Does wetland rehabilitation need mosquito control?

Webb_estuarinewetlands_SydneyOlympicPark_2014Mosquitoes can be more than a nuisance. They pose health risks but could also erode the good will of the community for wetland rehabilitation projects. Wetland rehabilitation needs mosquito control.

It’s a warm February evening. A small and anxious group of residents have gathered in a local community hall to discuss the implications of a local wetland rehabilitation project. Some are angry. One of the first questions comes from an elderly gentleman. Close to tears, he explains how his grandchildren no longer visit due to the plagues of mosquitoes that engulf his property day and night. “What are you guys going to do about it?” he pleads.

I learnt a valuable lesson that night. Trying to explain the best mosquito repellent to use doesn’t go down too well when an audience is facing some significant nuisance-biting impacts around their homes. It doesn’t matter how much DEET is in the repellent, it may well work but is it something you need to wear all day just to get the day to day jobs done around the house? Something more substantial is required and, with hindsight, should have been in place before the first waves of mosquitoes left the local wetlands.

Coastal wetlands are under threat

Sea level rise and climate change  is putting pressure on saltmarshes and urbanisation is eating away land that would otherwise accommodate a landward shift in estuarine habitats. There is nowhere for saltmarshes to spread to so they’re destined to be swallowed up by mangroves. While the mangroves are valuable themselves, they don’t provide the same critical habitats required by many of the internationally protected migratory shorebirds that rely on saltmarshes. Saltmarsh habitats could well disappear from much of the east coast in coming decades if sea levels rise as expected and mangroves continue their march landward.

webb_landinglightswetlandsEstuarine wetlands and mosquitoes

Saltmarshes are home to one of our most important pest and vector mosquito species. While it is important to remember that Aedes vigilax is an Australian native animal and just as much a part of our wetland ecosystems as fish and birds, there is little doubt that it can have substantial impacts with regard to nuisance-biting and the transmission of Ross River virus.

Historically, many of the saltmarshes along the east coast were drained or filled to enable increased cattle grazing (although much of it was under the guise of protection from flooding). Tidal flows were cut off with the construction of levee banks and installation of flood gates. Notwithstanding the impacts of grazing, without tidal exchange, the habitats became brackish water to freshwater dominated systems with a dramatic change in vegetation. Saltmarsh and sedgeland vegetation was steadily replaced by reeds and rushes. Invasive plants such as Phragmites quickly took over many of these wetlands.

webb_floodgates_march2011Bringing back the tides

To combat the degradation of wetlands and impending loss due to climate change, there has been some ambitious wetland rehabilitation projects planned. One of the largest in the southern hemisphere is the Hexham Swamp Rehabilitation Project. Much can be learned from the experience in this wetland just west of Newcastle, NSW, and applied to rehabilitation projects, not only in Australia but overseas as well.

Rehabilitation of Hexham Swamp involved the staged opening of existing floodgates to reinstall tidal flows to an otherwise freshwater system. Many aspects of this project were considered and it is unsurprising that one major issue was the possible impact of mosquitoes. Mosquito populations were something of legend in this area, enough so that there is a “big mosquito” outside the local bowling club affectionately known as “Ossie the Mossie” (coincidently, “Ossie” celebrated her 20th birthday in March 2014).

One of the important considerations when assessing mosquito risk was that there had been a dramatic transformation of the areas surrounding the wetlands in the last 20 years. What was once agricultural land was now residential. This is the same situation right along the east coast of Australia, the rapid urbanisation and swelling residential populations along the coast have put people in the firing line of Aedes vigilax.

The prospect of mosquito control was raised in the early stages of the rehabilitation planning but there was great reluctance from the local authorities to head down that path. The problem is that broad scale mosquito control and ecological rehabilitation are often seen at opposite ends of the wetland management spectrum. I’ve experienced this many times first hand, from scepticism regarding the non-target impacts of biological larvicides to “Apocalypse Now” jokes as helicopters go about routine spraying of local wetlands.

Webb_MosquitoCollectionsIs there such a thing as “environmentally friendly” mosquito control?

