Mosquito investigation, Paspaley Pearl facilities : Coburg Peninsula, Northern Territory, 31 October 1994

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Northern Territory Department of Health Library Services Historical Collection

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HISTORICAL COLLECTION

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DEPARTMENT OF HEALTH

~ AND COMMUNITY SERVICES

MOSQUITO INVESTIGATION

PASPALEY PEARL FACILITIES COBURG PENINSULA NORTHERN TERRITORY

31 OCTOBER 1994

For more information contact:

NT Department of Health and Community Services Medical Entomology Branch POBox40596

CASUARINA NT 0811 Telephone: 22 8333

Peter Whelan

Senior Medical Entomologist

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MOSQUITO INVESTIGATION .

· ~ PASPALEY PEARL FACILITIES COBURG PENINSULA

NORTHERN TERRITORY 31 OCTOBER 1994

PETER WHELAN

SENIOR MEDICAL ENTOMOLOGIST

NT DEPARTMENT OF HEALTH AND COMMUNITY SERVICES

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MOSQUITO INVESTIGATION COBURG PENINSULA OCTOBER 1994

1.0 BACKGROUND

In September 1993 there was a brief investigation of a mosquito problem at the Paspaley Pearl base at Raffles bay on Coburg Peninsula. Mosquito traps were set by Mr Geoff Wessels on the 10th of September 1993 and the results and conclusions from these traps were reported in a short paper by the Medical Entomology Branch of the Department of Heahh and Community Services. Th~ trapping indicated there was a particular problem with sah marsh mosquitoes (Aedes vigilax). There was no information available about the breeding sites in the vicinity of Raffles Bay except as indicated by a 1: 100 000 map, which indicated a potential breeding place north north west of the camp.

After receiving the report Mr Wessels smveyed the swamp approximately 1 kilometre north west of the Raffles Bay camp after a spring tide in late 1993. He found mosquito larvae over a relatively large area of approximately 1 hectare which was considered too large to control with larvicide at the time.

In October of 1994 Mr Harry Stevens of Paspaley Pearl in Darwin contacted the Department of Health and Community Services, advising that there had been continuing mosquito pest problems this year and requested a ground inspection to see if there was any practical method to either control the larvae or the adult mosquitoes in the area. An inspection was carried out in the Raffles Bay area on the 31 of October by the Senior Medical Entomologist Mr Peter Whelan accompanied by Mr Geoff Wessels and Mr Harry Stevens of the Paspaley Pearling Company. A brief inspection of the Port Bremer area was also carried out on the way back to Darwin from Raffles Bay.

2.0 INSPECTION OF THE RAFFLES BAY AREA

The visit was organised by Mr Harry Stevens of Paspaley Pearling with access to Raffles Bay by the Grumman Mallard flying boat operated by Paspaley Pearl On approach to the Raffles Bay area a low level circuit was made over the swamps in the general vicinity. A number of potential sources of Aedes vigilax were located from the air. (Map 1).

2.1 Potential Mosquito Breeding Places Raffles Bay Swamp A

The upper reaches of the tidal swamp A (Map 1) is approximately 1 kilometre north west of the Raffles Bay camp. An inspection from the air indicated a primeAedes vigilax breeding site that would be inundated by spring tides. The area was above the mangrove area, and is a fresh water/brackish water swamp in the wet season. At the time of the aerial inspection there was no freshwater or brackish water vegetation vi.stole in the swamp area, and there were numerous animal wallows and disturbances around the edges which were holding water from the last tide approximately three weeks previously. There was a relatively large area ( a few hectares) of open saline water that had partly receded. This area is probably an extremely

productive source of Aedes vigilax after spring tides or heavy rain in the early wet season.

After it is flooded by mid wet season rains it is likely to remain flooded for the duration of the wet season and well into the dry season. until it dries up in the mid dry season. Once it is full from wet season rains it is not likely to be a source of salt marsh mosquitoes because all of the egg laying areas would be flooded and there would be fish predation on any larvae. However this area is likely to be a variable source of various other fresh water species of mosquitoes if relatively thick brackish water reeds grow in this area and act to restrict fish predation. This brackish reed swamp area was not inspected by ground.

