was also captured as larvae at sites 1 and 2.
Culex annulirostris the common banded mosquito, was extremely common throughout the area, occurring at more than a thousand/trap night at site 4 (grassy, open swampy area near the river), site 8 ( the river itself, surrounded by stands of paperbarks and open forest), and at site 10 (Finniss River Homestead, an open, slightly raised plateau overlooking an extensive grassy swamp). By far the highest numbers were trapped at site 8, possibly due to Cx annulirostris concentrating along the river line. These numbers are high enough to present a significant health risk from arbovirus trans mission.
Nuisance species such as Coquillettidia xanthogaster and Mansonia uniformis were also extremely common throughout the trapping area. ~ xanthogaster occurred at over 750/trap night near site 7 (beside freshwater mangroves some kilometres from the Finniss River floodplain), and site 8 (in paperbarks beside the river itself). Ma uniformis occurred in more than a thousand/trap night at site 1 (a paperbark swamp with deep water and some reeds) and site 8.
Both species bite man but are not important disease vectors.
Two sections, 9 and 10, have been touted as areas for future subdivisions ( marked on map 1). Traps 3, 4 and 5 indicate the species range and relative abundance likely at this time of year for section 10. There were high numbers of the secondary malaria vector An bancroftii very high numbers of the nuisance species Cg xanthogaster and Ma uniformis and very high numbers of the major arbovirus vector Cx annulirostris Any development of this section would need to be done with due care to avoid exacerbating the already severe mosquito problem by impeding natural drainage creating water holding depressions, or creating water
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retention features and any general practice that can lead to changes in aquatic vegetation or nutrient levels. It is recommended that only large blocks be considered (eg 20 hectares) to minimise person/mosquito contacts. These comments also apply to section 9 , where traps 8 and 9 indicate a similar species range and slightly greater abundance, though there were fewer low-lying areas in this section and i t is probably more robust to development. Large blocks are s t i l l essential to limit human density.
The mosquito breeding in these areas is largely a natural phenomenon. Source reduction by engineering means is not feasible. Adulticiding and larviciding would also be impractical due to the extensive areas involved. Avoidance of mosquito bites by avoiding the principle mosquito breeding areas during the prime breeding season (April to July) and/or using repellents, protective clothing together with judicious use of insect proof tents and houses, are the only ways to reduce the impact of mosquitoes in this area.
3. 3 Coastal Areas
The terrain from the trapping sites in the inland areas to the coast is largely open forest with (at this time of year) cut-off pools along minor drainage lines and some paperbark swamps, likely to be the main breeding areas. There were very high numbers of Cx annulirostris and large numbers of
.Q_g_ xanthogaster and An bancroftii (see traps 11 and
12, Table 1).
The coastal area surveyed was from the mouth of the Finniss River to about 1 km north of Stingray Head. Most of this could only be traversed by 4WD, due to the deep sand roads to the north, and, because the bridge behind the beach is non-functional, the soft sand of the beach provides the present access to the south.
Here the major breeding was taking place in interdunal freshwater swamps (in the area behind five mile beach). The largest swamp was several square kilometres in area, completely covered in freshwater reeds(map 2). Cx annulirostris larvae were collected from most of these interdunal swamps, though some, for no apparent reason, had no mosquito breeding. Mimomyia metallica and Uranotaenia albescens larvae were also collected from the major swamp, and i t ' s quite likely Anopheles species were also breeding there (although none were found). There were also some brackish water breeding areas associated with the mangroves along the mouth of the Finniss < trap 15) and just north of Stingray Head ( traps 19 & 20). Some paperbark swamps in the black soil plains of the sections north (trap 18) and just south of the road <trap 13) were also significant potential breeding areas.
An farauti s. 1. the Australian malaria mosquito, was very common, in numbers of 50 or more/trap night at sites 13, 1 4, 16 and 18 ( all coastal or sub-coastal sites near ti dally influenced freshwater breeding sites). Site 18, with 166/trap night, had interdune swamps and tidally influenced creeks to the south, and a mangrove swamp to the north.
