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Northern Territory Department of Health Library Services Historical Collection
DL HIST
595.772 BOO 1987
BITING INSECTS SURVEY OF SECTION 2881
FINNIS RIVER REGION
DEPUTIIHT OF HEALTH 1110 COIIIIIUIITY SERYICES
HISTORICAL COLLECTION
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A BITING INSECT SURVEY OF SECTION 2881 OF THE FINNISS RIVER REGION, 10-12TH NOVEMBER, 1987
D. Booth & P. Whelan
Medical Entomology Branch
Dept of Health & Community Services
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A BITING INSECT SURVEY OF SECTION 2881 OF THE FINNISS RIVER REGION, 10-12TH NOVEMBER, 1987
1. Introduction
Section 2881, a subdivision of the old Finniss River Station, is an area of approximately 60 square kilometres overlooking the northern part of Fog Bay.
It is situated some 150 kilometers (by road) south-west of Darwin and is currently being developed for further subdivision.
Two officers from the Medical Entomology Branch conducted a survey there in mid November to determine the biting insects present. From this and observations on the topography and vegetation, and in conjunction with data obtained from surveys conducted in the Finniss region in May and August; estimates of the health risk represented by biting insects and the constraints they place on development are discussed.
2. Methods
Carbon dioxide baited EVS traps were set at representative habitat types in the section (Map 1).
Traps were run from before sunset to the morning of the following day on the 10 - 11th C sites 1-5 , 7) and the 11 12th of November (sites 6, 8-15). All mosquitoes in catches were identified. The biting midges were subsampled and 50 identified to species 1 eve 1.
Larvae were collected from various habitats (see Map 1), and either stored in 70% alcohol for later identification, or reared through to adults. Water salinities from some areas were also tested, and slides taken of representative habitats .
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3. Results and Discussions
3. 1 Mosquitoes
16 mosquito species were collected (Table 1). These included the main Top End arbovirus vectors Aedes vigilax, Ae normanensis, and Cx annulirostris;
and two species which may transmit malaria, Anopheles hilli and An meraukensis.
3. 1 . 1 The salt-marsh mosquito
A_g_ vigilax, the salt marsh mosquito, is a major
vector of epidemic polyarthritis. It breeds in brackish to saltwater pools in areas subject to tidal inundation. Thus large numbers were collected at site 15 (929/trap night) on the northern end of the section. The species was captured at all trap sites, but was particularly common at sites 1, 3,5 and 14; all near potential brackish or saltwater breeding areas.
Larvae were collected from site 5 and site 10 (salinity at the latter was 54. 3 ms/cm, equal to sea water).
This species fluctuates in numbers according to tide and rainfall changes. As our survey was three to four days after the last spring tides, we were too early to sample the population produced from the breeding areas created by those tides. However, i t can be deduced from the topography and vegetation that the northern end, associated with the large mangrove area, has by far the largest breeding areas for this species.
Other parts of the section, particularly in the vicinity of the small tidal creeks of the coast, would have occasionally high numbers of this species in the late dry or after the first rains.
3. 1.2 The common banded mosquito
Cx annulirostris, the common banded mosquito, was collected in small numbers at the inland trap sites (9,11,12,13,14). It is a known vector of epidemic polyarthritis and Australian encephalitis, and was implicated as a vector in a suspected case of Australian encephalitis in the Top End this year. This species breeds in freshwater swampy areas and low-lying grassy areas after rain. Its seasonal abundance elsewhere in the Top End is from the beginning of the dry to a few months after it. At site 12 in May several thousand were collected in one night, but none were collected there in November. As there are many areas in the section, particularly the freshwater swamps and areas subject to inundation in the southern end, suitable for large-scale breeding of this vector species, i t can be expected to occur in very significant numbers in the post-wet (April to July/August) within 2km of its major breeding areas.
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3. 1. 3 Anopheles mosquitoes
In May very large numbers of Anopheles mosquitoes, including the primary malaria vector in Australasia, An farauti s. 1., were collected just south of section 2881 including site 12. No Anopheles were collected at that site in November, but An hilli was collected at site 5, and An meraukensis at site 6. By comparison with the catch results from similar Top End localities, and judging from the many potential breeding areas in the section, i t is probable that high numbers of this genus can be expected in the early to mid-dry. The longevity of the various Anooheles species is unknown in this locality, and the receptivity of these species to malaria is uncertain. However, as An farauti s. 1.
is a proven vector of malaria elsewhere, and as malaria has been endemic in the Territory in the past, there is cause for concern regarding malaria transmission if tourist facilities are established in the area, particularly if near An farauti s. 1.
breeding sites.