SwampB

The mangrove area of swamp B (Map 1, Map 2) was inspected on the ground, from the mouth of the creek ( approximately I kilometre north west of Raffles Bay base), to approximately I kilometre west. There was a considerable area of upper high tide mangroves in this area which would be prime Aedes vigilax breeding sites. Most of the sites were on the landward margin of the area in old interdune depressions or in restricted drainage areas, where spring tides would reach but could not drain. There were potential mosquito breeding sites in other areas closer to the main drainage creek, and these were associated with old dune remnants, restricted drainage, :freshwater inflow or brackish marsh reeds (Shoenoplectus littoral is). All were dry but the mud was moist indicating recent tidal inundation approximately two to three weeks previously. The :freshwater seepage areas of mangroves were thickly forested and would provide difficult access for anybody with mosquito control equipment. These areas were up to I hectare in area.

The total area of mosquito breeding likely to occur in the area inspected was in the order of three to four hectares, often with impractical access for ground control mosquito larval operations. It is assumed that similar areas exist on the north side of this creek, and similar areas would also occur in the northern arm of this creek that runs parallel to the beach. The combined area is too large for any ground control operation of mosquito larvae. The only option for larval mosquito control in this area appears to be a helicopter or fixed wing aerial application of insecticide. This would need to be applied two to three days after each spring tide in the August to January period, and after episodes of rain over at least 25 mm in the October to January period. When inspected, there were many Aedes vigilax adults biting in the area and they were landing on peoples backs and resting on hats.

This mangrove creek system is the closest to the Raffles Bay base and hence it is likely to be the major source for any mosquito problem at the base. There is a dry Monsoon forest area on the southern side of the swamp B area which is dominated by Canarium species with Vitex, Pouteria and other thick canopy trees that would be good harbourage for adult mosquitoes and would facilitate their migration and dispersal to the camp area. It is this area that is likely to have the highest human biting rate area nearest to the camp.

Swamp C

The mangrove area at swamp C is relatively confined by higher ground, with no extensive low lying areas that are likely to retain tide water after spring tides. There may be small localised

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areas of salt marsh mosquito breeding in this creek system at the margins but they are likely to be minor compared to t4e other mosquito breeding areas to the north and the east.

SwampD

This mangrove system had an area of open salt marsh at the upper end of the mangroves, similar to swamp area A It is likely to be a major source of Aedes vigi/ax but as it is over three kilometres from Raffles Bay camp, it is not likely to be as important a source as swamp A and B mentioned above. However when there are large hatches of salt marsh mosquitoes in this area, and there are southerly to south east winds in the direction of the Raffles Bay camp, there is no doubt that this area will be a source for a least some of the mosquitoes that would be troubling the Raffles Bay camp.

SwampE

This swamp area contains similar areas to swamp D and again is likely to be a prolific breeding place of salt marsh mosquitoes as well as various :freshwater species in the mid to mid dry. It is relatively further away from the Raffles Bay camp, and hence not as important a source as the above areas.

SwampFandG

There is a site of similar habitat to swamps A, D and E adjacent to the mangroves in this area.

However the swamp area G is a perennial :freshwater swamp area. There were still green freshwater reeds evident in this area indicating remnant freshwater derived from the larger catchment and the flow lines leading into this area. The swamp area G is not likely to be a major source of salt marsh mosquitoes but it is likely to be a major source of freshwater mosquitoes throughout the dry season. This swamp system is over 5 km from the camp and so is not likely to lead to a major pest problem with freshwater mosquitoes at the camp. It would however provide a source of freshwater mosquitoes longer into the dry season than any other fresh and brackish water sites in the general area that would be likely to dry up by the middle of the dry season.