The hangover from the actual and perceived impacts of pesticide use more than 50 years ago is still present in the minds of many responsible for managing local wetlands. I say perceived as the development of environmentally sustainable mosquito control programs over the past two decades may not be appreciated amongst those charged with managing wetlands and wildlife.

I feel we need to continue building the case for the range of mosquito control strategies available for our coastal wetlands. Not only do we need to convince local authorities that mosquito populations can be minimised without adversely impacting the local environment but that mosquito control should be a critical consideration in wetland rehabilitation projects. It also has the potential to reduce mosquito-borne disease.

We know that the direct non-target impacts of Bacillus thuringiensis israelensis and s-methoprene are minimal and there is growing evidence that the indirect impacts on local wildlife due to reduced mosquito populations isn’t a major concern. Well-designed projects can also minimise the frequency of treatments while reducing peaks in mosquito activity.

It seems our coastal bats populations are mostly eating moths, not mosquitoes so there is unlikely to be any significant impact on these bats resulting from reduced food. There is no reason why the judicious use of larvicides can’t knock the top off abundant mosquito populations, reduce the pest impacts on local community and not pose a risk to local wildlife. Perhaps it should be considered a critical component of wetland rehabilitation?

redkneeddoteral_kooragangisland_march2015Mosquito control and wetland rehabilitation

In speaking with wetland managers, I try to instil with them the importance of mosquito control. There is a risk that swarms of nuisance biting mosquitoes may erode the good will in the community for wetland rehabilitation. These systems, particularly in the early phases of rehabilitation don’t represent pristine environments and while there may not be a desire to establish ongoing mosquito control programs, some control may prove useful while the wetland comes back into balance with the changed environmental conditions.

Rehabilitation takes time and while there is substantial breakdown of freshwater vegetation it is not going to be ideal for fish and other mosquito predators. It is likely to provide ideal conditions for mosquitoes. Over time, however, fish are likely to increase in both their abundance and penetration into the wetlands and greater tidal flushing will make many of the wetland habitats generally unsuitable for mosquitoes.

Perhaps there is benefit in undertaking control as a show of good will to the local community? After all, engagement with the local community will be critical in the success of wetland rehabilitation projects.

The restoration of tidal flows to Hexham Swamp resulted in an initial increase in the abundance of Aedes vigilax. These increases resulted in substantial nuisance-biting impacts. However, in subsequent seasons, the populations of Aedes vigilax levelled out to comparable levels to those of the surrounding estuarine wetlands. The net result has generally been that the long-term moderate increases in Aedes vigilax populations have been balanced by reductions in Culex annulirostris and Coquilettidia linealis populations as the wetlands shifted from freshwater to saline. The health of the wetlands, as well as the local estuary, is improving.

Mosquito control is only a short-term fix and if the rehabilitation of estuarine wetlands is not carefully planned, there may well be ongoing mosquito impacts. However, reducing any initial impacts will go a long way to ensuring continued engagement of the community with the local wetlands. Cost and the operational considerations may be a barrier for organisations unfamiliar with broadscale mosquito control but these issues can be overcome with the expertise that exists in many parts of the country.

In summary, it is important that mosquito management be considered in the planning process of major wetland rehabilitation projects. There is little doubt that such projects will be required into the future as saltmarsh habitats and other estuarine wetlands are threatened but protection wetlands shouldn’t mean increasing mosquito populations. A balance is required between conservation of environmental health and protection of human health.

A longer version of this article originally appeared in “Mosquito Bites” (the Bulletin of the Mosquito Control Association of Australia).

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What do bats eat more often, mosquitoes or moths?

batscatHave you ever thought about how hard it is to find pieces of munched mosquito in a pellet of bat poo?

Perhaps not but for many years I have. Luckily, I have been part of a wonderful research project that has overcome the difficulties of studying the diet of insectivorous vertebrates.

I’ve already written about the multidisciplinary research project, funded by a research grant from the NSW Environmental Trust, designed to determine the importance of saltmarsh mosquitoes (Aedes vigilax) to local bat populations on the Central Coast of NSW.  The hard work in this project was done by Dr Leroy Gonsalves but the 4 year project also included researchers from the Australian Catholic University, University of Sydney and Forest Science Centre (NSW Department of Primary Industries).

