The appearance of the Zika virus in our hemisphere, with all its attendant worries for expectant parents, casts a new and darker light on the presence of the Asian tiger mosquito. This ubiquitous pest of Mid-Atlantic gardens is closely related to the mosquito spreading the disease in Brazil (Aedes aegypti), and the tiger mosquito (Aedes albopictus) has been known to spread the disease in Africa.
My recent story on the tiger mosquito in the age of Zika prompted readers to wonder about garden features that might prove a haven for the mosquito. As a so-called container mosquito, it has evolved to reproduce in the smallest amounts of collected water, even in bottle caps. Thus, the species has found a nirvana amid the detritus of the human landscape, where so many people seem clueless as to how other animals live. It is no wonder that the tiger mosquito has spread across much of the United States since it arrived from Japan in the 1980s.
This is how the insect works: The mated female adult needs the protein of a blood meal for egg development. She then lays her eggs in standing water, they hatch into larvae that develop and turn into a pupa before emerging as a winged adult. The whole process can take little more than seven days.
Some potential breeding grounds are obvious — old tires, buckets and abandoned swimming pools — but with this sneaky pest, others are not as apparent: Corrugated PVC drainage pipe, a clogged gutter, a soda can buried in a rug of ivy, even folds in the tarp over the woodpile.
Folks who are aware of the problem are keen to minimize potential breeding sites before the mosquito begins to appear in May, but they have questions about how to deal with some common garden elements that could cause problems. Best to deal with three landscape elements in particular:
Mosquitoes prefer to lay eggs in stagnant and shallow water, so ponds that are deeper and with a robust filtration and circulation system are better placed against the pest. But the best way to rid your pond of mosquitos is to stock it with fish; they spend their whole day looking for food, and that wriggling mosquito larva presents the perfect snack.
I have had a fish pond for more than 20 years and never had a problem with mosquito larvae. I first stocked the pond with hardy goldfish and about six years ago added mosquito fish. The latter are smaller , growing to just three inches or so, are dark, almost black, and reach every nook and cranny in the pond. Both have proven winter hardy for me — all they need is a hole in the ice, which can be achieved with a floating heating element. I threw in five mosquito fish, which increased naturally to several dozen. If your local aquatic pond store or garden center doesn’t have them, common goldfish work just as well.
Even mosquito fish may not be able to pierce a mat of algae, so it’s important to keep on top of algae growth, which is a problem in early spring as the water warms. You clean up string algae by scooping it out and adding a fish-safe algaecide to the water (available at pet/pond stores). You prevent it by growing aquatic plants that will cover much of the surface of the pond and rob the algae of the light and nutrients it needs.
The popularity of rain gardens, in which storm water is captured before it enters the municipal storm sewer, has led to the parallel interest in rain barrels to capture water coming off roofs (we called them water butts where I come from). These barrels can become prime breeding sites for mosquitoes if you’re not careful. The slightest opening between downspout and barrel should be well screened. “I’ve seen some with not-good screening and [mosquitoes] breed like mad,” Jeannine M. Dorothy, an entomologist with the Maryland Department of Agriculture, wrote via email.
If you have a rain barrel, you should also use mosquito dunks containing a biological larvacide named Bt. One method of thwarting mosquitoes is to put a few drops of mineral oil or detergent on the water to break the surface tension they need to hatch. These “would not work so well and would need constant re-application, which homeowners are not likely to do,” she wrote. So screen the barrel, add a dunk and use the water for your garden.
It’s so much fun to see birds playing in and around these shallow basins, but you cannot neglect them. Even if we lived without mosquitoes, you should replace the water every two to three days for the sake of the birds. My neighbor in the community garden replenishes the bird bath every day. According to the Cornell Lab of Ornithology: “To provide a safe drinking and bathing environment, it’s important to change the water every day or two.” Good advice. If you are away from home for more than a week, you should flip the basin over to prevent rainwater accumulation, or store the bird bath under cover.
As we are out in the garden getting ready for the growing season, spend a moment or two thinking about other potential breeding areas for tiger mosquitoes this summer. Even without the threat of Zika, these mosquitoes are irritating in so many ways.
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