2.2 Inspection Of The Camp At Raffles Bay The pearl shell racks and the rope racks.

The under shed storage of the black rope shell cages was an ideal mosquito harbouring area, as it is shaded and cool and offered ample protection from the usually low humidity in this area. Mr Geoff Wessels indicated that there had been prolific harbourage of mosquitoes in this area after the last spring tide. It is most likely that these harbouring mosquitoes where Aedes vigilax. This is one of the main shore based work areas for the workers at the Raffles Bay area. Nearby is a dense group of small ropes hanging in a rack. Mosquitoes were observed harbouring in these ropes but many were non biting. A sample of these indicated that these were male and female Cu/ex sitiens. These mosquitoes breed in brackish to salt water pools and most probably originated from the retained water in the swamp A area from the last spring tide. At other times of the year these racks are likely to be important harbourages for the salt marsh mosquitoes.

Septic tanks and other sources

The septic tanks in the Raffles Bay camp were inspected. All of the tanks inspected were fully sealed. An inspection inside three of them indicated that there were no mosquitoes harbouring inside these tanks. They all had screw caps. There was one wash/shower area that had an overflow tank into a banana patch and had created a polluted surface pool This water had been purposely directed into the banana patch. There were no mosquitoes breeding in the pool but it is a potential source of mosquitoes and should be inspected regularly to ensure that it does not become a source of other mosquito species. Rectification of this pooling could be facilitated by removing the surface water with a constructed subsoil soakage drain.

There was a fish pond near one of the living quarters that has been maintained with freshwater.

There were no mosquito breeding in the fish pond when it was inspected and Mr Wessels indicated that it had been previously treated. Mosquito breeding in this fish pond could be prevented by introducing a few native fish into the pond.

2.3 Mosquito control avoidance or self protection measures.

a) Larval mosquito control

There is little practical scope for larval mosquito control in this general area because of the size and limited accessibility of the mosquito breeding areas.

b) Adult mosquito control large scale evening control

Adult mosquito control in the evenings and at night is not likely to have any carry over effect from one day to the next. Adult mosquito control to reduce numbers in the general camp area would require large scale fogging of the monsoon forest area and the mangrove areas to the north of the camp. With the very large areas involved and the lack of access and the repeated requirements of such operations, this would appear to be impractical.

c) Adult mosquito control, small scale fogging in the harbouring areas during the morning.

There is scope to do limited fogging of selected areas of the Raffles Bay camp in the mornings where mosquitoes harbour during the day. Those areas that would be useful to fog would be the rope racks, the rope baskets in the shed and any dense vegetated areas such as the sweet potato patch and the banana patch. The salt marsh mosquitoes will not disperse into these areas during the day so fogging of these areas in the morning would give relative freedom from salt marsh mosquito bites during working hours. However with the approach of evening, dispersal of salt marsh mosquitoes will occur from the mangroves and the breeding areas so that there will still be pest problems during the evening.

Suitable equipment for this fogging operation is the Fogmaster or TriJet fogger (see appendix 1 ). This equipment uses a reslin insecticide and water mix. This insecticide is a synthetic pyrethrin which is identical to the insecticide that was used for the disinsection of overseas aircraft on arrival in Australia. Its toxicity to humans is very low.

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The best results for the fogging of mosquito harbouring areas are likely to be achieved if the areas are fogged in the mid morning around eight o clock when the sun is up and the salt marsh mosquitoes have sought daytime harbourage sites, rather than the early morning when the mosquitoes may still be active during the high humidity period. The net area should be fogged with the equipment a few metres away from the nets and ropes so that the fog drifts through the nets assisted by light breezes. It is probably not necessary for the net areas to be repeatedly fogged during the day as one fog should be sufficient. The operation should commence up wind of the areas to be fogged and the operator should use a respirator. This operation should only take a few minutes and can be reoccupied a few minutes after the fog has dispersed.

d) Personal protection.

There is likely to be very high numbers of salt marsh mosquitoes in the evenings in the Raffles Bay area at least from September to January. These mosquitoes will be most numerous eight to nine days after spring tides from September to January or rain in excess of 20 millimetres from November to January. Once the brackish marsh swamps are full of water in the mid wet seasons, they are not likely to be significant sources of salt marsh mosquitoes until the following dry season because all the egg laying areas will be under water and most of the sites will have considerable fish predation.