Our most recent publication, titled “Mosquito Consumption by Insectivorous Bats: Does Size Matter?“, is the fourth paper to be produced from this study. We’ve probably left the best until last as this research has really opened some doors to our understanding of mosquitoes and their importance in the diet of Australian bats in coastal environments.

Doggett_AedesvigilaxThere had already been some qualitative studies that hinted at the possibility that saltmarsh mosquitoes were an important prey source for insectivorous bats. Studies have shown that the “feeding buzzes” (the sounds bats make when they’re hunting food) had been recorded over saltmarsh habitats. The conclusion was made that these bats were most likely feeding on mosquitoes. Some of our studies have indicated that the abundance of mosquitoes can influence the use of foraging habitats of bats. As with previous studies, this conclusion was made by recording the “feeding buzzes” of bats or radio-tracking bats while also sampling the abundance of mosquito and moth populations.

What was missing was a quantitative assessment of mosquito consumption by bats. How many mosquitoes are eaten by bats? How many bats species were eating mosquitoes? How important are mosquitoes, compared to other insects, such as moths, to the diet of bats?

This was the critical question. It was also a little tricky trying to work out how to get an answer. While the circumstantial evidence appeared to be strong, we needed actual evidence that bats were eating mosquitoes.

bathouseWhat did we do?

There are a few different ways you can determine what bats are eating. The classic approach is through visual examination of fecal material. That basically means searching through bat shit under the microscope looking for bits of insect and then identifying it. This technique has been used successfully. However, it only took us 15 minutes of looking through bat shit to realise this wasn’t going to cut it. We could clearly identify pieces of insect but what were they? We never found a piece of “entomological material” in the samples that was definitively from a mosquito. We needed another approach.

Since we know there was insect material in the feces, perhaps we could use molecular methods to identify them? Instead of looking for bits and pieces of chewed up mosquito, why not look for mosquito DNA?

Fecal samples were taken from five eastern Australian bat species; Vespadelus vulturnus (little forest bat), Vespadelus pumilus (eastern forest bat), Miniopterus australis (little bent-wing bat), Nyctophilus gouldi (Gould’s long-eared bat) and Chalinolobus gouldii (Gould’s wattled bat). These bats are relatively common in coastal regions of NSW and range in size from 4-14 g. Yes, these bats are small!

How did we collect the bat feces? Bats were collected using harp traps set in forest habitats adjacent to the estuarine wetlands. Bats that were trapped were then transferred to small bags. Bats were kept in the bag for at least 1 hour and then released. Fecal pellets were then collected from the bottom of the bag and returned to the laboratory for testing.

Five sub-samples from each individual bat were tested for the presence of mosquito and moth DNA using molecular techniques. We even conducted some quality control experiments and found that our molecular techniques could identify mosquitoes, even if they comprised around 5% of the total contents of the fecal pellet. We were confident that if a sample was negative, it was highly unlikely that moth or mosquito material was contained within the fecal pellet.

Leroy Gonsalves removing a little forest bat from the harp trap. All set to collect some bat poo! (Photo by Doug Beckers)

What did we find?

Across the five bat species collected, fecal samples were collected from 52 individual bats. From the samples, a total of 40 prey taxa was identified from the bat feces. Moths were the most frequently detected prey, present in the feces of 49 of the 52 (approx 94%) bats tested individuals. We were expecting this, moths are probably a pretty tasty treat for a bat.

Interestingly, mosquitoes were only detected in the feces of two bat species (Vespadelus pumilus and Vespadelus vulturnus). Of those bats, mosquitoes were collected from 2 of 10 Vespadelus pumilus individuals and 11 of 20 Vespadelus vulturnus individuals. All the individuals had eaten moths.

With regard to mosquitoes, only 2 of the 5 bat species tested had been eating mosquitoes and of those two species 20-55% had recently consumed mosquitoes. By comparison, all individuals of these two species tested had been eating moths.

Little Forest Bat (Vespadelus vulturnus) (http://museumvictoria.com.au/melbournemuseum/discoverycentre/wild/victorian-environments/dry-forest/little-forest-bat/)

Little Forest Bat (Vespadelus vulturnus) (Photo: Museum Victoria)

What does this mean?

