Messing with microbats
Microbats go largely unnoticed due to their tiny size and quiet, nocturnal habits. But in Dr Leroy Gonsalves, these winged creatures have found a friend.
"People don't give a lot of thought to microbats. I guess they tend to go unnoticed as they only come out at night and can be quite small.
The bats I worked with for my research are tiny - the smallest we have in Australia. Most of them weigh about four grams and are about the size of a small matchbox.
I studied their diets in wetlands on the Central Coast for a project funded by the NSW Environmental Trust.
Local residents had experienced nuisance biting impacts from a locally abundant mosquito species, the saltmarsh mosquito, which is also a known vector of Ross River virus. A local residents group requested the local government control adult mosquito numbers by spraying wetlands with a larvicide, Bti, which kills a high proportion of the larvae of saltmarsh mosquitoes.
However mosquitoes may also be food for up to 15 insectivorous bat species, six of which are threatened species and protected by legislation in NSW. Before the government could give the go-ahead for spraying, they wanted to know if it was going to be problematic for bats in the area.
I joined researchers from the University of Sydney and the Department of Primary Industries to investigate bat diets to determine just how important mosquitoes are as a food resource for local bats.
We tackled this question using several approaches.
Firstly, we surveyed the local wetlands where the mosquitoes breed to identify which bat species may be feeding on mosquitoes, and measure how many mosquitoes were present and whether this changed throughout the warmer months.
The next step was to examine the diets of those bats that spent more time in areas with saltmarsh mosquitoes. We did this by trapping bats every fortnight in summer and autumn and collecting their droppings, also known as guano, so we could see what they were eating.
Because bats chew their food very well and we were looking for a specific type of mosquito.
We couldn’t use traditional approaches that involve staring down a microscope and identifying insect remains to broader groups such as moths, beetles or flies.
Instead, we used an approach that at the time was still early in its development. It involved using DNA to identify what specific insects the bats were eating. The method was very successful and we were able to identify two bat species that were feeding on saltmarsh mosquitoes. Interestingly, both species were the smallest in our study area, weighing 4 grams and 4.5 grams each. They both also consumed moths and a range of other insects. A limitation of the DNA approach is that it only tells you whether a bat has eaten a certain type of insect and not how many.
Radio tracking bats
The final step in the project was to use another metric to infer importance of mosquitoes to bat diet. We used habitat selection, and radio-tracked bats to learn where they spent most of their time hunting and whether that corresponded to areas with high numbers of saltmarsh mosquitoes. To do this, we attached little tags to their backs and released them.
We were then able to track their movements at night, both when mosquito numbers were high, and when they were considerably lower. We found that bats would spend most of their hunting time in wetlands when mosquito numbers were really high, but the numbers of other insects were similar to other habitats. The interesting result was that the bats shifted to feed in other areas, also with lots of mosquitoes, when mosquito numbers dropped in the wetlands.
Our results demonstrated that saltmarsh mosquitoes are important to the smaller bat species in the local area and that these are likely to be most sensitive to broad scale mosquito control.”
Dr Leroy Gonsalves completed a Bachelor of Environmental Science (Honours) and a PhD at ACU. Explore science at ACU.