There are a number of avoidance and self protection measures that could reduce mosquito attacks in the evenings. These include:

1. Staying inside screened areas during the mosquito active period.

2. Wearing light coloured long clothing.

3. Relaxing in the evenings in well lit areas.

4. Using mosquito coils in those areas with some protection from the wind or when there are only light breezes.

5. Using a personal mosquito repellent, preferably in a cream or gel form and containing DEET.

6. Avoiding areas of dense planing of shrubs and trees near residential and relaxation areas. Tall spreading trees are preferable to medium to small dense trees.

7. Locating any animal holding yards nearest to the brackish marsh swamp to the north, to divert mosquitoes.

3.0 MOSQUITO INSPECTION PORT BREMER AREA 3.1 Creek inspections

An inspection of the tidal influenced creek was made in the general area of the Port Bremer camp. The nearest creek lines to the north ( Site 1 and 4, Map 3) had depressions at the mouth of the creeks just behind the frontal dune. These depressions will hold water after spring tides or after heavy rain. These sites could produce small concentrations of salt marsh mosquito breeding and due to their close proximity to Bremer camp could contnoute to a small salt marsh mosquito problem at the camp.

The creek at site 2 drains the camp area and has a relatively small area near its mouth to the bay which could be filled by spring tides and still retain tidal water behind the frontal dune.

However it appears that this depression can only be reached by the highest of the spring tides and may not fill this year. This depression could be filled by the first heavy rains of the wet season and because of the saline soils, produce sah marsh mosquitoes and other freshwater mosquitoes.

The creek at site 3 is relatively steep and has a depression at the mouth of the creek that has probably been filled two or three weeks previously by the last spring tide. This site has probably produced salt marsh mosquitoes that have affected the port Bremer camp in the last few weeks.

Site 5 and 6 were not inspected but the points at which the creek lines meet the mangroves are potential salt marsh mosquito breeding areas. These particular sites are about 1.5 km from the port Bremer camp and if they do produce sah marsh mosquitoes, their control could mean relative freedom from mosquitoes at the Point Bremer camp. It is suggested that the above mention sites be inspected approximately 3 days after the spring tides in the late dry season. If mosquito latVae are detected in these areas they could be controlled by relatively small pressure sprayer equipment using Bacillus thuringiensis var israe/ensis (Bti).

3.2 Septic tanks and soakage trenches.

There are a number of septic tanks in the Port Bremer camp that were obviously having problems with the infihration trenches. The infihration trench at the managers house was in the process of repair and there was a minor pool of effluent that had a small amount of mosquito breeding. These mosquito latVae were Cu/ex annulirostris, the common banded mosquito.

The toilet and shower block between the managers house and the mess had a small pool of effluent at the beach of approximately 200 mm diameter. Although there was no mosquito breeding at the time of inspection, this site could possioly be a small and minor source of mosquitoes in this area at other times.

The toilet block near the power supply had a relatively large pool of about 2 square metres with significant weed growth. This particular site was breeding Cu/ex annulirostris mosquitoes and could be a minor source of mosquitoes for the camp area.

The septic tank at the mess was overflowing through the inspection hole and was being periodically pumped out to the beach. This particular septic tank appears to be overloaded and has limited ability for the soakage trench to dissipate effluent. It is possiole that this septic tank will need to be relocated so that it has sufficient infiltration that would be unaffected by tides.

All the above septics indicated they had _potential problems with infiltration in the beach sands.

It appears necessary to reconstruct most of the infiltration trenches for these septics to achieve a better disposal of waste water.

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4.0 CONCLUSIONS

There is little practical opportunity to control the larvae of the salt marsh mosquitoes that periodically cause a pest problem at Raffles Bay, because of the large area of the breeding sites associated with the creek north west of the camp, and the relatively inaccesS1ole nature of the breeding sites. The only practical method to control the larvae in this area is for aerial application of insecticides with specialised application equipment.

Some relief from the pest problem is posS1ole by a combination of judicious fogging of mosquito day harbouring areas in and near the camp, together with self protection measures.

PI Whelan

Senior Medical Entomologist DH&CS

8 November 1994

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APPENDIX 1 Page 2

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