Melbourne’s rainwater tanks overflowing… with mosquitoes

By Mary O’Callaghan

As recently as the mid-1960s, domestic rainwater tanks were discouraged or even banned by some local councils in Australia because of concerns about environmental risks such as mosquito breeding. Between 1897 and 1955, Brisbane had 7 dengue outbreaks, and mosquitoes breeding in rainwater tanks were regularly found to be the cause. By the 1990s, almost no rainwater tanks remained in most of Australia’s urban areas.

How times have changed. Now, thanks to severe drought and the prospect of a drier future, and spurred on by subsidies and regulation, at least 30 per cent of homes in Brisbane, Melbourne, Adelaide and Sydney now have a rainwater tank installed.

But with the proliferation of tanks in our cities comes an increased risk of outbreaks of mosquito-borne diseases. And now, unlike in the early part of the last century, tanks are not being regularly checked for breeding mosquitos.

During 2013–14, we inspected rainwater harvesting systems in 417 Melbourne homes. The results gave us a shock—we found mosquitoes breeding in 21 per cent of them.

Public health warning flag or time bomb?

The fact that 21 per cent of tanks were infected with mosquitoes is a serious concern for Melbourne health officials and a warning flag for other capital cities.

To put the figure in context, the Queensland Government previously stated that its acceptable threshold is 1 per cent because of the risk of a dengue outbreak.

Dengue is currently confined to Queensland but, as the global climate continues to warm, it could become a problem further south.

Unsealed overflows the biggest culprit

The most common access routes for mosquitoes were found to be the tank inlet and the tank overflow, either because there was no mosquito mesh covering the opening or the mesh was damaged.

Unmeshed overflows were the biggest culprit, representing about 60 per cent of the mosquito-infected systems. This was despite good meshing of the inlet.

In about 22 per cent of cases, the mesh on the inlet was in poor condition and this allowed the entry of mosquitoes to nearly the same extent as having no meshing at all.

Even with adequate mosquito meshing on both the inlet and the overflow, 5 per cent of tanks still had mosquitoes breeding in them.

How the mosquitoes are gaining entry to these tanks requires more detective work. There could be a hole in the mesh, or perhaps the mesh is not sitting properly.

Wet systems are more of a risk and harder to check. In a wet system, pipes from the gutter run down the wall, under the ground, and up into the tank. The pipes can hold stagnant water that could become a habitat for breeding mosquitoes.

Blocked gutters can also cause water to pond, providing a breeding site for mosquitoes, with the eggs being washed into the tank.

5 tips for keeping mosquitoes from breeding in and around your tank

Seal both the inlet and the overflow with proper mosquito mesh, which is available from any hardware store. Poor meshing is as bad as no meshing, so check it regularly and replace the mesh if it is damaged or not well fitted. If your overflow water pools on an open grassy area, this can be a breeding ground for mosquitoes. Ideally, your overflow water should be diverted to a covered drain. Clean your gutters. Mosquitoes can breed in small pools of water that form in a blocked gutter. The eggs can then be washed into the tank. Be vigilant. Inspect your whole system regularly.

Want to learn more? Read the full article on this research in ECOS.