Most importantly, it highlights the dangers of generalisation when it comes to bat biology. We’ve clearly demonstrated in this study that there will be species-specific relationship between bats and preference for foraging on mosquitoes. This relationship is most likely driven by the size of the bat. Based on our work, the small bat species are more likely to feed on mosquitoes.

This makes sense as other studies have shown that when it comes to insectivorous vertebrates, larger predators tend to eat larger prey, rather then greater numbers of smaller prey.

With regard to bat conservation, the results of our study haven’t identified a bat species that relies heavily on mosquitoes. However, in regions where smaller bat species are threatened or endangered, consideration should be given to the possibility that if mosquitoes are locally abundant, they may provide a relatively more important food source for these species. Local authorities should be mindful of this potential when designing broad-scale mosquito control programs. Perhaps they should be doing more to encourage moth populations?

If they’re good enough for bats, perhaps moths are good enough for us? Canberra’s Ironbark Cafe cooks up “Brandy Flamed Bogong Moth Frittata”. (News Limited)

It has been exciting working on this aspect of the project. It has been great to see Leroy develop this innovative use of molecular techniques to investigate bat diet (as well as helping understand the ecological role of mosquitoes). Many ecologists are heading in this direction and it will be interesting to see how new technologies open up new opportunities. When we started this project, this approach hadn’t been used extensively, and certainly not to determine weather bats were eating mosquitoes.  This technique has now been used widely, including to determine if bats are eating agricultural insect pests and can be used to track bats and their response to seasonal fluctuations of these pest insects. It has also been used to investigate the diet of snakes.

The full citation our the paper is:

Gonsalves L, Bicknell B, Law B, Webb C, Monamy V (2013) Mosquito Consumption by Insectivorous Bats: Does Size Matter? PLoS ONE 8(10): e77183. doi:10.1371/journal.pone.0077183

The abstract is below:

Insectivorous bats have often been touted as biological control for mosquito populations. However, mosquitoes generally represent only a small proportion of bat diet. Given the small size of mosquitoes, restrictions imposed on prey detectability by low frequency echolocation, and variable field metabolic rates (FMR), mosquitoes may not be available to or profitable for all bats. This study investigated whether consumption of mosquitoes was influenced by bat size, which is negatively correlated with echolocation frequency but positively correlated with bat FMR. To assess this, we investigated diets of five eastern Australian bat species (Vespadelus vulturnus Thomas, V. pumilus Gray, Miniopterus australis Tomes, Nyctophilus gouldi Tomes and Chalinolobus gouldii Gray) ranging in size from 4-14 g in coastal forest, using molecular analysis of fecal DNA. Abundances of potential mosquito and non-mosquito prey were concurrently measured to provide data on relative prey abundance. Aedes vigilax was locally the most abundant mosquito species, while Lepidoptera the most abundant insect order. A diverse range of prey was detected in bat feces, although members of Lepidoptera dominated, reflecting relative abundance at trap sites. Consumption of mosquitoes was restricted to V. vulturnus and V. pumilus, two smaller sized bats (4 and 4.5 g). Although mosquitoes were not commonly detected in feces of V. pumilus, they were present in feces of 55 % of V. vulturnus individuals. To meet nightly FMR requirements, Vespadelus spp. would need to consume ~600-660 mosquitoes on a mosquito-only diet, or ~160-180 similar sized moths on a moth-only diet. Lower relative profitability of mosquitoes may provide an explanation for the low level of mosquito consumption among these bats and the absence of mosquitoes in feces of larger bats. Smaller sized bats, especially V. vulturnus, are likely to be those most sensitive to reductions in mosquito abundance and should be monitored during mosquito control activities

6th International Congress of the Society for Vector Ecology

Culex_molestus_Photo_StephenDoggettThis month, medical entomologists from across the globe will come together in California for the 6th International Congress of the Society for Vector Ecology. With thanks to a travel grant provided by the Bill and Melinda Gates Foundation, two of my (recently completed) PhD students will be attending and presenting work on the role of mosquitoes in urban environments and mosquito-borne disease risk.

