Ord River Arboviruses-mosquito Captures During 197677

J . Ausr. en[. SOC., 1981, 2 0 41-57

47

ORD RIVER ARBOVIRUSES-MOSQUITO CAPTURES DURING 1976/77 A. E. WRIGHT Deparrmenr of Microbiology, University oJ Western Australia, Queen Elizabeth I1 Medical Centre, Nedlands, Western Australia, 6009.

Abstract During 1976/77 four field trips, each of one month's duration, were made to the Ord Valley to collect mosquitoes in the continuing study of arbovirus ecology. Results indicate that Culex annulirostris Skuse continued to dominate the mosquito fauna of the area, whilst Coquilletridia xanthogaster (Edwards), Mansonia uniformis(Theobald) and Anopheles bancrojiii Giles appear to be increasing in numbers following stabilisation of the lake margins and prolific growth of aquatic plant species. Limited trials suggested that new double-chambered aluminium traps, particularly when used with sharply inflexed entrance funnels, were superior to the chicken-baited traps used previously. Further trials with three different bait animals indicated that rabbits attracted more mosquitoes than chickens and that both of these caught more than guinea-pigs. When chickens and rabbits were combined in the same traps they attracted significantly more mosquitoes per trap than when used separately.

Introduction Previous publications have reported results of entomological (Liehne, P.F.S. et al. 1976c), serological (Liehne, C.G. et al. 1976b) and virus isolation (Liehne, C.G. et al. 1976a) studies in tropical north-west Australia between 1972 and 1975. This work has continued in order to further define the problems of arbovirus ecology in the Ord Valley of the Kimberley, where the Australian Government created its largest tropical man-made lake in 1973 (Stanley 1972). Results of entomological work conducted in four field trips to this site during 1976 and 1977 are presented here. Materials and methods Details of the study site can be found in a previous paper (Liehne, P.F.S. rt a/. 1976~);its geographic location is also shown in this paper and in Stanley (1975). The previous scheduling of field trips (1-8) to coincide with the changes of season from dry to wet and vice-versa was maintained, with the exception of field trip 9 (June/July, 1976) which was undertaken in an attempt to locate foci of Murray Valley Encephalitis (MVE) virus active in the middle of the dry season. Whenever possible, efforts were made to trap over a wider area than previously. From trip 10 onwards the Irrigation Area (Area 40) was trapped regularly and extensively following a marked reduction in 1974 of insecticide application resulting in an increase of mosquito numbers in this area. The Packsaddle Plain Area (Area 20) was trapped regularly on all four trips, as this area was generally found to yield the highest mosquito numbers, although it had not been trapped extensively on trips 1-8. The Parry's Creek Area (Area 90) was trapped several times on trips 9, 10 and 11 following high virus isolation rates from the few samples obtained on trips 1,2 and 3 in 1972(unpublished data). The north-eastern corner of Lake Argyle and nearby creeks (Area 50), and the old Wyndham town area (Area 70) were trapped several times during trip 12. The Lake Argyle trapping was commenced following the discovery of Cx. annulirostris Skuse larvae on the shallow shores of the lake during trip 11. Animal bait trapping for mosquitoes at Wyndham was incidental to attempts to capture Culicoides spp. by human bait catches in the area. Figs 1-5 show locations of all trap sites used regularly during trips 9-12. Many of these are the same as those used by Liehne el a/. during trips 1-8, although all sites have been renumbered to accommodate the increase in area trapped. Adult mosquitoes were generally collected by the chicken baited drum trap method previously used (trips 1-8, Liehne, P.F.S. et a/. 1976~).Six new traps of similar design were built to enable the coincident use of chicken and suitable mammalian bait (rabbit or guinea-pig) (see Figs 6, 7). This was achieved by incorporating a removable partition into the design of the traps. These were constructed of 16 gauge, 25 mm L-section aluminium, 20 gauge aluminium sheet, aluminium rivets and fibreglass flyscreen wire. Flyscreen wire sections of the 'mammalian bait' half of each trap were reinforced internally with 12 mm square wire mesh. Desirable attributes of light weight, rigidity and durability were thus improved when compared to the previous chicken baited traps. These were constructed of 3 mm diameter steel wire, galvanised iron sheeting and fibreglass flyscreen wire. Dimensions of 75 x 27 x 27 cm were chosen for the new aluminium traps so as to enable maximum use of space in the back of the station-wagon used to transport traps in the field. Entrance funnels were so constructed as to be inter-changeable, enabling trials to be undertaken comparing entrance funnels of different inflexion.

In addition to the normal capture of mosquitoes to monitor numbers and provide samples for virus isolation, trials were set up to enable comparisons to be made of the: (i) relative attractiveness to mosquitoes of the three bait animals (chickens, rabbits and guinea-pigs); (ii) relative effectiveness of entrance funnels of three different inflexions (for details see Table 2).

