It's a question that pops up all the time.
"When the new wetland is finished, will there be an increase in mosquitoes?"
No-one likes mozzies and it's logical that more water could mean more mosquitoes. Those living close to a new wetland, pond or rain garden are understandably concerned that, come summer, they could be inundated with the buzzing blood-suckers thanks to the ACT Healthy Waterways project over the fence.
So do constructed wetlands, ponds and rain gardens attract mosquitoes and can we expect numbers to grow once they are operational?
In 2014 researchers at the University of South Australia set out to answer that question, specifically about wetlands (which are very similar to ponds and not at all the same as a rain garden but we’ll deal with that later).
They sampled mosquitoes on a monthly basis at seven constructed wetlands around Adelaide both prior to, during and after construction. Although they found annual fluctuations in numbers (due to seasonal variations like rainfall and temperature) they determined that there was no change in the number of mosquitoes post-construction compared with prior.
In fact, they found that numbers decreased as the wetlands matured and plants and natural predators became more abundant.
You can read the full report What is happening to the mosquito abundance in the constructed wetlands? [PDF 912 KB].
Now, let's get back to rain gardens.
A rain garden is basically a depression densely planted with native sedges and grasses. Mostly dry, it will fill to a maximum of about 30cm during a storm. It will then drain within three to four hours: not long enough for mosquito lavae to complete its seven to twelve day life cycle.
In conclusion, while it’s true that mosquitoes need water to breed, the evidence suggests that we can expect no more or less mosquitoes after a wetland, pond or rain garden has been constructed than before. What’s more, as the waterbody matures, mosquito numbers should decline.
The more common cause of mosquitoes is standing water in backyards – things like uncleared gutters, ponds, pot plant dishes and neglected pools.