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Mosquito Risk Assessment: Ploughman’s Creek Constructed Wetlands, August 2009Climate and rainfallThere are a number of key climatic differences between Orange and the threereference sites that must be taken into account when comparing the abundance anddiversity of mosquito populations across the four locations. These climaticdifferences are important considerations and extrapolation of data from the referencesites to the Orange district should be done with caution.Notwithstanding the distance between Orange and the three reference locations(approximately 100km to Forbes, 350km to Leeton and 350km to Griffith), there aredifferences in elevation (Orange 950m compared to Forbes, 230m, Leeton 140m andGriffith 135m), rainfall and temperature.The long-term average total monthly rainfall for Orange and the three referencelocations is shown in Figure 2. Of note is the substantially greater rainfall recorded atOrange compared with the reference locations. This rainfall distribution wouldtypically indicate that Orange may be predisposed to more favourable mosquitohabitats when compared to the other three locations. However, given the absence oflarge floodplains and/or wetlands associated with the major waterways, favourablehabitats for major pest mosquitoes are not common in the region.The relatively high elevation of the Orange district results in generally coolertemperatures compared to the three reference locations as indicated in Figure 3.While mean daily maximum and minimum temperatures per month are relativelysimilar for Forbes, Leeton and Griffith, temperatures recorded at Orange betweenDecember and March each year are generally 4-6oC cooler.Studies have shown that temperature is a significant predictive factor for mosquitoactivity and, even in the presence of favourable mosquito habitats and high rainfall,mosquito populations, particularly the most common freshwater species found inNSW, will not reach substantial levels at cool climates.The comparison of climate between Orange and the three reference locationshighlights some notable differences. While the higher rainfall may predispose theregion to potential increases of mosquito populations, the cooler climate may limitthe magnitude of population increases. The use of the three reference locationsprovides important information in the absence of local mosquito data. However, theextrapolation of mosquito risk from the data provided by reference locations shouldbe approached with due caution.Page 6

Mosquito Risk Assessment: Ploughman’s Creek Constructed Wetlands, August 2009100Orange90GriffithAverage Monthly Rainfall nJulAugSepOctNovDecFIGURE 2. Mean long-term total monthly rainfall for four key inland mosquitomonitoring locations in NSW including Orange, Forbes, Griffith and Leeton.403530Orange Mean Max TempOrange Mean Min TempGriffith Mean Max TempGriffith Mean Min TempForbes Mean Max TempForbes Mean Min TempLeeton Mean Max TempLeeton Mean Min FIGURE 3. Mean long-term daily maximum and minimum temperatures per monthfor four key inland mosquito monitoring locations in NSW including Orange, Forbes,Griffith and Leeton.Page 7