5 492 240

195

-

41 (0.75)

2 (0.04)

-

3 (0.05)

1 (0.02) -

-

1 780

l(O.06) l(O.06) l(O.06) 15 (0.84) l(O.06)

-

2(0.11) 2(0.11)

-

538 (30.22)

396 (7.21)

-

-

-

-

48 (2.70) 1 169 (65.67)

-

1 (0.02) 6 (0.11)

-

1 (0.02)

-

Trip 10 10.10.7612.11.76

477 (8.69) 21 (0.38) 50 (0.91) 4493 (81.81)

-

l(O.06) l(O.06) -

Trip 9 22.6.7619.7.76

*Figures shown are numbers of each species captured with percentagesof totals captured in brackets.

TRAPPING EFFORTS (Trap nights)

TOTALS

Aedes (Finlaya)notoscriptus (Skuse) Aedes (Macleaya) iremulus (Theobald) Aedes (Muridus) alternans (Westwood) Aedes (Neomehiconion) lineatopennh(Ludlow) Aedes (Ochlerotatus)normanensis (Taylor) Aedes (Ochlerotatus)vigilax (Skuse) Aedes (Stegomyia) kathermensisWoodhill Aedeomyia catasticta Knab Coquillettidiaxanthogaster (Edwards) Mansonia (Mansonwi&s) uniformis (Theobald) Culex (Culex) annulirostris Skuse Culex (Culex) bitaenwrhpchus Giles Culex (Culex)fatigans Wiedemann Culex (Culex) sitiens Wiedemann Culex (Culex) starckeae Stone and Knight Culex (Culex) vicinus (Taylor) Culex (Culiciomyia)pullus Theobald Culex (Lophoceraomyia)hilli Edwards Triperoides(Rachwnoromyia)punctolateralis (Theobald) Anopheles (Anopheles) bancroftii Giles Anopheles (Cellia) amictus Edwards Anopheles (Cellia) annulipes Walker Anopheles (Cellia) hilli (Woodhill and Lee)

Species

81 (0.35)

224

22 892

2 (0.01)

13 (0.06)

45 (0.20) 5 (0.02)

-

1 (-)

-

9 (0.04) 41 (0.18) 7 (0.03) 22 301 (97.40) 4 (0.02) 285 (1.24)

-

I(-)

1 1 (0.05) 34 (0.15)

10 (0.04) 23 (0.10) 18 (0.08)

14.4.774.5.77

Trip 11

Field tripidates

245

8 618

23 (0.27) 4 (0.05) 5 (0.06) 60 (0.70) 5 (0.06) 12 (0.14)

-

563 (6.53) 229 (2.66) 143 (1.66) 6 829 (79.24) 2 (0.02) 198 (2.30) 7 (0.08)

l(O.01)

21 (0.24) 452 (5.24)

l(O.01)

-

12 (0.14) 51 (0.59)

Trip 12 16.11.7714.12.77

TABLE1 MOSQUITOES CAPTURED* DURING FOUR FIELD TRIPS TO THE ORD RIVER, 1976-1977

904

38 782

81 (0.21) 3 (0.01)

13 (0.03j

291 (0.75) 200 (0.52) 34 792 (89.71) 6 10.02) I 417 (3.65j 7 (0.02) 5 (0.01) 3 (0.01) 105 (0.27) 4 (0.01) 6 (0.02) 106 (0.27)

1 0 9 i i2.83)

1 (-)

24 (0.06) 75 (0.19) 18 (0.05) 12 (0.03) 57 (0.15) 459 (1.18)

TOTALS

4

n z

F

5

m

?

m

P

ORD RIVER MOSQUITO CAPTURES

49

This was achieved by putting the traps (two or three depending on the comparisons made) at different subsites 20 to 30 m apart at the same time on a given night. This minimised the possibility of mosquitoes attracted by the odour of an avian bait entering a trap with a mammalian bait, and vice versa. Traps were then rotated between subsitesm subsequent nights. Mosquitoes captured were identified and processed for virus isolation as described earlier (Liehne, P.F.S. ei al. 1976~).

FIG.l a r d River study area, showing figures 2-5 as insets.

50

A. E. WRIGHT

FIG.2-Wyndham (Area 70) and Parry's Creek (Area 90) on the lower Ord floodplain showing trap sites.

Results and discussion (Q General The numerical dominance of Cx. annulirostris noted by Hodgkin and Britten (1955) and Liehne, P.F.S. et al. (1976~)was confirmed in this study. However, one of the secondary aims of the study since trip 10 has been to determine how much, if any, of

ORD RIVER MOSQUITO CAPTURES

51

this dominance is an artifact of the trapping methods used (i.e. the almost exclusive use of chickens as bait). As a result rabbits and guinea-pigs have been used to provide an insight into any previous trapping bias. Overall results of these trials are shown in Tables 3 and 4. The unusually large numbers of Cx. annulirosrris captured during trip 1 1, and to a lesser extent trip 12, were largely due to an increased trapping effort in the Packsaddle

FIG.3-The main Irrigation Area (Area 40) in the Ord Valley showing trap sites.

LHDIYM ‘2 ‘V