You may get a break from dealing with outdoor pests in most parts of the country during the cold months of winter, but in the warmth of your heated home, houseplants pests can still be a problem. Often these trouble-makers have hitchhiked in on plants that spent the summer outdoors. But they can also come in on newly purchased plants. At this time of year, when many of us are purchasing plants for holiday decorating or receiving them as gifts, it's easy to bring in some unwanted visitors as well.
It is always a good idea to give any plant coming indoors a thorough going over before it takes up residence with your other houseplants. Check under the leaves and along the stems of plants that summered outside before you bring them back indoors. You may even want to give them a couple of precautionary sprays with a mild insecticide like insecticidal soap just to be on the safe side. And if you can give them a spot at least ten feet apart from your other plants for a couple of weeks, you may keep a beginning pest problem confined and easier to treat.
When you buy new plants, give them a similar look-over before you bring them home. Pass over any with signs of insect infestation. And what should you be looking for? Some of the most common houseplant pests are spider mites, mealybugs, aphids, fungus gnats, and scales.
Spider mites, which are spider relatives, not true insects, thrive in the hot, dry air of our heated homes. Although they're tiny, the feeding of these sucking pests on the undersides of leaves can cause a lot of damage to plants. Look for leaves that are stippled with yellow and, when the infestation is severe, leaves encased in fine webbing. Hold an infested leaf over a piece of white paper and tap it sharply; if you see tiny, moving, dark dots on the paper, this will confirm your identification.
If you catch a spider mite infestation early, you may be able to bring it under control by spraying all leaf surfaces with water from the kitchen sink sprayer every few days. For larger infestations, control mites with sprays of insecticidal soap or horticultural oil. Repeat treatment at least twice at five day intervals to make sure you mop up mites newly hatched from eggs. It's probably best to discard severely infested plants.
To help reduce the chances of problems with spider mites, keep the humidity high around your plants. Line a waterproof tray with pebbles and fill with enough water to come up about half the depth of the pebbles. Then set your plant pots on the pebbles. The evaporating water will raise the humidity level around the plants, but the pots won't be sitting in water.
Aphids are pear-shaped, soft-bodied insects that feed by sucking, usually on the newest growth, and are often found in large numbers. Sometimes you'll notice the sticky ″honeydew″ they excrete, which turns a dark sooty color when mold grows in it. Because aphids are generally found clustered on the tender new growth of plants, start by pinching off and destroying these heavily infested parts of the plant. The kitchen sink sprayer will often take care of a mild infestation. Insecticidal soap sprays are effective for bigger pest populations.
Mealybugs look like flattened, oval, cottony masses on the undersides of leaves or tucked into the crevices where leaf stalks meet stems. They are covered with a white, powdery wax that looks like finely ground meal; hence their name. This waxy material may extend out from their bodies in lacy filaments. Mealybugs feed by sucking, weakening plants and causing yellowing leaves, distorted growth and dropping foliage. Like aphids, they may also secrete sticky ″honeydew.″ As many as 600 eggs are laid in a mass of cottony wax, so it's easy to see how a population explosion can happen quickly. While the adult insects may appear immobile, they crawl slowly and can spread from plant to plant, especially when leaves touch. Eggs hatch out onto mobile crawlers. This is the stage that is most vulnerable to insecticide sprays, because the newly hatched crawlers lack a protective waxy coating.
Use a cotton swab dipped in rubbing alcohol to remove individual insects and egg masses, repeating weekly until all of the pests are gone. Or you can dislodge mealybugs by washing plants with a soft brush or cloth dipped in a solution of two teaspoons mild dish detergent in a gallon of warm water. If your plant is heavily infested, you can spray insecticidal soap or a mixture of one part rubbing alcohol to nine parts water directly on the mealybugs. Be sure to test these treatments on a small section of the plant before treating the entire plant since the foliage of some plants may be sensitive them. You'll need to make repeat applications according to label instructions to catch insects at a susceptible stage in their life cycle. It's probably best to discard severely infested plants.
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.
Make sure that any product you use is labeled for use indoors and lists the target pest as well as the plant you plan to treat. Even when using such low-toxicity products as insecticidal soap or horticultural oil, it's important to always read and follow all label instructions and precautions. The foliage of some houseplants may be sensitive to injury from certain products - many ferns, palms orchids, African violets, and begonias, for example, can be damaged by pesticides, even insecticidal soap - so check the label for information on plants to avoid treating with a particular product.