A n ban c r of t i i was pre s e n t i n v e r y 1 a r g e numb e rs w i t h o v e r- 1000 /trap night at sites 14, 16, 17 and 18 - that is, all the
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coastal trap sites near shaded freshwater breeding areas. The high numbers at sites 14, 16 and 17 are probably from the major interdune swamps which were characterised by extensive areas of freshwater reeds. An hilli was quite common (20 - 60/trap night) at the salt water trap sites 15, 16, 18, 19 and 20. An annulipes s. 1. and An novaguinensis occurred in large numbers near freshwater breeding sites (site 17 for both, 11 for An novaguinensis ), and An meraukensis was collected from near a paperbark swamp (site 12) and freshwater i nterdunal swamp C site 14).
These are among the highest catches of An farauti s. 1. and An bancroftii from the Top End in recent years and there is no doubt the general area surveyed is highly receptive to malaria. The peak period of receptivity would be probably from April to July, but the breeding areas are so extensive that Anopheles species are likely to be present throughout the year. The risk of transmission increases with increasing longevity of potential vectors. Nothing is known of the variation in seasonal longevity of mosquitoes in the Finniss district. Thus to avoid exacerbating the malaria transmission risk any land development in these sensitive areas must not impede natural drainage. Source reduction, however, is not practical due to the extensive nature of the mosquito breeding areas and the importance of these areas in the natural ecology.
Some 20 people live in weekender type accommodation or fishing vessels at the mouth of the Finniss. 'The rest of the coastal area is only inhabited by short-term visitors such as campers, fishermen and tourists. However, in the event of an imported malaria case in any frequently used locality or more permanent camping or living area, the risk of transmission is high and a number of steps should be taken. Fogging is not likely to be effective as the area to be treated is prohibitively large and access to i t is difficult. Instead people should either leave the area for three weeks, or be advised to take added precautions to avoid being bitten by mosquitoes, and mass drug treatment may need to be considered. Visitors to the area could be advised of the danger by a temporary (3 weeks) sign erected on the main access road.
Cx annulirostris a major arbovirus vector, was common throughout the coastal area. There were nearly 5000/trap night at site 11 (inland open forested area approximately 2 km from the Finniss River floodplain), 3500 at trap 17 (near the major i nterdune swamp), 3500 at trap 18 ( near paperbark swamps) and 500 1000 at the other trap sites. I t was collected as larvae from the interdune freshwater swamps. The nuisance biters Ma uniformis and ~ a n t h o g a s t e r were also in large numbers, though in contrast to the inland areas, .Q_g_
xanthogaster was more abundant. It was usually collected at greater than 100/trap - but at site 12 (beside an open water lake fringed with paperbark and with areas of freshwater reeds) over 1000 were trapped. The 10 traps from the coastal regions produced only about 10% of the Mansonia uniformis produced by the 10 inland traps, even though freshwater, reedy swamps were abundant in the coastal area. This may represent different habitat preference between these two usually associated species or just be a seasonal variation.
Some Aedes vigilax were trapped < site 14, site 20) and they probably would occur in larger numbers with the high tides later in the year. The coastal mudflats and saltwater
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marshes associated with this species were limited in extent, and unless there is major saltwater inflow into the major interdune swamp, which drains into the Finniss River, the numbers of this species should not be as severe as other areas. This could easily be ascertained by a survey there in October/November.
Very high numbers of Aedes daliensis a crab hole breeder, were collected at site 20 ( a mangrove swamp) . There appeared to be two forms of this species (pinned in NT Department of Health and Community Services reference collection).
Source reduction, adulticiding and larvaciding are impractical methods of minimising vector numbers here due to the extensive areas involved and their importance in the natural ecology.
However, any development here should be done with due regard to the risk of exacerbating breeding areas, that is, natural drainage lines should not be blocked and water-holding depressions or features should not be created. Low human
density should be maintained by maximising subdivision size.
People living in the area and visitors need to be aware of the health risks presented by mosquitoes and mosquito avoidance procedures should be adopted (long clothing, use of repellents and insect screens). Such public education is at -present through general mosquito awareness programs such as those used in 'Mosquito Week', which is timed to coincide with the increase in mosquito numbers, and outdoor human activity, following the wet season.
