Monday, June 15, 2015

Mosquito Season

This is the time of year when people start asking questions about repelling mosquitoes, especially about the effectiveness of those "mosquito plants" that turn up in garden centers and big box stores. Supposedly, having a few of these around the deck will keep mosquitoes at bay. Unfortunately, this is not true. The plants are a geranium bred to produce citronella oil. Citronella oil has been shown to be effective as a repellant when rubbed on the skin. Its mere presence is unlikely to have any effect on skeeters.

Mountain Mint from the Lady Bird Johnson Center gallery
Those large citronella candles have been shown to work fairly well,  but much depends on such factors as the presence of wind, how many people are outdoors together, and how many mosquitoes are in the vicinity. Plus, you have to replace them as they burn down, and the burning wax adds a lot of carbon pollution to the atmosphere.

It appears that the application of citronella directly to the skin may be a better approach. Other natural plant oils that have a lemony fragrance also seem to work. Lemon balm oil is reported to be more effective than DEET, for example. (DEET is the most widely used synthetic repellent, found in products such as "Off.") Rose-scented geraniums produce both a citronella-like oil and geraniol, a rose-like scent. Even catnip has shown some repellent effect.

For those interested in native plants and traditional remedies, mountain mint (Pycnanthemum) has shown repellent activity, and you can find it blooming along roadsides around the summer solstice and later in the season at higher elevations. It's about 30 inches tall and the leaves around the blooms look like they have been daubed with white paint. It is adaptable as a garden plant, though can be rambunctious.

In all of these cases, the repellent plant is most effective if its essential oil is rubbed on the skin, and the effect lasts only about 30 minutes. However, this would be sufficient time for a stroll around the garden, for example.

Another important strategy is eliminating the places where mosquitoes can breed on your property. This includes obvious things like a wheelbarrow left out and collecting rain, to un-noticed things like a sag in your gutter, where water can stand. Asian tiger mosquitoes, to name but one of several species found around here, can breed in a tablespoon of water.

If you have a pond of any size, it should be stocked with goldfish, mosquitofish (Gambusia affinis) or small tropical fish such as guppies. The tropical fish will not survive a winter in the Tennessee Valley, but they will multiply all summer and turn your garden pond into an effective mosquito trap by consuming all the larvae that manage to hatch. If you don't want fish you can use "Mosquito Dunks" a commercially available product based on a species of bacteria that kills mosquito larvae without harming other organisms. (This product is comparable to Dipel, or Bt, used on vegetable gardens, and in fact is a different strain of the same bacterium.)

One article I found online mentioned using "native" fish in your garden pond for mosquito control. While this might be good advice in some locations, it is definitely bad advice in Tennessee, where the possession of any native non-game fish species "away from its native habitat" is prohibited by law. It is also illegal to keep game fish in captivity. There are a few "rough" fish species that are both native to the state and allowable by the fishing regulations, but why bother seeking them out when goldfish and guppies are available widely? (Large farm ponds can be stocked with certain game fishes, but we are talking here about garden ponds much smaller than the average swimming pool.)

Although it might seem counterintuitive, do not use pesticides to control mosquitoes, even the so-called "safe" products made for "kitchen and bath" use, and those intended for spraying around the patio before your party begins. All these products will accomplish is reducing the population of natural mosquito predators, such as spiders, dragonflies, and praying mantis. The active ingredients used in these products are harmful to aquatic organisms and definitely should not be used near a pond or any natural waters.

Mosquito traps range from highly efficient to totally ineffective. In particular, the untraviolet "bug zappers" that have been around for years rarely catch mosquitoes, as the mosquitoes are not attracted to the UV light. The more effective traps rely on chemical lures, often merely carbon dioxide, to draw mosquitoes to their untimely deaths. This kind of equipment, originally developed for the military, can be as expensive as a nice gas grill, and it also runs on a propane tank requiring regular replacement. Unless you live in the Okeefenokee, it is probably an unwise investment.

Screened porches and lawn tents with mosquito netting are among the most effective permanent solutions for outdoor living, and the long term cost is probably much lower than repellents or traps. Tents should be taken down and stored dry when not in use, which will greatly increase their lifespan.

Mosquitoes find a potential human target by following the trail of carbon dioxide that we exhale. If you are fond of beer or carbonated beverages, your carbon dioxide output will be higher than that of other folks, and you are likely to be bitten more frequently. However, the mosquito decides upon landing whether to bite, based on chemicals on the surface of your skin. People who have not bathed in a day or two are far more likely to be bitten than the more fastidious. (This may help explain the popularity of bathing in ancient Rome, where malaria was a well-known killer.) This also explains why the most effective repellents, whether natural or synthetic, are applied directly to the skin, and probably accounts for the claims made for commercial products that were not intended as repellents but do appear to work for some people. (Avon Skin-so-Soft is often cited.)

Bottom line: approach the problem with common sense. Temporary and all-natural protection is available for when you want to enjoy a cool drink on the porch and you live in the suburbs. If you live near a creek and spend a lot of time outdoors, you need additional control measures. If you are throwing a cookout party for friends and relatives, use citronella candles and offer everyone the repellent of your choice.

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