Mosquito Risk Assessment: Ploughman’s Creek Constructed Wetlands, August 2009MOSQUITO POPULATIONS OF THE ORANGE REGIONWhile many regions of NSW have been part of the NSW Health ArbovirusSurveillance and Mosquito Monitoring Program, there has not been any mosquitosampling undertaken within the area covered by this mosquito risk assessment. Norecent scientific studies or technical reports could be found that may providequantitative data on the extant mosquito fauna.Although the previous section has identified that the cooler temperatures of theOrange district is a possible limiting factor in the potential for large mosquitopopulations. In the absence of any mosquito fauna surveys, it is difficult to makepredictions regarding the abundance of potential pest species. However, frominformation on mosquito fauna from other regions within inland NSW somepredictions of general mosquito fauna can be made.An assessment of adult mosquito collections undertaken at Forbes has identified atotal of 15 mosquito species (Table 1), with the species of greatest concern, due tolikely nuisance biting impacts and potential public health risks, Anopheles annulipes,Coquillettidia linealis, Culex annulirostris, Culex quinquefasciatus and Aedesnotoscriptus. These species are associated with a range of different habitatsincluding natural and urban freshwater environments. The actual impacts of thesespecies is expected to vary with the relative abundance of populations dependent onfactors such as the quantity and distribution of rainfall, flooding events, speciesspecific dispersal patterns, prevailing winds and the distribution of appropriate refugesites. An important factor in determining future mosquito diversity and abundance, aswell as pest impacts, may be the creation of new wetlands if they represent asubstantial change in the availability of potential mosquito habitat.Compared to the abundance of mosquitoes at other locations such as Leeton andGriffith, mosquito abundance at Forbes is considered relatively low (Figure 4). Thesubstantially greater abundance of major pest species such as An. annulipes andCx. annulirostris at Leeton and Griffith compared to Forbes is the result, primarily, ofavailable habitat provided by agriculture but also higher mean temperatures. BothForbes and Orange are surrounded by different types of agriculture compared toLeeton and Griffith where flatland crops associated with flooding irrigation (e.g. rice)are more likely to provide opportunities for mosquito production. In addition, Orangeand Forbes are located at a higher elevation than Leeton and Griffith and resulting ingenerally cooler temperatures, this is particularly the case for Orange thatexperiences a much cooler climate than Forbes.This data suggests that the species recorded from Forbes provides a reliableindicator of mosquito diversity in the Orange region. However, the substantiallyhigher rainfall and cooler temperatures of Orange make it difficult to confidentlypredict the abundance of pest species in this region. While the cooler temperaturesmay limit the abundance of mosquitoes, the higher rainfall may provide morefavourable mosquito habitats. It is difficult to make an assessment on currentPage 8

Mosquito Risk Assessment: Ploughman’s Creek Constructed Wetlands, August 2009productivity of aquatic habitats but the steep topography throughout much of theOrange region indicates there are few opportunities for low-lying floodplains orwetlands. The absence of substantial bushland areas, where mosquitoes associatedwith ephemeral ground pools created following rainfall can be productive, is alsolikely to limit the abundance of mosquitoes.Overall, this information suggests that the mosquito species most likely to representcurrent and future pest impacts in the Orange region are: Culex annulirostris is a medium sized, light to dark coloured mosquito with abanded proboscis. Larvae are commonly collected from a range of freshwaterhabitats from flooded grasslands to permanent, well-vegetated wetlands. Thismosquito is of increasing concern as constructed freshwater wetlands areincreasingly incorporated into urban coastal developments. Coquillettidia linealis is a medium sized, dark mosquito and has the potential tobe an important pest close to extensive natural or constructed well vegetatedfreshwater wetlands. The larval biology of this species differs markedly frommost other mosquitoes in that the larvae have a modified siphon that attachesto the roots and/or stems of aquatic vegetation. Culex quinquefasciatus is a medium sized pale brownish mosquito and anothervery common pest species in urban areas. The larvae of this mosquito areusually associated with, habitats with a high organic content such as drains,sullage pits, septic tanks and other water holding and water storage areas. Thisspecies is more likely to be associated with a constructed wetland if there arehigh levels of organic content within inflows. Anopheles annulipes is a medium sized, spindly grey mosquito. The larvae aretypically associated with freshwater wetlands, particularly those wherefilamentous algae forms thick mats on the water surface. Aedes notoscriptus is a small to medium sized dark mosquito with conspicuouspale banded legs. This mosquito is often a serious nuisance pest in residentialareas. The larvae are usually associated with small water holding containersaround dwellings such as tins, pots, ornamental ponds, blocked guttering andtyres, as well as water holding plants (e.g. bromeliads) and tree holes.It is likely that there are many other species of mosquito present in the greaterOrange region. However, these species may only be found at very low abundancesor may only be collected following particularly favourable climatic conditions. Theeggs of many Aedes spp. are desiccation resistant and can remain in soil or on rocksor debris for many years until flooding occurs. These species may cause some shortterm nuisance-biting impacts but do not represent the same potential pest risks asother species.Page 9