4. Summary
1. Several important disease vectors were trapped in high densities in the Finniss River district.
2. In the inland area these were Cx major arbovi rus vector, An bancrofti i vector, and other Anopheles species malaria.
annulirostris a a secondary malaria which may transmit
3. In the coastal area An farauti s. 1., the primary Australian malaria vector, and other potential malaria vectors - An bancroftii An meraukensis An hilli and An novaguinensis - were very common. No higher numbers of An farauti s. 1. or An bancroftii have been trapped anywhere in the Northern Territory in recent years. The coastal area is thus highly receptive to malaria reintroduction. Cx annulirostris occurred in high numbers at the time of the visit, and Ae vigilax a major vector of epidemic polyarthritis, was collected and may be expected in higher numbers later in the year.
4. The xanthogaster
nuisance species Ma uniformis were abundant in coastal and inland
and areas.
5. Mosquito control by source reduction, larviciding or adulticiding would be impractical for all these species due to the large breeding areas involved and the importance of these areas in the natural ecology.
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6. Personal protection through use of repellents, protective clothing and exclusion of mosquitoes from accommodation areas by insect screens - is the only effective way to reduce human/mosquito contact and thus health risk in these areas. Camping sites should be sited away from densely vegetated areas and at least a kilometre from swamps breeding mosquitoes. Mosquitoes can be expected to be most abundant from April to July.
7. The health risk presented by mosquitoes would be exacerbated by any development which interrupts natural drainage lines, creates breeding areas through water storage or uncontrolled effluent release, or increases in the density of habitation. Any subdivision or major development proposal should be detailed enough to allow evaluation of the likely effect of such development on numbers of the disease and nuisance mosquito species.
Tr-ap Sites:
Species
An annulipes s.!.
An bancr-oftii An far·auti s.l.
An hilli An mer-aukensis An novaguinensi:- Ad catastida
Ma uniform is
Tp rnagnesianu:- Ur- nivipes
Ae daliensis*
Ae rmi Ae sp 76 Ae vigila;;
C: annulirostris Cx bitaeniorhynchus C~{ fr·audatr·ix
io • 1 '
LX s111ens C~{ sp 32 C}; SP 167 C~{ squamosus Cx vicinus Total
435 344
1210 680
517
14
360
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24
3
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4
51
31
24 347
35 581
851 1204
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16
2258 1432 1523 2214
t possibly two types (pinned in NT collection!
Inland 5
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740
10
1001
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9
412 7
21 21
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32 1512
784 4032
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572 i666 6840 9
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24
00 157
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11
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07
34 12
126
54
284 374 1026
255 54
524 1107 4673 1512
Ji 10
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10
20
1853
cn::c j l J j 2790 134 t 4 1084
17
103
7
9 1r: g 858 1548
15
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1493 3089
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73 134 1231 2091
18 106 28 61
28
71 i,J
129 193 220
791 1322
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46 9
1408 5981
1082
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42
4284
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5784 105
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24 3
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91
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34
34
195
650 1033
t/7
8210 481 182 144
79
5043 5
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116 107 31 50932
Table 2. Larvae collected in the Finniss River district, 19 - 20 Hay, 1987
Species Site (see Description Maximum
maps 2 8. 3) Density
Cx annul irostris 4 depression in 1/dip
black clay soil
14 inter dune swamp 1/ dip 3.2 km from major inter dune 5/dip
jetty swamp
Cx halifaxii 3 wheel ruts in 1/dip
black clay soil
Ae notoscriptus 13 ice-cream bucket 50/ dip
An meraukensis 1 shaded paperbark 5/dip
swamp
2 open grassy 1/ dip
swamp
Ur albescens 3.2 km from major inter dune 1/dip
jetty swamp
Hy met a 11 i ca 3.2 km from major inter dune 1/dip
jetty swamp
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Section Boundary
Finniss River Survey - Inland Area Trap and Larval Collection Sites
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