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Gardening Articles: Health :: Houseplants

What's Bugging Your Houseplants? (page 3 of 4)

by Susan Littlefield

Fungus Gnats

Just about anyone who grows houseplants has probably encountered fungus gnats, sooner or later. A cloud of tiny, dark, delicate-bodied flies rises up from a plant when you disturb its leaves. While fungus gnats generally don't do much damage to plants, they are a nuisance as they flit about.

Female flies lay their eggs in the potting medium of your plants. They are especially attracted to mixes high in peat moss, a characteristic of many mixes used for indoor plants. The tiny, white, worm-like larvae that hatch out feed on algae, fungi and organic matter in the top couple of inches of potting mix, as well as on the roots of plants. In small numbers, they don't usually cause much damage, but when their population is high or plants are at the vulnerable young seedling stage, plants may be harmed.

To bring a burgeoning infestation under control, allow the top couple of inches of potting soil to dry out between waterings. Fungus gnat larvae need moisture to survive; keeping the growing mix drier will decrease survival and make the mix less attractive to egg-laying females. Covering the surface of the growing mix with a thin layer of sand will also make it less enticing to females as a place to lay eggs.

Adult flies are attracted to the color yellow. Cut plywood into a 4 x 6 inch rectangle and spray it with bright yellow paint. Coat one side of the wood with a sticky substance such as Tanglefoot (available at garden stores) or petroleum jelly. Place the wood, sticky side up, horizontally on top of the soil. Flies landing on the yellow wood will be trapped. When the traps become covered with insects, scrape them off and re-coat the wood.



Scale insects look like small brown bumps on the stems and leaves; under each shell or ″bump″ is a sucking insect dining on the plant. Leaves of infested plants may be yellowing, limp, and covered with sticky ″honeydew.″ Scales are most vulnerable to sprays at the crawler stage of their life cycle, when the tiny, newly hatched young are moving over the plant looking for a place to settle down and feed.

If you catch an infestation early enough, you may be able to bring it under control by diligently removing the scales by hand with a toothpick or tweezers, a cotton swab dipped in rubbing alcohol, or by dabbing at them with a soft cloth dipped in a mild dish detergent solution as recommended for mealybugs. This technique is also useful to bring down the pest population before spraying.

For bigger infestations, make several applications of an insecticide labeled for use against scales on houseplants, such as insecticidal soap or horticultural oil sprays, repeating at ten day intervals. To be most effective, examine your plant carefully and time your sprays when the colorless to yellowish crawlers are active. Unless you pick them off, dead scales can remain on the plant for several months or longer. If scales are still alive, they will exude some liquid when crushed.

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