Mosquito Risk Assessment: Ploughman’s Creek Constructed Wetlands, August 2009TABLE 1. Total number of mosquitoes collected at trap sites located at Forbes,NSW, 1995-96 through until 2000-2001.No. trap weeks per yearAedes alboannulatusAedes alternansAedes notoscriptusAedes sagaxAedes theobaldiAedes vittigerAnopheles annulipesCoquillettidia linealisCulex annulirostrisCulex australicusCulex cylindricusCulex molestusCulex orbostiensisCulex quinquefasciatusMansonia uniformisWeekly 1481601811400Number of mosquitoes per trap s notoscriptusAnopheles annulipesCoquillettidia linealisCulex annulirostrisCulex quinquefasciatusFIGURE 4. Mean trap night abundance of five common mosquito pests collected atForbes, Griffith and Leeton as part of the NSW Arbovirus Surveillance and MosquitoMonitoring Program.Page 10

Mosquito Risk Assessment: Ploughman’s Creek Constructed Wetlands, August 2009MOSQUITO RISK AND CONSTRUCTED WETLANDSThe areas of Australia likely to be at greatest ‘mosquito-risk’ from constructedwetlands are those where natural wetlands are non-existent or few and, generally,there are relatively small mosquito populations. In such areas, particularly in arid orsemiarid regions, the establishment of wetlands can provide a high-risk situation withhabitat for pest and vector mosquitoes, and perhaps for vertebrate reservoirs ofmosquito-borne pathogens, that presents a major public health concern to localcommunities. Elsewhere, in regions with greater inherent mosquito habitat,constructed wetlands can still be a health concern, providing additional breeding andharbourage sites and bringing vector mosquitoes into closer association withpathogen reservoirs such as birds that are roosting or nesting, or mammals visitingfor water or shelter.In the absence of site-specific mosquito sampling, due caution must be exercisedwhen assessing the potential future pest impacts associated with the constructedwetlands based on off-site mosquito data. However, while the magnitude of mosquitopopulations may be difficult to predict, the mosquito species of greatest concern inthe local area can be identified.The three species most likely to colonize the constructed wetlands areCx. annulirostris, Cq. linealis and An. annulipes. Of the mosquito species most likelyto exist in the local area, these three species are best suited to the newlyconstructed wetlands. However, each of these three species exhibits uniquebiological and ecological requirements that may influence the rate of colonization ofwetlands and generation of problematic populations.It is important to note that currently, there does not appear to be any substantialwetland areas that already support populations of these species. While themosquitoes may be present in the Orange region, there is no evidence from aerialphotography that substantial habitats occur close to the residential areas. There are,however, some areas along Ploughman’s Creek that appear to be potential mosquitohabitats but, as the constructed wetlands (particularly Escort Way Wetland,Sommerset Park Wetland and Burendong Way Wetland) will replace these existinghabitats, there may be no significant net increase in potential mosquito habitat.As previously stated, it is difficult to assess the current mosquito productivity of areasalong Ploughman’s Creek based on aerial photography alone. While there appearsto be some moderately large areas of macrophytes, many factors will influence howfavourable they are for mosquitoes and, as a consequence, make it difficult todetermine if they currently represent a locally significant mosquito habitat.As there are unlikely to be large existing populations from which colonization ofnewly constructed wetlands can occur, the five newly constructed wetlands mayrepresent a significant increase in potential mosquito habitats and, as aconsequence, contribute to overall increases in mosquito populations.Page 11