3. 1. 4 Other mos qui to speci es
Other mosquito species collected in the area include low numbers of the arbovirus vector, Ae funereus, low numbers of the vector species Ae normanensis, low numbers of the container or treehole breeder Ae not o s c r i pt us and l_ ow numb e rs of t he ·p es t s p e c i e s Coquillettidea xanthogaster Only the latter species was very common in earlier surveys in the Finniss region. ~ notoscriptus is one of the most common domestic species of mosquito, and i t s presence often indicates poor rubbish disposal, though i t can also exploit treeholes for breeding and this the more likely source of this species in the section.
3. 2 The mosquito breeding areas
3. 2. 1 The brackish and saltwater breeding areas
The many mosquito species collected reflect the diversity of habitats in the section. These include the brackish water· creeks on the western side of the section (sites 5,6,8,15) from which Cx sitiens (site 8) and Ae vigilax <site 5) larvae were collected. Only site 1 5, with its extensive salt flats and low lying areas subject to tidal inundation, produced large numbers. The other areas could be more important in the first months after the wet, particularly as they provide ideal conditions for the primary malaria vector, An farauti s. 1..
3. 2. 2 The freshwater breeding areas
The freshwater areas in t he southern part of the section (marked as areas subject to inundation and as permanent and semi-permanent swamps on Map 1) are likely to produce very large numbers of mosquitoes
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after the wet season. These areas were fairly dry at the time of the survey, and only the comparitively unimportant species Cx sp. 67 was collected (site 11 >. However, very large numbers of Cx annulirostris and other freshwater breeders, including Anopheles species such as An bancroftii and An meraukensis and nuisance species such as Hansonia uniformis and ~
xanthogaster can be expected to exploit such conditions in the early to mid dry season.
3. 2. 3 The major mos qui to breeding areas
The major mosquito breeding areas in order of importance can be summarised as:
- salt and brackish water breeding areas to the of site 14 (the biggest mosquito plagues expected here, particularly in the late dry, wet> ;
north can be early
freshwater breeding sites in the southern part of the section (high numbers of disease vectors can be expected here in the early to mid dry>;
brackish water breeding areas near (in order of importance) site 12, site 6, sites 8 and 10, and site 5. These sites are particularly important from the point of view of poten~ial malaria vectors, and will produce high numbers of -these mosquitoes in the early to mid dry.
The best site for a tourist facility, of view of avoiding the considerable and vector mosquitoes, is near the sites 6 and 7.
3. 2. 4 Mosquito harbourage
from the point number of pest coast, between
Areas of monsoon forest are scattered throughout the section. Apart from providing breeding areas for some of the minor Aedes treehole breeding species, they also provide good harbourage areas for vector species.
The monsoon forest areas are likely to harbour large numbers of Ae vigilax in particular, after large emergences in the late dry and earlt wet season. This harbourage favours mosquito longevity and thus the likelihood of virus and protozoan transmission. For this reason any proposed tourist accommodation area is best sited away from the monsoon forest areas, or at least have a buffer of relatively open forest between the monsoon forest and any proposed accommodation and outdoor recreation areas.
3. 3 Biting midges
3. 3. 1 Biting midges collected
Although many Culicoides species were collected in this survey, pest levels were indicated only at the northern end of the section. Biting midges are not
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known to spread diseases among humans, but their pest status is well known in the Territory. Only~
ornatus was trapped at greater than a hundred per trap. It is known to breed in the neap tide zone and is associated with mangrove species such as Rhizophora stylosa and Aegiceras corniculatum.
No control methods are available short of destruction of the mangrove areas.
Some taxonomic difficulties were posed by some Culicoides species collected. Such specimens are stored in the N. T. collection for later examination.
The biting midge numbers are highest two days after a neap tide, and lowest just before a neap tide, when this survey was done. Hence much higher numbers than are indicated by these trap collections can be expected after neap tides.
3. 3. 2 Major pest areas
The major areas of the section likely to have serious order of pest levels of biting midges are, in
importance:
- the general area north of extensive . suitable mangrove breeding,
- areas near the mangrove belt the section,
site 2, where there are areas f o r ~ ornatus
on the eastern edge of
- areas near the creek lines at which traps 13 and 10 were set.
~ ornatus is capable of dispersing more than 2 km from its mangrove breeding areas, with the highest densities of biting midges occurring from the mangroves to approximately 1 km away (depending on wind, humidity, host availability, and available resting sites>. Tourist facilities should not be placed in this biting midge zone.
