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Q. I’m puzzled by the presence of a few fruit flies hovering around my house plants this winter. How is this possible, when there are no fruits or vegetables nearby? How can I eliminate the flies?
A. Small flies hovering around house plants are not fruit flies. They are fungus gnats. The gnats lay eggs in potting soils. The resulting larvae, tiny white worms, feed on organic matter in the soil and on plant roots.
Fungus gnats are commonly brought into homes from eggs or larvae in the soil of newly purchased house plants or plants brought back indoors after a summer outside.
Sometimes, they are in potting soils used for house plants.
For a few years I had significant infestations of fungus gnats in the flats of transplants I grow under plant lights. The infestations disappeared when I changed the purchased soil component of my seeding and transplanting mix.
A primary prevention measure is to check bag labels carefully to make sure the planting mix you purchase for house plants (and indoor seeding and transplanting) is clearly indicated for indoor use.
Fungus gnat life and breeding proliferates in wet soils. Letting a top layer of soil dry before watering again, moderately, is helpful. To trap adults I have used yellow sticky traps, available at garden centres and from most seed and garden catalogues. The ones I used are Safer’s Sticky Stiks Houseplant Traps
Fruit flies are most troublesome in kitchens around fruit and kitchen compost containers during late summer and early autumn. For this indoor pest, homemade traps of apple cider vinegar with a drop of vegetable oil in shallow glass jars work well to keep their numbers down.
Yellow sticky traps attract and trap fruit flies as well as fungus gnats. In the fall, I often place one or two beside the kitchen sink.
Sticky traps should be used only indoors. Outdoors, they will attract and destroy beneficial insects as well as flying pests.
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