Keeping Bugs Out Of Your Pond

The main breeding areas for mosquitoes in the city are not wetlands, but instead the numerous small puddles and accumulations of water around people's houses, e.g. rainwater in eaves, buckets, or even the drain pans for potted plants. The Asian tiger mosquito, which is fairly new to North Carolina and was reportedly transported to this country in a ship carrying old tires, needs very little water to reproduce.

The tiger mosquito is also the one that is active even during the day. Native mosquitoes generally become active at dusk. So part of the mosquito problem comes from poor regulation of overseas shipping, and the cumulative effect of homeowners allowing water to stand in containers on the patio and elsewhere. In wetland plantings, there are two main ways to prevent use for breeding by mosquitoes.

If the wetland has standing water year-round, it can be stocked with predators of mosquitoes. Fish are the most effective, specifically the native gambusia, also called mosquito fish, which is related to guppies and is common in our urban creeks. A couple of caveats regarding mosquito fish are in order. They may suppress desirable species such as aquatic insects and amphibians, and their tendency to quickly become numerous may alter the pond's chemistry in a way that encourages growth of algae. The fish, however, are so effective at mosquito control that they effectively turn wetlands into mosquito traps. Other predators that reduce mosquito populations include predacious beetles, bugs and spiders, dragonfly and damselfly larvae and adults, and water striders.

The larvae of dragonflies and damselflies would need a refuge of standing water in order to survive in the wetland. Water striders can fly, which means they will come and go. Amphibians can also prey on mosquito larvae. Tadpoles eat mosquito eggs. Some kinds of salamander larvae eat mosquito larvae. To insure that water remains year-round, a refuge can be made by digging a hole a few feet deep and wide somewhere within the wetland, where rainwater can collect and remain through droughts. Though dense wetland vegetation and mud would typically keep people away from any deeper water, a metal or wooden grate could be placed over the refugia if there are any safety concerns. The other approach is to design the wetland so that water doesn't linger for more than a few days, thereby preventing any mosquito larvae from maturing into the adult stage. Adding protection are the many natural predators of mosquitoes that the complex ecology of a diverse wetland planting promotes. Possibly the most effective way of limiting mosquitoes is through soil fungi and bacteria that prey upon them.

At Indian Trail Park, where there are wetland gardens of varying ages, I noticed that a one year old garden had lots of mosquito larvae in its standing water, while the surface water in a garden established elsewhere in the park five years ago had none. The difference, according to Nolan, might be that the older garden had developed a population of two naturally occurring soil fungi, Coelomomyces and Laegenidium. These cause diseases in mosquito larvae, and tend to build up in the soil over time. There is also a soil bacterium that can be purchased and applied to the standing water, to kill the mosquitoes there.

A product called Mosquito Dunks has Bacillus thuringiensis israelensis, which produces a mosquito larvicide and comes in the shape of donuts that can be thrown into standing water, remaining active for a month or more. Specific to mosquitoes and other light flies, it can be bought at some local plant nurseries.