Mosquito Risk Assessment: Ploughman’s Creek Constructed Wetlands, August 2009Nuisance bitingIt is extremely difficult to quantify the impact of nuisance biting in a specific area dueto the spatial and temporal fluctuation in mosquito abundance resulting from the mixof different mosquito species, their respective habitats and the environmental and/orclimatic conditions that trigger population increases. The NSW Mosquito Monitoringand Arbovirus Surveillance Program identifies average trap densities of adultmosquitoes greater than 100 per trap as high with over 1,000 per trap as very high toextreme when exceeding 100,000 per trap.At Forbes, adult mosquito collections rarely exceeded 100 mosquitoes per trap nightand there is currently no evidence to suggest that mosquito populations at Orangeare substantially higher. However, even in areas where there is a low baseline ofmosquito activity, even small increases in relative abundance may result innoticeable increases in nuisance-biting rates.The tolerance level of individuals to nuisance-biting varies substantially and is oftendependent on the extant mosquito populations and previous personal experiences.However, there are strong indicators that nuisance biting alone can have negativeimpacts on a homeowner’s standard of living as well as the economic impacts onresidential, recreational and tourist developments.In areas where there is a change in the contact between mosquitoes and humans,where residential developments encroach on wetlands or newly constructed orrehabilitated wetlands increase mosquito populations, pest impacts may be morenoticeable. Even in regions where mosquito populations are considered low, it is therelative change in local mosquito abundance and/or diversity that is often theconcern for local communities.Public health risksRoss River virus (RRV), Barmah Forest virus (BFV) and Murray Valley encephalitisvirus (MVE) are the most serious disease-causing pathogens spread by mosquitoesin Australia and are classified as notifiable diseases by NSW Health. There are otherarboviruses transmitted by mosquitoes but these rarely cause human disease inAustralia.There are, on average, approximately 5,000 human cases of RRV and BFV per yearacross Australia. While the symptoms can vary greatly between individuals, andinclude fever and rash, infection with either of these viruses may result in a conditionknown as polyarthritis with arthritic pain in the ankles, fingers, knees and wrists.Generally, the rash tends to be more florid with BFV infection but the arthritic pain isgreater with RRV infection.Page 12

Mosquito Risk Assessment: Ploughman’s Creek Constructed Wetlands, August 2009TABLE 4. A summary of mosquito species most likely to occur at Orange and knownarboviruses previously isolated from elsewhere in NSW and the human biting riskthey represent.Mosquito speciesRoss RivervirusBarmahForestvirusMurray ValleyencephalitisvirusOtherarboviruses1 Aedes alboannulatusAedes alternans Aedes notoscriptus Aedes sagaxAedes theobaldiBitingpest2 Aedes vittigerAnopheles annulipes Coquillettidia linealis Culex annulirostris Culex australicusCulex cylindricusCulex molestus Culex orbostiensisCulex quinquefasciatus Mansonia uniformisOther arboviruses include Stratford virus, Edge Hill virus, Kunjin virus, Kokobera virus and Sindbisvirus. These viruses, although known to cause mild human disease, are not considered serious publichealth risks2Mosquito has been documented as a human biting species (Russell 1990)1Both viruses have the potential to be spread by mosquitoes likely to be present in theOrange region (Table 4), particularly Cx. annulirostris, generally considered to beone of the most important pest species of inland regions. The transmission cycles ofthese viruses require the presence of suitable reservoir hosts such as nativemacropods (i.e. kangaroos and wallabies or waterbirds), and there is a greater risk ofdisease transmission in rural and semi-rural areas compared with major urbancentres.It is important to note that while Cx. annulirostris is generally considered to be themajor vector of arboviruses from inland regions, mosquito species expected to beboth present in Orange and potentially associated with the constructed wetlands,such as An. annulipes and Cq. linealis, are known to carry and transmit diseasecausing pathogens. One of the key nuisance-biting pests and vectors of disease,Ae. notoscriptus, is likely to be present in Orange but this species will not be directlyassociated with the constructed wetlands.Page 13

Mosquito Risk Assessment: Ploughman’s Creek Constructed Wetlands, August 2009While BFV tends to be more prevalent in coastal regions, RRV is, generally, equallycommon in both inland and coastal regions as indicated by human notification data.From 1994 to 2008, an average of 24 cases per year of BFV were reported frominland areas compared to 298 cases from coastal regions while, over the sameperiod, 257 cases of RRV were reported from inland regions compared to 404 casesfrom coastal regions.The most important factors influencing the number of cases from each region aredifferences in local mosquito fauna (e.g. the coa

sites to the Orange district should be done with caution. Notwithstanding the distance between Orange and the three reference locations (approximately 100km to Forbes, 350km to Leeton and 350km to Griffith), there are differences in elevation (Orange 950m compared to Forbes, 230m, Leeton 140m and Griffith 135m), rainfall and temperature.