4. Conclusions
1. There are several significant habitats in Section 2881 likely to be sources of large numbers of mosquitoes that may spread diseases such as epidemic polyarthritis, malaria, and Australian encephalitis.
These areas fluctuate in importance according to seasonal factors.
2. At the time of the survey (November) the areas inundated by Spring tides in the northern section were producing the only major levels of biting insects.
Ae vigilax, a potential vector of epidemic polyarthritis and an avid man-biter, was the dominant species.
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3. The brackish water breeding areas (sites 3,5,6,8,12,15 and sites such as 10 and 13 at the interface of the numerous freshwater creeks and tidal areas on the eastern side of the section) could be expected to produce the primary Australasian malaria vector, An farauti s. 1.. This species was collected just south of Stingray Head in high numbers in May - but was not collected there in November.
There is thus cause for concern regarding the risk of malaria transmission if tourist facilities are established within 1. 5 km ( the flight range of this species) of the areas of brackish water pooling.
4. The brackish water areas above, and the freshwater areas in the southern part of the section (sites 9, 11 and areas marked on Map 1 as subject to inundation) can be expected to produce large numbers of the arbovirus vector Cx annulirostris particularly in the first months after the wet. In our present survey very few of these potential breeding areas held water, and not surprisingly few freshwater species were trapped.
5. The above mosquito breeding areas are natural features and mosquito control would be impractical due to the extensive areas involved and the high costs, both financial and ecological, of any control measures. Some of the smaller breeding areas Ceg at site 5 and site 8) could be eliminated by engineering if an intensive development was planned, but i t is usually more economic to avoid rather than eliminate mosquito breeding areas. People visiting or living in the area can protect themselves, particularly in the evenings, by using repellents, by wearing protective clothing, by using insect screens around living areas, or by avoiding the areas of high mosquito activity.
6. The biting midge, ~ ornatus was collected at pest levels in the northern end of the section and may be expected to occur a within a couple of kilometres of the mangrove belt at the northern and eastern parts of the section, and near the creek lines at sites 13 and 10, particularly just after neap tides. Intensive development should be kept south of site 14 and away from the mangrove belt.
7. Constraints on any development should include block size limitation so that lot size is large enough that human density is low near the larger mosquito breeding areas. To minimise the risk of disease transmission blocks should be at least Sha or larger. Any major developments such as tourist facilities should be sited at least 1. 6km from any major mosquito breeding areas. Stormwater, septic systems, water storage facilities, and rubbish disposal should be managed to avoid mosquito breeding.
Water retention features should have formalised edges and steep (1: 2 at least) slopes this allows wave action and inhibits vegetation growth along margins.
No development plans or Preliminary Environmental
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Report for this section have been sent to the Branch as yet, so more detailed comments cannot be made.
8. The Medical Entomology Branch is available to provide advice on the effect of any development plans on mosquito breeding, and for information on areas likely to be problem mosquito breeding areas.
David Booth Peter Whelan
18th December, 1987
Tabie 1. E:itir:g ir:mts collected in car·bori dioxide baited traps in Section 2881 - Finnis coastal am on the 10-im Nov., 1987.
~OS~UITOES
An hi 11 i Ar: ur·aukefISi s Cx annu1ir·o:.tr·is
B1TIN6 MlD6ES C. or·natus C. he¥.i t ti
Sf' rir- ur-natus Af.
C. sp r:r ur-nat!Js Bt C, :.p nr· ur·nat us Ct C. sp nr· or·natus Dt C. austropa1~:a!is C. pa11 iiJuthura~
C. i~1sacu1atu:.
C. subi11acuiatus
Tota 1
t i n NT co 11 e ct i or:
Sites ar·e shown on Nap 1
17
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17
16
200
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62
14
27
27
TRAP SITES 5
10
11
45
10
11 12
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14
13
15 Total
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652 107
20 43 24
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Table 2. Larval collections in Section 2881, Finniss River District 10 - 12th November, 1987
Map Site
5
8
10
Description
Brackish/salt water coastal discharging over sandy beach Brackish water (3.5 ms/cm) coastal creek discharging over sandy beach
Saltwater creek (54ms/cm) discharging into mangroves
Species
Ae vigilax
Cx sitiens
Ae vigilax
Map 1. Survey Area - Section 2881 of the Finniss Region - Trap and larval collection sites used in survey
Native
• Foint
Fog
Bay
Stingray Head
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'l'rap site Mangrove area 8
'i. Larval collection
site Area subject to
inundation
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Permanent or semi- permanent swamp Salt flat
Dirt road or track
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scale (km)
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