The Society for Vector Ecology was established in 1968 to bring together individuals interested in the management of vector-borne disease. This includes professionals mostly involved in mosquito research, mosquito control and surveillance operations and communications. Every four years, the society holds a congress either in North America or Europe. The couple of congresses that I’ve attended have been fantastic and I’m greatly disappointed not to be able to attend this year’s meeting.

The 6th International Congress of the Society for Vector Ecology is being held 22-27 September in La Quinta, California, USA. You can have a look at a PDF of the program here.

Although I won’t be able to make it, two of my PhD students will be attending after being awarded travel grants by Bill and Melinda Gates Foundation. They will be presenting some of the work they completed as part of their PhD candidature.

The titles and abstracts of their presentations are below.

Understanding the ecological importance of mosquitoes to insectivorous bats and the implications for mosquito-borne disease management in coastal Australia

Leroy Gonsalves, Bradley Law, Cameron Webb, Vaughan Monamy and Brian Bicknell

Manangement of mosquito-borne disease risk in coastal Australia faces many challenges. Urbanisation is increasing the size and proximity of the community to productive mosquito habitats. Coastal wetlands are also the focus of conservation and rehabilitation efforts. Mosquitoes associated with these wetlands, in particular the saltmarsh mosquito, Aedes vigilax, are abundant, widely dispersing and key vectors of Ross River and Barmah Forest viruses. These mosquitoes may also represent an abundant prey resource for threatened and endangered insectivorous bat species and local authorities are reluctant to approve broadscale mosquito control programs due to concerns regarding indirect impacts on local bat populations. A combination of diet analysis, radio-tracking and prey abundance studies were undertaken. Analysing prey DNA within guano collected from 52 individuals representing five local bat species demonstrated that bats consumed a diverse range of prey dominated by lepidopterans. Consumption of Ae. vigilax was restricted to two species, Vespadelus pumilus and V. vulturnus. Radiotracking of 13 V. vulturnus individuals during periods of relatively large and small population abundances of Ae. vigilax, together with monitoring of prey abundance, revealed that foraging ranges of bats shifted in response to mosquito abundance (and no other prey). These findings suggest that there are species-specific relationships between bats and mosquitoes and that there may be site-specific strategies required to balance mosquito management and bat conservation.

The biology, distribution and genetics of Culex molestus in Australia?

Nur Faeza A Kassim, Cameron E Webb & Richard C Russell

The Culex pipiens subgroup of mosquitoes includes some of the most important vector species involved in mosquito-borne disease transmission internationally and four species within this subgroup are found in Australia. One of these species, Culex molestus, is thought to have been introduced into Australia in the 1940s. Closely associated with subterranean urban habitats, this mosquito has the potential to cause serious nuisance biting impacts but also may cause significant public health risks through the transmission of endemic arboviruses. Exotic pathogens, such as West Nile virus, may also pose a potential threat to biosecurity of Australia. Our review of the literature has confirmed that the current Australian distribution of Cx. molestus is limited to areas south of latitude -28.17ºS. However, given that the mosquito is established in habitats south of the corresponding zone in the northern hemisphere, there is potential for Cx. molestus to spread north into QLD and NT. Molecular analysis of the mosquito indicated that Australian Cx. molestus shared stronger genetic similarity with specimens from Asia than specimens from Europe or North America. Laboratory and field studies have shown that the mosquito is uniquely adapted to urban environments through the expression of autogeny (ability to lay their first batch of eggs without a blood meal) and stenogamy (ability to mate in confined spaces). Culex molestus is active throughout the year and the current trend towards increased water storage in urban areas of Australia has raised concerns of increased nuisance-biting and public health risks in the future. However, the results of our studies indicate that there may be biological and ecological barriers that may lessen the importance of this mosquito in urban mosquito-borne disease cycles. A delay in blood feeding resulting from their obligatory autogeny, combined with limited access to potential reservoir hosts, may reduce the likelihood of them playing a significant role in pathogen transmission.

Public Lecture: Are mosquitoes good for anything?

LIFEINPARK_8SEP2013On Sunday 8 September I’ll be giving a public lecture as part of the “Life in the Park” annual event at Sydney Olympic Park. Along with presentations by researchers working within the wetlands and bushland of Sydney Olympic Park, I’ll be talking about the ecological role of mosquitoes.

