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Gardening From Alaska

Fungus Gnats or Fruit Flies
by Jeff Lowenfels
by Jeff Lowenfels


Jeff is the Past President of the Garden Writers of America, a columnist with the Anchorage Daily News, Host Alaska Gardens and Supporter of Plant a Row.

December 18, 2005

This time of year I get lots of calls and emails asking about pesky little “fruit flies” that “just started to appear” literally before readers’ eyes.

This is the indoor season of the fungus gnat, ¼ to 1/8 inch long black flies that “suddenly” appear indoors after the first few frosts. These are not fruit flies. If you look closely they will resemble either a tiny mosquito or a very small house fly depending on what kind of gnat infests your indoor soils. Both lay their eggs in damp locations, primarily on soil. In about 5 days the eggs hatch and small translucent-white, legless larvae appear. Each has a black head and a hearty appetite for dead roots which, along with fungus and other organic matter, sustains them until they reach ¼ of an inch or so. This takes 10 to 14 days and is the only time fungus gnats cause any damage to plants. If there is not enough other favored food, they will eat root live root hairs and root branches. The rest of the time they are merely nuisances though they can carry pathogens from plant to plant.

After a couple of weeks the larvae pupate for 4 or 5 days in tiny white, silky cocoons in the soil. The adults subsequently emerge, flies that live about 10 days. The females hover from pot to pot laying to 300 eggs in batches of up to 30.

Do the math: If enough get started, they can quickly become more than just a nuisance that seem to be attached to the CO2 in your breath or the moisture in your sink drains. Leaves can start to yellow, fall off and plants actively growing can be stunted.

Fungus gnats can be a real greenhouse problem anywhere because of the moisture required in most. In homes they are particularly attracted to peaty material and bark-based potting soil. Most of the potting soil sold here contains both and so our homes are particularly attractive to fungus gnats.

Plants most susceptible to damage from fungus gnats are small ones, seedlings in particular, where there is not enough dead root material to satisfy the population of larvae. As for house plants, fungus gnats love the soil of African violets, cyclamen, pelargoniums and poinsettias. Carnations, too, are very susceptible to fungus gnat damage.

Actually the damage is all caused by the activities of the larvae. As root hairs are destroyed by their chewing and populations grow, root branches are next in the line of fire and the plant starts to yellow and drop leaves. For the ultra curious, the transparency of most of these larvrae allows you to see the food in channels inside individual “maggots.”

Letting soil dry and making sure there pots are not standing in water will help control populations, but creates a certain modicum of risk of possibly letting the plants go too dry to the point of no return. Covering the soil with circles of newspaper frustrates the egg laying process.

You can trap larvae by cutting small potatoes in half and placing the cut side on the soil. The larvae will congregate there and can be removed in a few days.

There are a number of chemical treatments, most not suitable for houseplants. There are some neem products that work as a soil drench and are safe to use.

Or you can release your very own army of fungus gnat larvae-eating nematodes to decimate populations. You buy a sponge loaded with the microscopic nematodes, soak it in water and then apply the water to your plants. They heat-heat seek the target larvae and that is pretty much the end of the problem.

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