Beyond the pest mosquitoes that we all know (and hate) is a diverse range of species that rarely, if ever, bite and have adapted to every nook and cranny in our local environment. If you’ve ever asked “what are mosquitoes good for?”, then why not come along to this free event and find out! More information, including a map, is here.

The buzz of bat conservation

Illustration by Golly Bard (http://www.etsy.com/shop/GollyBard?ref=seller_info)

Illustration by Golly Bard (Check out some of the other wonderful illustrations of plants and animals by Golly Bard at her Etsy site)

What is the ecological role of mosquitoes?

There is a huge gap in our knowledge of mosquitoes and their place in the world. There are very few studies that have investigated what environmental “good’ they may do. This would actually be a pretty handy thing to find out. A better understanding of their ecological role may assist balancing the objectives of mosquito control and wetland/wildlife conservation that all too often appears to be (or is perceived to be) in conflict. The results of our newly published study in PLoS ONE may help in understanding the ecological role of one mosquito species in coastal wetlands.

There have been plenty of studies that have documented mosquitoes in the diet of a range of animals such as predatory aquatic macroinvertebrates, fish, frogs, lizards, birds and bats. However, the importance of mosquitoes in the diet of these animals either hasn’t been quantified or can often be overstated. One example often cited is that tadpoles eat mosquito larvae. When we tested some common Australian tadpoles, results demonstrated that they rarely consume mosquito larvae. We need to build our understanding of how important mosquitoes are (or perhaps are not) to local ecosystems.

There are almost 80 species of bat in Australia. While fruit bats may be the most commonly encountered, insectivorous bats are a less well known but diverse group of bats. These often tiny bats (often weighing less than 10g) are usually found in small and far less conspicuous groups. Many of them are found in close proximity to humans and many even be present in in major cities like Sydney. Many are listed as threatened or endangered.

The management of these bat species faces many challenges. Notwithstanding the threat of urbanisation and direct impacts on roost and foraging habitats, indirect impacts may also be important. One potential prey in the diet of bats that may be regionally important is the mosquito. Could broadscale mosquito control programs impact local bat populations?

Saltmarshes have already been identified as important habitats for the saltmarsh mosquito, Aedes vigilax. The abundance of these mosquitoes can dramatically increase in response to tidal flooding of coastal wetlands and nuisance-biting impacts can be widespread in the local community as these species disperse widely from local wetlands. This mosquito is also the major vector of Ross River virus in coastal Australia. There are about 5,000 cases of human illness caused by Ross River virus each year across Australia.

An example of saltmarsh habitats in Empire Bay, Central Coast, NSW

An example of saltmarsh habitats in Empire Bay, Central Coast, NSW

Mosquito-borne disease is an ongoing concern in many parts of Australia, particularly coastal regions. There is a range of environmentally sensitive mosquito control strategies available that have been demonstrated to provide effective mosquito control without directly adversely impacting the local environment. However, studies have shown that it is preemptive, not reactive, mosquito control efforts that have a significant impact on reducing mosquito-borne disease risk. Some concerns have been expressed by local authorities regarding the potential indirect non-target impacts on insectivorous bats of reducing local mosquito abundance.

In 2007, a multidisciplinary research project, funded by a research grant from the NSW Environmental Trust, kicked off to investigate the importance of saltmarsh mosquitoes to local bat populations on the Central Coast of NSW. The 4 year project included researchers from the Australian Catholic University, University of Sydney and Forest Science Centre (NSW Department of Primary Industries). The hard work in this project was done by PhD candidate Leroy Gonsalves (Dr Gonsalves now!).

Leroy Gonsalves (Photo ACU Media)

Leroy Gonsalves (Photo ACU Media)

The project was undertaken in a region where mosquitoes had been identified as both a nuisance-biting problem and potential public health risks but local authorities were reluctant to undertake broadscale mosquito control. For full details, see the Living with Mosquitoes document I produced for local councils in the region.

Our previously published studies analysing bat call recordings have shown that the activity of insectivorous bats can be influenced by the abundance of prey as well as the structure of local habitats. Bats use sound for the detection and capture of prey, as well as navigation, and, as calls are species-specific, an analysis of recordings can determine the bat species present in local habitats and what they’re doing there.

In this newly published paper, we radio-tracked Vespadelus vulturnus (little forest bat), a species known to be a predator of saltmarsh mosquitoes. The activity of these radiotracked bats could be monitored across a range of habitats (including saltmarsh and adjacent coastal swamp forest) when mosquito populations were at naturally relatively high and low abundances.

Bats were collected using harp nets set along flyways in the coastal swamp forest close to the estuarine wetlands. The collected bats were then fitted with small radio-transmitters and released. The activity of each bat was tracked over a period of up to 10 days whereby triangulating the signal direction could record where the bats were active, as well as allowing the identification of day roost sites. This data was then analysed together with prey (mosquito and moth) abundance data recorded using light and carbon dioxide baited traps.

An example of a harp net set to collect microbats (Photo Doug Beckers)

An example of a harp net set to collect microbats (Photo Doug Beckers)

The results were interesting.

When mosquito abundance was high, the saltmarsh was selected preferentially for foraging instead of the coastal swamp forest. However, at times of low mosquito abundance, the bats spread their activity across a range of habitats. The results suggest that this species of bat may be moving into habitats of high mosquito abundance to feed.

Little Forest Bat (Vespadelus vulturnus) (http://museumvictoria.com.au/melbournemuseum/discoverycentre/wild/victorian-environments/dry-forest/little-forest-bat/)

Little Forest Bat (Vespadelus vulturnus) (Photo: Melbourne Museum)

This is the first radio-tracking study to demonstrate a shift in habitat use by an insectivorous bat species in association with fluctuations in the abundance and distribution of a particular prey population. The shift in habitat use by the little forest bat suggests that, for this species, the saltmarsh mosquito may be an important food item (at least at times when mozzies are abundant).

Does this mean that broadscale mosquito control activities should stop? There is no doubt that the results of our research indicates that the little forest bat moves into areas where the saltmarsh mosquito is abundant. However, whether reducing the abundance of mosquitoes will have ecological implications is yet to be shown. The bats in our study site certainly weren’t eating enough to reduce nuisance-biting impacts of the mozzies!

The Saltmarsh Mosquito (Aedes vigilax) (Photo: Stephen Doggett)

The Saltmarsh Mosquito (Aedes vigilax) (Photo: Stephen Doggett)

The implications for wetland management are likely to change from location to location. In areas where there is abundant alternative prey such as moths, or mosquitoes not targeted by control activity, are present, reducing populations of key pest species like the saltmarsh mosquito may be achieved without any significant ecological impact. It is important to note that there is no evidence from our studies that the little forest bat is targetting the saltmarsh mosquito specifically. That mosquito was most abundant in this local area but in some regions along the coast, there are equally abundant mosquitoes (e.g. mosquitoes associated with freshwater or brackish water environments), many of which are not the focus of control efforts.

Perhaps an adaptive management approach is required whereby careful monitoring of bat populations is recommended and that, in areas where local bat populations may be shown to be susceptible, avoid control activities during the lactation period of bats, a time when their energetic demands are greatest. As these periods generally fall in late spring and early summer, as opposed to peak periods of  mosquito-borne disease risk that generally fall in late summer and early autumn, an integrated approach to management of both public health and environmental health can be achieved.

One of the key findings, with implications for further research directions and mosquito management, is that species-specific studies are required to understand the ecological role of mosquitoes. This result indicates that the importance of local mosquitoes for insectivorous bats is likely to be specific species (perhaps both the bat species and mosquito species), as well as no doubt varying from region to region. Managing not just the habitats but the insects associated with them may be important. Maintaining suitable habitats for bats under the pressures of expanding urbanisation along much of coastal Australia is critical. It will take more than just retaining these remnant habitats, it will be the strategies used to manage these wetlands and surrounding habitats that will also be important.

You can read the media release regarding this project and publication at the University of Sydney news page.

The full citation for our new paper is: Gonsalves L, Law B, Webb C, Monamy V (2013) Foraging Ranges of Insectivorous Bats Shift Relative to Changes in Mosquito Abundance. PLoS ONE 8(5): e64081. doi:10.1371/journal.pone.0064081